Rough running - Need help and advice

[SharkPilot]

This may be a bit long-winded but I will be as brief as possible.
The car: 1991 #1135 with 23,000 miles. The only mods are a DRM chip and K&N filter with open lid.
I have owned it for three months and it has been running great.
The other day while idling at stop light it started to miss. Just a randon occasional miss. As soon as I started moving it went away. This happened at the next two stops. At the time the engine was cold.
On the way back from lunch and going home that evening it ran fine.
The next morning it had a rough idle and was very rough accelerating from a stop. The car runs great at cruise. In fifth gear at 65-70mph it's very smooth and gets 23mph instant mpg. It has low power and just sounded 'flat' when accelerating.
I had gotten fuel at a different station the night before it missed and suspected trouble there.
Next day I siphoned 10-12 gallons out and put 12 fresh in from my usual station. No change.
I replaced the plugs because I had been wanting to anyway.
No change.
Fuel pressure is 44psi at idle. Low according to the svc mnl but when I unplug the vacuum hose from the fuel pres. regulator, the pressure goes right to 55 and the idle does not change. Still rough with misfires.
Connected a scan tool and have no codes. All of the sensors seem to be returning normal readings. The long term block learn is 133 left and 134 right. Short term is 128 on both sides.
The oxygen sensors read very low at idle with spikes up to rich then back to lean. When the engine is revved up to ~2000 rpm the readings look more like the book says they should. Kind of like a sine wave rich to lean and back.
Yesterday at work we hooked up a smoke machine to check for vacuum leaks. When we pulled off the cannister purge line on the right side of the plenum charcoal particles came out. A lot of charcoal. That seems very bad to me as that stuff looks and feels pretty abrasive. How much has been sucked through the purge valve into the engine?? YIKES!!
We did find a vacuum leak at the PCV connection where that boot connects to the hard line. We put a clamp on there to get the car back home and that stopped the leak. A new boot will be ordered. We also plugged off the evap line. There were no other vacuum leaks that the smoke showed.
The idle was a tad better but still rough. The long term block learn was still 133 & 134. My co-worker, a GM master tech, said that on the way home it should start to run better as the ECM adapted and relearned now that the vacuum leak was gone. After an eight mile drive home in heavy traffic with a short run at 65 mph things are not much better. It's still rough at idle and has very low power on acceleration.
As of right now I am lost and not sure where to turn next. Does anyone have any advice or ideas?
Many thanks for reading through all of this and my apologies for the length but I wanted to be as thorough as possible.

SharkPilot

[tomtom72]

Okay being only a three year owner now I'll take a stab until some of the more senior guys like Crabs & Jeffvette & Z Factor see this!:o

Those symptoms sound a lot like what I went thru when my primary injectors started to have shorts in the coils....the gettin gas & then a slight miss going to a major miss. It seems you haven't gotten to the major miss, multi-injector coil failire. You may have just one that is acting out. I had four on one bank quit in rapid progression...like a day!

Symptoms for me: one entire bank's plugs were clean as snow = no fuel or very little. Car smelled lean out of the exhaust from that bank. Car had miss that got worse as the temp got to closed loop. Oh, I had no scan tool to use until late in the game. I ohmed the wire to coil pack to wire combos & checked for spark energy on each wire & all was okay. I changed out plugs & did a vac gauge & still no indications of solutions to the miss. I swapped fuel filters after getting readings a bit low like you did get...how's the F/F???

Finally borrowed a scan tool and my fuel numbers pointed to the coil failure. I posted up the numbers here & the senior guys did the diagnosing.

Okay now I apologize for being long winded!:o I figure if we compare diagnostic steps maybe we arrive at your root cause??

There are tooooo many spots that a vacuum leak can originate from on an LT5 & I assume that you checked the MAP hose, els at the I/H's, the lower PCV holder, the top cover for the PCV's, the T/Body vac hoses ( two on the left & one under the right side that ya can't see). The TPS was okay, IAC okay, IAT okay, Oh I even did the chart on the charcoal system on mine...I don't like what you found with that! Your miss is only "open loop" or you get it in "closed loop" too? The book's snap shot of ECM data is in closed loop so I figure your miss is the same in both? If it gets worse in closed loop then I still say injector(s) coil(s) are going out to lunch, if they are the OEM injectors.

Okay now I'm at the end of my abilities. Lets see what the senior guys have to say.:thumbsup:

:cheers:
Tom

[Tom]

The injector coil resistances are very easy to check. Go to Marc Haibeck's website and he has an article posted (http://www.zr1specialist.com/HAT%20Web/articles/FI%20Resistance.htm) that describes doing the check.

My 90 currently has a slight miss at idle and runs fine the rest of the time. I have one failing primary injector which measures 6.5 ohms and the other seven primary injectors measure 10.8 ohms.

[ZR1 MK]

The injector coil resistances are very easy to check. Go to Marc Haibeck's website and he has an article posted (http://www.zr1specialist.com/HAT%20Web/articles/FI%20Resistance.htm) that describes doing the check.
My 90 currently has a slight miss at idle and runs fine the rest of the time. I have one failing primary injector which measures 6.5 ohms and the other seven primary injectors measure 10.8 ohms.
The electric test is the first stage of diagnosis. The mechancial test is the second part of injector testing. At this AGE, its best to just get new RC Engineering injectors.

[tccrab]

Okay being only a three year owner now I'll take a stab until some of the more senior guys like Crabs & Jeffvette & Z Factor see this!:o
Tom:
Thanks for the nod, although I don't think I'm anywhere near as technically proficient or experienced as Jeffie or Mr. Z. :worship:

I answered to this post over on the CF a couple of days ago. My armchair diagnosis was failing primary injectors, same as you.

Mr. Shark Pilot, get off of your duff and measure the resistance of the primary injector coils.
If I'm wrong I'll eat my hat. :mrgreen:

TomC
'90ZR1 #792
a.k.a "Crabs"

[SharkPilot]

Thanks for all of the help and pointers, guys.

Tom, I will be pulling the plenum this weekend to clean all of the charcoal out of the evap hose and valve. I'll measure the injector resistance then.

I have been looking on the svc mnl and can't find any values for checking the coils. Is there a secret place to locate those?

This will be my first plenum pull but from what I have read around here it's really not all that bad. It will be interesting to see all of that stuff down in there. I'll keep you posted.

Thanks again for the help,
SharkPilot :worship:

[tccrab]

Thanks for all of the help and pointers, guys.

Tom, I will be pulling the plenum this weekend to clean all of the charcoal out of the evap hose and valve. I'll measure the injector resistance then.

I have been looking on the svc mnl and can't find any values for checking the coils. Is there a secret place to locate those?

This will be my first plenum pull but from what I have read around here it's really not all that bad. It will be interesting to see all of that stuff down in there. I'll keep you posted.

Thanks again for the help,
SharkPilot :worship:

Sharkie:

Use your volt meter and measure the coil pairs from spark plug end to spark plug end, the pairs are:
1/6
2/3
4/7
5/8
They should measure around 20k ohms. What you are looking for are large differences. Obviously the shorter spark plug wires will have less resistance, so expect a variance, but not a lot.

TomC

[Jeffvette]

This may be a bit long-winded but I will be as brief as possible.
The car: 1991 #1135 with 23,000 miles. The only mods are a DRM chip and K&N filter with open lid.
I have owned it for three months and it has been running great.
The other day while idling at stop light it started to miss. Just a randon occasional miss. As soon as I started moving it went away. This happened at the next two stops. At the time the engine was cold.
On the way back from lunch and going home that evening it ran fine.
The next morning it had a rough idle and was very rough accelerating from a stop.

Typically things that will cause a miss are spark related, could be a coil, spark plug or a plug wire. I've had instances of all three. The other option is fuel. You get a puddling effect and the fuel doesn't want to ignite properly or it's to lean and doesn't fire properly.



The car runs great at cruise. In fifth gear at 65-70mph it's very smooth and gets 23mph instant mpg. It has low power and just sounded 'flat' when accelerating.

The LT5 is a very efficient design and will mask most problems. WHat you should try to do is overload the engine. Go into 4th or 5th gear at a low speed and see if you can get it jerk, chuggle or stumble.

I had gotten fuel at a different station the night before it missed and suspected trouble there.
Next day I siphoned 10-12 gallons out and put 12 fresh in from my usual station. No change.

ok

I replaced the plugs because I had been wanting to anyway.
No change.

How did the old plugs look? Wet? Sooty? Powder white?

Fuel pressure is 44psi at idle. Low according to the svc mnl but when I unplug the vacuum hose from the fuel pres. regulator, the pressure goes right to 55 and the idle does not change. Still rough with misfires.

Ok


Connected a scan tool and have no codes. All of the sensors seem to be returning normal readings. The long term block learn is 133 left and 134 right. Short term is 128 on both sides.
The oxygen sensors read very low at idle with spikes up to rich then back to lean. When the engine is revved up to ~2000 rpm the readings look more like the book says they should. Kind of like a sine wave rich to lean and back.

The O2's at a idle condition will fluctuate very quickly. They ill start don low at 100 mv and go up to 800's and then start the cycle all over again. What you are looking for is a slow reading/change.



Yesterday at work we hooked up a smoke machine to check for vacuum leaks. When we pulled off the cannister purge line on the right side of the plenum charcoal particles came out. A lot of charcoal. That seems very bad to me as that stuff looks and feels pretty abrasive. How much has been sucked through the purge valve into the engine?? YIKES!!

The canister on the 91 is all the way at the back. I almost find it hard to belive that it has travled that distance, but there is always a first time. I would like to know more info on this.


As of right now I am lost and not sure where to turn next. Does anyone have any advice or ideas?
Many thanks for reading through all of this and my apologies for the length but I wanted to be as thorough as possible.

SharkPilot


1st step would be to look at the old plugs, if you don't have them or remember the order they came out of the car, pull the new ones and inspect them. Check to see if any of them are wet or show signs of a lean misfire.

2nd, you can pull the plug wires while the car is running and listen for how strong the spark is. It should be making a loud poping noise.

3rd, be prepared to pull the plenum. http://www.pnwzr1.net/LT5_tear_down.htm

[SharkPilot]

Update...

I just pulled off each plug wire with the engine idling and there was no change when #'s 1 & 6 were pulled. Those two share a coil. So it looks like I have a failed coil. Either that or a very coincidental failure of two injectors. With the plenum off I will check the reistance through all of the injectors just to see what I have.

Jeff,
If you'd like I will try to capture some images of the charcoal. I already have a new evap hose for under the plenuim along with a new canister. I can photograph what comes out of the tubing along with what I blow out of the hose leading up to the engine.
The tech at work was rather surprised too when we saw that charcoal in the evap tube.

So I'm headed out to do the plenum.
Once again, thanks for all the help. :worship:
I will keep you all updated.

SharkPilot

[SharkPilot]

Update 2

The plenum is off. I found good and not so good.

The good:
All of the injectors test with 12.1-12.6 ohms resistance.
I don't see any oil leaks down in there. Though there is some shredded insulation way down in the valley. I guess some critter once called that home?

Not so good:
I found charcoal all the way up to and beyond the purge valve. When I rub a finger inside the runners on the plenum I can feel grit on my skin. I blew a few particles of charcoal out of the engine side of the purge valve tube. The one that leads to the plenum.
Is it normal to have some oil in the plenum? I have oil in the end of the purge tube and can see evidence of oil on the secondary throttle plates. There is a film of oil inside the plenum.

I took a couple of photos of my findings but do not how how to post them here. If anyone is curious or would like to host them please let me know.

Right now I plan to order a coil and some gaskets on Monday. Before I put it together I will flush out the plenum and clean as much of that stuff out as I can.

Right now it's time for a break.
SharkPilot

[tccrab]

Update 2

The plenum is off. I found good and not so good.

The good:
All of the injectors test with 12.1-12.6 ohms resistance.
This is great news! Guess I'll have to eat that darned hat after all. Anybody have a good recipe?

I don't see any oil leaks down in there. Though there is some shredded insulation way down in the valley. I guess some critter once called that home?
This is more common than you'd expect. Something about this engine attracts those little critters.

Not so good:
<snip>

Set up a free account with http://www.photobucket.com and upload the pictures there. Then cut and paste the links for the pictures into your post.
The online services like Photobucket automatically size the pictures for you so that they display correctly in a forum format.

It's common to find a little oil inside the plenum and also down in the valley below the plenum. Retorque the bolts to the breather box and check the hose clams on the breather box hoses (it's common for them to come loose). Give it a good clean under there, take the time to follow all the hoses and pipes and get to know what's going on under there. Check the secondaries for proper operation, all you need is one of those hand held vacuum pumps to apply vacuum in the appropriate places.
Hopefully JeffVette will chime in on the charcoal issue, there wasn't any in mine, but I have a '90, my systems are slightly different than yours.

TomC
'90ZR1 #792

[Jeffvette]

Update...

I just pulled off each plug wire with the engine idling and there was no change when #'s 1 & 6 were pulled. Those two share a coil. So it looks like I have a failed coil. Either that or a very coincidental failure of two injectors. With the plenum off I will check the reistance through all of the injectors just to see what I have.

http://images.corvetteforum.com/images/smilies/thumbsup.gif

Keep diagnostics simple.


Jeff,
If you'd like I will try to capture some images of the charcoal. I already have a new evap hose for under the plenuim along with a new canister. I can photograph what comes out of the tubing along with what I blow out of the hose leading up to the engine.


I'll pm you my email and you can send the photos over and I'll host them.

[Jeffvette]

Is it normal to have some oil in the plenum? I have oil in the end of the purge tube and can see evidence of oil on the secondary throttle plates. There is a film of oil inside the plenum.

Small difference between a little oil and excessive. You should have a light coating on the inside of the plenum. You can start by replacing your PCV valves, and then go really wild and make a oil/air sperator catch can.

[Jeffvette]

Want to bring this one back up to the top.

So what did you ever find on the charcoal? Replace the lid on the canister? or?

[SharkPilot]

I replaced the complete canister along with the evap valve under the plenum. The hose going from the canister toward the engine was packed with charcoal so I worked to blow that out. That took some doing because of how packed in there it was. I had to remove the rear wheel housing to get at the pipe. After shaking it around to loosen up the charcoal it fianlly all blew out.
That charcoal is pretty soft so I doubt that it caused any trouble going through the engine. At first I was concerned about that.

Jeff, I sent you a few photos of the charcoal last year. Did you receive them? If not I still have them on file.

Jim

[ZWILD1]

Hey Jeff, This all sounds like what I have been seeing in my car. I have spark to all plugs. No matter how I drive it all plugs are white and clean. Car just seems off and after replacing fuel filter and then both O2 sensors I get the rough idle and mis after it warmed up. I am going to test injectors, wires and coils. Coils and wires were replaced at 78,000 to 80,000 miles car now has 103,000.

[karterdon]

If you are going to replace 1 coil - I would replace them all - MSD has a replacement coil - I found them from summit racing for about $46 per coil. And since I have the plenum off - I would also replace the injectors - Since the originals were prone to failure due to age and ethanol.