Rough Idle/Hesitation/Backfire

[Gary Yarbrough]

I'm hoping for some help with my 90 ZR-1. It idles just a little rough (not too bad), but it really hesitates if I floor it from idle. It will also sometimes backfire. If I smoothly and slowly give it the gas or start from about 2500 RPM, it is fine.

Things I've checked:
MAP hose is fine.
No vacuum leak (well...I've got the usual very slow leak).
Plugs & Wires are ~1 year old with ~5,000 miles.
Injectors are Accel with about 3,000 miles.
I have a chip from Jim VanDorn, otherwise the car is stock.

About two weeks ago, I checked codes because I got a brief SES light while cruising on the free way and had a left oxygen sensor code. I replaced the sensor and that problem seems resolved. I've had this idle issue for a while now and would really like to get it resolved. Any suggestions would be greatly appreciated!

[Jeffvette]

Coil(s)

[XfireZ51]

DVM the TPS sensor. Also, while the car is idling jiggle each of the three wires on the TPS.

[Gary Yarbrough]

Please excuse my ignorance, but what does "DVM" mean?

[DTZ ZR1]

I somewhat of the same problem, changed plugs it was a little better. After I ran it for a minute or two, I took plugs out and one was wet. Put them in started the car and pulled the plug wires off one at a time. When I got to #4 and #7 plugs there was no change in the idle and you did not hear the spark jump either. I am in process of changing them out. Good luck.

DT

[secondchance]

Please excuse my ignorance, but what does "DVM" mean?

Digital Volt Meter. I believe you are looking for .54 volts.

[Gary Yarbrough]

Thanks for the suggestions so far. When I pull the plug wires 1 at a time, all of them do make the idle rougher. However, there are some with a very strong loud spark (I can actually see it) and others that seem much weaker. So, I'm thinking I need new coils as suggested. So, I have a related question. Given that the plugs and wires are about 1 year old, should I also replace them when I do the coils or are they most likely okay?

[secondchance]

Thanks for the suggestions so far. When I pull the plug wires 1 at a time, all of them do make the idle rougher. However, there are some with a very strong loud spark (I can actually see it) and others that seem much weaker. So, I'm thinking I need new coils as suggested. So, I have a related question. Given that the plugs and wires are about 1 year old, should I also replace them when I do the coils or are they most likely okay?

As for plugs, depends on what was installed and visual inspection. Plugs can easily replaced without pulling the plenum. Visual inspection and replacement if they look bad. I ran AC Delco platinum plugs about 5 years ago. Car ran real bad right after installing them. Upon visual inspection, it turned out one plug had cracked insulation.

If not plugs then pulled the plenum. At that point check wires for resistance. Generally it should should be good for 3-4 years at least.

[Sgreg]

Wires should be fine. Take a close look at each plug to determine whether they need replacing.

[XfireZ51]

Thanks for the suggestions so far. When I pull the plug wires 1 at a time, all of them do make the idle rougher. However, there are some with a very strong loud spark (I can actually see it) and others that seem much weaker. So, I'm thinking I need new coils as suggested. So, I have a related question. Given that the plugs and wires are about 1 year old, should I also replace them when I do the coils or are they most likely okay?

If you find that the weak spark comes from a pair of plugs with a common coil,
ie 1-6, 3-2, then that would be a clue on needing coils.

[Gary Yarbrough]

Well....I changed the coils today and the problem is still there. On the upside, I now have a very strong spark when I pull the plug wires. So, it's plugs tomorrow (if I get a chance). I have three questions:
What's the proper torque for the plugs when i put them in?
How do I get out the rear plug on the passengers side?
Any other suggestions for what I should be looking for?

Thanks for any help I can get!

[XfireZ51]

DVM the TPS sensor. Also, while the car is idling jiggle each of the three wires on the TPS.

? Anything?

[Gary Yarbrough]

Maybe the TPS is my problem. I saw three wires: blue, white and black.
Between the back and the other two, I get ~4.5 and 4.0 volts. Does that sound possible and what's next?

[Kb7tif]

Well....I changed the coils today and the problem is still there. On the upside, I now have a very strong spark when I pull the plug wires. So, it's plugs tomorrow (if I get a chance). I have three questions:
What's the proper torque for the plugs when i put them in?
How do I get out the rear plug on the passengers side?
Any other suggestions for what I should be looking for?

Thanks for any help I can get!

Did you use d545 coils?

[Gary Yarbrough]

I used Accel coils.

[ZR1Vette]

Rear plug... bit of a challenge but basically you need a plug socket, short (1 1/2") extension, universal and then a longer (4") extension

Have you checked your ALDL/DIC for any fault codes?

[tomtom72]

Just thought you might look this over about #8 plug r&r...

http://www.zr1netregistry.com/ZR1_techtips.htm

This is the ticket. 20 N-m ( 15 lb-ft ) on the 90 plugs w/the flat seats & flat washer.

:cheers:
Tom

On the TPS, Black is sensor ground, Dark Blue is the signal to the ECM, and Grey is 5v reference signal from the ECM. ckt #'s 452, 417 & 416 respectively. Therefore you should read the TPS signal from the TPS to the ECM at the Dark Blue wire. A back probe on the Dark Blue (417) should yield a reading between .46v & .62v. If I understand the book correctly.

[XfireZ51]

Gary,

With key ON, the TPS signal (between black wire and blue)
should be around .54v. Once you have it set up, read the TPS signal voltage as you open and close throttle with motor off but key ON. See if the voltage change is smooth or changes in a "choppy" manner. Does the reading change abruptly? Also, make certain the TPS switch assembly is securely attached and not loose. Move switch wires around and make certain voltage readings aren't changing.

[Gary Yarbrough]

I checked the TPS voltage. I get 0.37v. With smooth pushing of the accelerator, the voltages fairly smoothly (maybe 0.5 or so v increments). Flooring it, it goes from 0.37v directly to ~2.5-3.5v. Time for a new TPS?

[tomtom72]

Gary, I have a 90 and I know for sure that my TPS is adjustable...to what degree I don't know, but the screws that hold it in place are in slotted holes, therefore I conclude it's adjustable.

You may want to look closely at those screws as you maybe able to make an adjustment. If you say that the needle on the VOM moves thru it's sweep at a smooth progression then maybe an adjustment is in order.

Just a fwiw, try it, in the end you can always go spend the money if the adjustment don't work.

:cheers:
Tom

[secondchance]

Gary, I have a 90 and I know for sure that my TPS is adjustable...to what degree I don't know, but the screws that hold it in place are in slotted holes, therefore I conclude it's adjustable.

You may want to look closely at those screws as you maybe able to make an adjustment. If you say that the needle on the VOM moves thru it's sweep at a smooth progression then maybe an adjustment is in order.

Just a fwiw, try it, in the end you can always go spend the money if the adjustment don't work.

:cheers:
Tom

It is adjustable. Loosen up both screws just enough to be able to rotate TPS and rotate until you get .54 volts. FSM gives you an acceptable range but gurus tell me .54 is optimal.:thumbsup:

[tomtom72]

Stupid of me not to include this. When I scan my 90 at idle, in closed loop, the TPS does read .54v on my scanner. So jsut make your adjustment at idle and make sure you are up normal operating temp.

Sorry I type faster than I think.....:o

:cheers:
Tom

[XfireZ51]

Gary, I have a 90 and I know for sure that my TPS is adjustable...to what degree I don't know, but the screws that hold it in place are in slotted holes, therefore I conclude it's adjustable.

You may want to look closely at those screws as you maybe able to make an adjustment. If you say that the needle on the VOM moves thru it's sweep at a smooth progression then maybe an adjustment is in order.

Just a fwiw, try it, in the end you can always go spend the money if the adjustment don't work.

:cheers:
Tom

Gary

As TT has said, loosen the TWO bolts on the side of the TPS and adjust with IGN ON so the DVM reads about .54v. Then tighten bolts
and check WOT TPS signal. Should be at 4.5v or higher. If not,
swap in new TPS.

[secondchance]

When you say back fire, do you send detonation in the plenum or hear it at the tail pipe?

I never had this situation with fuel injected cars but way back when cars had carburators, cracked ign. wire would make gas vapor explode into the air filter housing.

If it is coming through the tailpipe, its coming from uncombusted fuel detonating when it his catalytic converter.

I very much doubt TPS, even if off a little, would do this.

Lastly, I struggled with a similar symptom a few years ago. In my case it was hairline crack on one plug (very difficult to see). Cracked plug wire will behave in similar manner.

[Gary Yarbrough]

Here's where it will stand today:
I've got the TPS at the correct voltage. Operation seems smooth.
I've changed all plugs other than the dreaded #8. I'll need a new spark plug socket to put it in...will pick one up today if they have them at Sears.
All new injector coils.

Took the car for a spin: some improvement but still not perfect.
Then I changed my chip from a JVD chip to a DRM chip. Seemed a little better (or I may be imagining things). However, I feel that the JVD chip was better at the top end.
Best I can tell, it is now running okay above 2K RMP. Any chance getting the last plug in could cure the problem?

As for the backfire, it is hard to tell. I'm always in the car when it happens and I have some hearing loss that prevents me from knowing which direction sounds are coming from with any degree of accuracy.

Later, I'll get my wife to rev the car and see if I can pinpoint the noise.

Thanks for everyone's help!

[secondchance]

Here's where it will stand today:
I've got the TPS at the correct voltage. Operation seems smooth.
I've changed all plugs other than the dreaded #8. I'll need a new spark plug socket to put it in...will pick one up today if they have them at Sears.
All new injector coils.

Took the car for a spin: some improvement but still not perfect.
Then I changed my chip from a JVD chip to a DRM chip. Seemed a little better (or I may be imagining things). However, I feel that the JVD chip was better at the top end.
Best I can tell, it is now running okay above 2K RMP. Any chance getting the last plug in could cure the problem?

As for the backfire, it is hard to tell. I'm always in the car when it happens and I have some hearing loss that prevents me from knowing which direction sounds are coming from with any degree of accuracy.

Later, I'll get my wife to rev the car and see if I can pinpoint the noise.

Thanks for everyone's help!

Best way I found for removing #8 plug is to get a socket w/ rubber insert specifically designed for spark plugs (this allows plug to be pulled out of the hole after being unthreaded. Also, combination of short extension, swivel joint,added to long extension. With this combination, I can twist the plug despite odd angle required.
Also check all plugs and see if they appear uniform.
I am sure we (with help from gurus) can sort this out.:thumbsup:

[XfireZ51]

I use a breaker bar with a short extension and a swivel. Just need it enough to "break" plug. Then I use a 3/8" fuel hose on top of spark plug to turn it the rest of the way and pull it out of hole. Works for putting it back in as well.

[4DSZR1]

Best plug advice I ever got and it works GREAT

Socket
swivel
1" extension
swivel
Long extension


when its loose push the plug wire in and it will grab the plug

[Gary Yarbrough]

Thanks for all the advise. I got plug #8 changed and I would say that the problem is about 98% cured. I can easily take off with no hesitation with either the DRM or JVD chip. I just get a little touch of hesitation if I'm rolling below 1200-1500 RPM (hard to tell exactly...at about 8 mph) and floor it. Maybe that's not a good idea anyway or maybe I needed new plugs too... Also, there is no more backfiring! Thanks again for all the help!

[secondchance]

Thanks for all the advise. I got plug #8 changed and I would say that the problem is about 98% cured. I can easily take off with no hesitation with either the DRM or JVD chip. I just get a little touch of hesitation if I'm rolling below 1200-1500 RPM (hard to tell exactly...at about 8 mph) and floor it. Maybe that's not a good idea anyway or maybe I needed new plugs too... Also, there is no more backfiring! Thanks again for all the help!

Well, I am glad to hear 98%.
You might consider ('cause I am no guru) running a bottle of BG K-44 fuel system cleaner. You may have some deposit build up on intake valves. Now, some may chime in and talk about negative effect of this product I am not aware of so see general reaction first.
At least in my case, after injector replacement, a bottle seems to have cleaned up intake and combustion chamber and took mine to 100%.:thumbsup:

[Jeffvette]

Thanks for all the advise. I got plug #8 changed and I would say that the problem is about 98% cured. I can easily take off with no hesitation with either the DRM or JVD chip. I just get a little touch of hesitation if I'm rolling below 1200-1500 RPM (hard to tell exactly...at about 8 mph) and floor it. Maybe that's not a good idea anyway or maybe I needed new plugs too... Also, there is no more backfiring! Thanks again for all the help!


Take it up to 6th at at about 40 mph and mash the gas, it should pull smooth if everything is good.

[Gary Yarbrough]

I suppose I still have a problem. The car revolts if I floor it in 6th at 40mph. It will accelerate smoothly with gentle throttle. Any suggestions about what to try next?

[Jeffvette]

Gary these are steps you should take, skip the ones you have done already. Steps go from easiest to more difficult.

1) Inspect spark plugs. Looks for cracks in the porcelain.
2) Ohm out plug wires
3) Ohm out injectors
4) Ohm out coils


The only other thing outside of these issues is a failing ECM or DIS module. Eliminate as much as possible and let me know on the ECM, I have a test box I keep around for things like this.

[FU]

5) Do a compression test.

[Gary Yarbrough]

I'll probably be Saturday before I can Ohm out the plug wires (I'm on jury duty...).
However, I wonder if it might be something else. My belt tensioner is rattling a bit more than what I think is normal. I got my wife to floor the car for me while I had my head under the hood. I noticed that I get a sort of clapping sound when it gets floored from idle and the noise is in sync with a very large movement of the tensioner. Could a failing tensioner be my problem?

[Jeffvette]

Could a failing tensioner be my problem?

No.

[Kb7tif]

Thanks for all the advise. I got plug #8 changed and I would say that the problem is about 98% cured. I can easily take off with no hesitation with either the DRM or JVD chip. I just get a little touch of hesitation if I'm rolling below 1200-1500 RPM (hard to tell exactly...at about 8 mph) and floor it. Maybe that's not a good idea anyway or maybe I needed new plugs too... Also, there is no more backfiring! Thanks again for all the help!

Your running good now. Leave well enough be.
You have new accel coils, injectors, wires. Plugs?
You chip is pry tuned for top end not flooring it in 6th at 1K rpm.

[Gary Yarbrough]

Yeah, I've got new coils, injectors (~6 months old), and plugs. The wires are, however, a year old. Tomorrow, I'm putting the other chip back in to see if it will pull better in 6th at 1K rpm. I've got mixed feelings...I like for things to be perfect. But maybe I need to follow the DIMWIT rule in this case?

On another topic, how seriously should I take the belt tensioner issue? It really moves around a lot and does make some noise.

[flyin ryan]

On another topic, how seriously should I take the belt tensioner issue? It really moves around a lot and does make some noise.Jeff already gave you an opinion but if you are that worried about it just change it out & call it done. My 90 jump's around a bit too, I put a long TQ. arm/bar on it while it was running to take off say 70-80% of the load...didn't change the world all that much :cool:.

[Gary Yarbrough]

Don't get me wrong, I fully trust Jeff's opinion that the belt isn't causing the hesitation and I am very grateful for his and everyone else's advise! I was just asking if it is a problem in a more general sense. The loud clapping sound that seems to be coming from it when I floor it from idle doesn't seem like a good thing. But, I've got a lot to learn...

[flyin ryan]

Don't get me wrong, I fully trust Jeff's opinion that the belt isn't causing the hesitation and I am very grateful for his and everyone else's advise! I was just asking if it is a problem in a more general sense. The loud clapping sound that seems to be coming from it when I floor it from idle doesn't seem like a good thing. But, I've got a lot to learn...The learning never stop for all of us Gary. Your keeping your ears open & paying attention, that's all good ;). This noise your talking about from idle, is it when your in neutral/park, under load/power or both?

[Gary Yarbrough]

Code 63! Today I took the car for a long drive (120 miles) and I got the dreaded SES light. When I got home, I pulled the codes and got code 63 (it flashed this code three times). I just changed the left oxygen sensor. Could this mean that I need to change the right one too? Could this be the source of my issues?

Hopefully, tomorrow I'll get time to do all of the other checks that Jeff suggested. I'm so swamped right now that it is hard to get anything done!

[bldavis11]

I would say you're up for the starboard oxygen sensor!

[GregCrowe]

Start on this page and keep reading:

http://zr1netregistry.com/forum/showthread.php?t=1777&page=16

Those Accell coils are garbage. They made my car run like complete crap, backfire, etc.

[Gary Yarbrough]

Crap! I changed the second 02 filter and it's no better (well...it actually seems to run stronger on the top end, but it could be my imagination since I haven't slept much this week). It is now "backfiring" again. The "backfire" is coming from the plenum area for sure. Hopefully, tomorrow I'll get time to ohm out everything. I've got to get some gaskets ordered, I've run out of spares... I sure hope that my Accell coils aren't the problem.

[flyin ryan]

Got to say I feel for ya'...your having a tough go of it Gary. Just keeep telling yourself, 'It's something, It's something'...just have to pin-point it :dontknow:. Keep your chin up, bud :)

[flyin ryan]

When is it backfiring 'Exactly' Gary? Off idle, light load, Trottle 'Tip-In', W.O.T. (Wide open throttle) under load (power) :confused:.

[Gary Yarbrough]

The backfire only happens if I am at idle and I go to W.O.T. If I'm just parked and I go from idle to W.O.T. it only does it some times. If I'm coasting (say 7-8 mph) and go to W.O.T. it does it 80-90% of the time. So I'd say that it is worse under a load.

[ittlfly]

Start on this page and keep reading:

http://zr1netregistry.com/forum/showthread.php?t=1777&page=16

Those Accell coils are garbage. They made my car run like complete crap, backfire, etc.

Did you read this link as suggested by Greg? Especially pages 17 & 18? Your problem sounds very similiar.

[Gary Yarbrough]

I did read the link as suggest by Greg.

I checked the resistance of the coils I removed and here is what I got:
0.6/5.7K
0.6/5.69K
0.7/5.88K
0.6/5.62K

So, I don't think coils were my problem to begin with. However, I could be wrong and would be glad to hear it if I'm thinking about this the wrong way.

Maybe today I'll get time to pull the plenum back off and ohm out the plug wires and new coils.

I have noticed something else that might be relevant. When I'm sitting and I go from idle to W.O.T., I can hear a bit of a sucking noise for maybe 1-2 seconds. Could I have a vacuum leak or is a sucking noise from the throttle body normal?

[ittlfly]

The numbers for resistance look good. But you seem to have a problem under load. That's why I would still suspect the coil(s).

[flyin ryan]

Not saying this is it, but if this was happening when I'm on the dyno, first thing I'd check would be the timing...sounds like the timing is too far retarded? Do you have any idea what your base timing is, by chance?

[Gary Yarbrough]

I have no idea what the base timing is. I have notes on the car from about 25K miles and nothing to suggest that the camshafts have been retimed. However, I can't say for sure...

Now, I have a slight coolant leak. I had a chevy dealer replace the radiator about 2 months ago, so I'm a bit surprised. The leak is coming from somewhere near the bottom of the radiator. So, I spent my pulling the radiator shroud. I'm going to try to find the leak tomorrow when the sun is back out. I figure I should do the leak first and then the hesitation...

[flyin ryan]

I have no idea what the base timing is. I have notes on the car from about 25K miles and nothing to suggest that the camshafts have been retimed. However, I can't say for sure...

I'm just talking about the 'Base' ignition timing at this point, Gary. Not the 'Cam' timing, of coarse depending where one or more cams are in at, it could be that also...? For now I would concentrate on the basics.

[Gary Yarbrough]

I'm not sure about the "base" ignition timing. I'm not even sure how to find out.

For now, fixing the hesitation is on "hold" because my radiator failed. About 8 weeks ago, I had a local GM dealer put on a new radiator. It seems that the new radiator has failed at the lower passengers side seam where the side tank connects to the aluminum. I've pulled it and am waiting for confirmation that it was a manufacturing defect. Then, I'll take a week or so for them to get another replacement...

[Gary Yarbrough]

I got my radiator and will be installing it by Saturday, so I'm back to trouble shooting the hesitation. While the car is drained of coolant, I took the opportunity to remove the plenum this morning. I'll be checking everything this afternoon/tomorrow.

My dad also convinced me to take a closer look at my MAP sensor. I noticed something that I need some opinions on. I'll try my best to describe this. Using a flashlight, I took a look inside the hole where the hose connects on the bottom of the sensor. While it was clean inside, I noticed that at the bottom of the hole, it appears to me that there is a small piece missing in the plastic. It looks as if I can see inside the sensor to some electronics. Since I don't know how the sensor is supposed to look, I suppose this could be normal, but the hole is irregularly shaped. Does it sound like I need a new MAP sensor?

[XfireZ51]

I got my radiator and will be installing it by Saturday, so I'm back to trouble shooting the hesitation. While the car is drained of coolant, I took the opportunity to remove the plenum this morning. I'll be checking everything this afternoon/tomorrow.

My dad also convinced me to take a closer look at my MAP sensor. I noticed something that I need some opinions on. I'll try my best to describe this. Using a flashlight, I took a look inside the hole where the hose connects on the bottom of the sensor. While it was clean inside, I noticed that at the bottom of the hole, it appears to me that there is a small piece missing in the plastic. It looks as if I can see inside the sensor to some electronics. Since I don't know how the sensor is supposed to look, I suppose this could be normal, but the hole is irregularly shaped. Does it sound like I need a new MAP sensor?

If you are describing a vacuum connection that isn't sealed, then you need a new sensor. Trying to envision what you are describing. :confused:

[Gary Yarbrough]

I think that's what I'm describing. Let me try to do better. If the connector for the hose were a glass (let's say), then the missing piece would be in the bottom of the glass, not the sides. So, it is sealed in the sense that the hose is making a seal. It is just an issue of whether there should be a hole at the bottom or not.

[jeasen]

I had the same problem a couple of weeks ago with my ZR1. I found that I had water in my gas. Add some gas dry to your gas tank to see if this fixes the problem before you embark on a much harder and costly fix.

I think this is going to be a common problem now that the gas stations are charging more for premium fuel. The price for premium is no longer 20 cents more than regular and people are not buying it like they use to. Therefore the gas stations do not have the turn over like they use to have and I think we will see more water in the gas problems.

I hope this helps.
Jeasen

[Z51JEFF]

Ive heard nothing possitive about the Accel coils so if you havent replaced the plenum yet I would replace those coils.As far as the slapping noise,a miss will get the DM flywheel jumpin around and just might get the cam chains to make a racket.When I had an injector die I got a noise that sounded like a rod knock but knew it was the flywheel.

[Gary Yarbrough]

I'll be checking the Accel coils soon. I just finished checking the wires and here is what I got:
1: 8,700
2: 7,730
3: 7,720
4: 7,760
5: 6,370
6: 6,320
7: 5,260
8: 7,000

That seems like a lot of variability to me. However, I need some more experienced opinions as to whether I need new plug wires.

[Gary Yarbrough]

The new Accel coils check as follows:

0.9/7.35K
0.8/7.10K
0.8/7.10K
0.7/7.30K

[A26B]

First, doesn't look like you got an answer re the MAP sensor. Here's a photo of the underside. Suck on the tube & see if it holds vacuum. If it does, nothing broken inside.


http://i190.photobucket.com/albums/z224/A26B/MapSensor001.jpg

Plug wire resistance: All are within spec. Longer wires should have more resistance, all is good there except #8 is a little high but should be fine.

I don't read where you ever did a VISUAL spark test. Put a plug in one wire at a time, lay it on the cam cover or plenum, start the engine, pull the plug away from the ground slowly. A strong spark should be blue and 3/4"~1" jump. A weak spark will be orange. You should try that as it costs nothing and has often revealed the reason for a miss ocurring under load at low rpm. Such a miss shows up most often when accelerating in 6th gear from 50mph. If all is good, acceleration should be smooth with no bucking.

Such a miss is often not aparrent at high rpm or normal driving except for sub-par performance, unlike a dead-miss. A bad coil or wire/coil tower/plug with corroded connections are generally the problem.

[jeasen]

On Sunday and Monday I checked to many coils to count. I took my multi-meter to the auto parts store and checked them right there on their counter. I made them order in more before before I bought a set of four. From what I read coils should read .5 to .6 on the primary side and in the 5.5 to 6.0 on the secondary side. Coils that read more than the above on either side are out of spec and should not be used.

I installed the coils I bought yesterday but have not finished to let you know how my ZR1 runs with the new coils. I should finish up tonight and have a report for you by tomorrow.
Jeasen

[Gary Yarbrough]

First, doesn't look like you got an answer re the MAP sensor. Here's a photo of the underside. Suck on the tube & see if it holds vacuum. If it does, nothing broken inside.


Plug wire resistance: All are within spec. Longer wires should have more resistance, all is good there except #8 is a little high but should be fine.

I don't read where you ever did a VISUAL spark test. .

How much vacuum should the MAP sensor hold?

When I get the radiator on tomorrow, I'll do the visual spark test.

[jeasen]

I forgot to mention that I did find one dead coil (no ohm reading at all between the secondary terminals). I had questionable spark coming from one side of two other coils. Anyway I decided that if I was going to pull the plenum to replace a dead coil then I was going to replace all four coils. I figured that it would only be a matter of time before the other coils failed. I changed coils on other cars but I got to tell you this was by far the hardest (longest) job to date but the results were well worth the work.

I did drive the car last night. The motor idles much smoother and I did not detect any high speed miss. The car pulls really hard all the way to the redline.

Jeasen

[Gary Yarbrough]

I did drive the car last night. The motor idles much smoother and I did not detect any high speed miss. The car pulls really hard all the way to the redline.

Jeasen

That's great news! I hope it keeps working for you.
I hope I'm so luck tonight if I get mine back together.

[Gary Yarbrough]

Weeks later....

I got my new radiator on and the cooling is better than ever. I've still got the hesitation. I did a "rough" visual spark test and everything looked good. I say rough because I wasn't exactly sure how to do it. Should it be idle only or should the car be rev'd up?

[tccrab]

Weeks later....

I got my new radiator on and the cooling is better than ever. I've still got the hesitation. I did a "rough" visual spark test and everything looked good. I say rough because I wasn't exactly sure how to do it. Should it be idle only or should the car be rev'd up?

Gary:

The "Visual Spark Test" will tell you all you need to know at idle.
One of these "Noids" are very handy and can potentially save you from any electrical related problems from allowing that much voltage to go to ground on top of your motor.
http://www.oreillyauto.com/site/c/detail/DOR4/86579.oap?partType=N0665&parentPartType=C0103

TomC
"Crabs"

[Jeffvette]

Weeks later....

I got my new radiator on and the cooling is better than ever. I've still got the hesitation. I did a "rough" visual spark test and everything looked good. I say rough because I wasn't exactly sure how to do it. Should it be idle only or should the car be rev'd up?


Doing it at idle is fine. And it's not so much a visual, but you should be able to hear it arcing.

[Gary Yarbrough]

Thanks for all the advise! At idle the spark sounds "loud and clear"... Does anyone know if there is a way to test the ECM before I send it off to have it rebuilt?

[XfireZ51]

Thanks for all the advise! At idle the spark sounds "loud and clear"... Does anyone know if there is a way to test the ECM before I send it off to have it rebuilt?

Is the motor running? Is this just a miss at idle? Why would the ECM need to be rebuilt? Is there a code?
Seriously guys, the ECM and ignition module are core systems not just for running the motor but even more importantly, from an OEMs perspective, emissions. So these components are pretty robust with other peripheral components more likely to fail first. I just see many here jumping
to the ECM problem as a conclusion for an idle miss etc. Particularly if the motor hasn't been scanned and datalogged. I'm not doggin anybody but just my .$02. I'll get off my soapbox now.

[Pete]

What brand of plugs are you using.

If i had to guess by your symptoms,Bosch.

Also where did you gap the plugs?

Also why in gods name would you wanna put it in 6th gear @ 40mph and floor it. :jawdrop:

Pete

[Gary Yarbrough]

The issue is deeper than rough idle. It is a hesitation in the ~1500 to 2200ish RPM range.

Pete, you guessed right about the plugs. Am I correct in assuming that I've gone wrong with these? As for the gap, I don't recall off the top of my head but I set them at whatever the stock gap was.

[secondchance]

I agree w/ XfireZ51. Before going into ECM based on my personal experience;

1. Check for cracked insulation on plugs. What the heck! Try a new set of standard Delco plugs - only $30 top.
2. You said MAP hose is fine. Is it factory? Factory hose is a hard plastic with rubber boot at both ends only. I thought my MAP hose was fine but found out it was collapsing at low RPM (high vacuum condition) after aging w/ fuel vapor and heat degradation.

I apologize if this had been discussed already.

[XfireZ51]

The issue is deeper than rough idle. It is a hesitation in the ~1500 to 2200ish RPM range.

Pete, you guessed right about the plugs. Am I correct in assuming that I've gone wrong with these? As for the gap, I don't recall off the top of my head but I set them at whatever the stock gap was.

I doubt hesitation is spark related. If you are hitting the accelerator and its "bogging" that's a sign of air but not enough fuel. As for the backfire, is it in the exhaust or the intake?

[Gary Yarbrough]

I doubt hesitation is spark related. If you are hitting the accelerator and its "bogging" that's a sign of air but not enough fuel. As for the backfire, is it in the exhaust or the intake?

Backfire is on the intake end of things.

[secondchance]

Backfire is on the intake end of things.

That sure sounds like one of the cylinder firing during intake stroke - possibly arcing?

[Pete]

The issue is deeper than rough idle. It is a hesitation in the ~1500 to 2200ish RPM range.

Pete, you guessed right about the plugs. Am I correct in assuming that I've gone wrong with these? As for the gap, I don't recall off the top of my head but I set them at whatever the stock gap was.


Bosch will do all the symptoms you describe above.

Change them with a set of regular resistor AC-Delco 41-602 plugs $10 gap them at .035 and enjoy.
Lesson learned, more doesn't always mean better.

Pete

[secondchance]

Bosch will do all the symptoms you describe above.

Change them with a set of regular resistor AC-Delco 41-602 plugs $10 gap them at .035 and enjoy.
Lesson learned, more doesn't always mean better.

Pete

Very well could be. Corvettemasters in Orlando tried a set of Bosch Platinum 4 (I think) in my car back in 2000 and they hated it. Went to stock plug which was FR2 LS? It has been a while...

[bans25]

Old thread but I am working through the same issue with my car... What did this end up being on your Z?

[jeasen]

On my car the issue was with the ignition coils. I looked back at my posts on this thread and found that one of the coils was dead and others were out of spec. I replaced the coils with new ones that were in spec and the car ran fine. If you look back on this thread you can find the specs for the coils. Do not just buy new coils and leave it at that. Test them before you buy them. Many new ones are not within spec and should not be used in our application. Good luck with your problem.

P.S. Being our coils are under the plenum and subject to a lot of heat and cooling it is important to start with good coils. Hope this helps.:)

[Pete]

Most common issue for this is in our cars is spark plug wires,fuel injectors.

If you take your plenum off i would replace wires and spark plugs and while under there ohm injectors,check for leaky injectors and check coils.

If you "Search" on here you will find all the procedures.

On the spark plugs you don't need the fancy plugs that cost $10-$20/each just regular resistor plugs are fine $2each.

Pete