Need Advice: Miss, low rpm stumbling

[EWest]

Hey guys, I am new to the forum and relatively new to the ZR-1 scene. I was hoping for some help with my '90. I have seen quite a few posts about engine hesitation etc., but none exactly like mine, so here goes:

There is a pretty severe miss during cold idling, and sometimes less severe while hot. At low rpm acceleration there is a severe stumble / hesitation up until about 3000rpm, when she takes off like nothing is wrong. WOT at low rpm usually causes severe stumbling and backfire through the plenum, but oddly on occasion she accelerates fine. The less throttle you give, the less severe the stumble seems. It seems that the lower the rpm the worse it is, especially in higher gears.

Idle is at about 800 rpm.

I replaced the fuel injectors with the rebuilt fic ones, and had to pull the plenum again to fix a vacuum leak in the intake. No change. Plugs and wires are new. I'm kicking myself for not replacing the coils while I was in there, but I did the "pull the plug wire and check the arc" trick and all seemed to have good spark.

I realize there are sensors etc. that I should test, but where should I start?

Thanks.

[Tyler Townsley]

Check the fuel pressure. Usually you can use a tester at tool loan program at some auto parts places.

Tyler

[scottfab]

I agree fuel pressure is a good place to start. One thing these cars don't like is to get into hard long ~1G cornering (exit and on ramps) with <1/4 tank. They can suck air even with the little tub that surrounds the pumps.
Sucking are = pump death.
Static test of the pressure should be > 50psi and hold fairly well.

You should also tell us how heavily modified the car is.
When things fail some just choose to rip things out.
Do you have secondary actuators?
Stock chip?
exhaust?
Gas is old? (car been sitting)

[EWest]

Car is completely stock to my knowledge including exhaust. The car had been sitting off and on for some time, but I've cycled through several tanks of gas at this point.

[scottfab]

Car is completely stock to my knowledge including exhaust. The car had been sitting off and on for some time, but I've cycled through several tanks of gas at this point.

Let us know what the results are of the pressure test.

[EWest]

OK I did the static fuel pressure test, and the pressure was 45 psi with the test connector jumped. I bled the line, took off the jumper and started the engine - back to 45 psi and held. Thought that was odd as I figured the primary pump would be at fault due to the symptoms. Both pumps are operational as they read 9.74 A while doing the test.

Any thoughts?

[sammy]

did you try the test that's used in the fsm . attach the gauge to the shrader valve and just turn the key to on for 2 sec then off and see if the pressure holds for about 5 min . before going down ?????? there are some threads that tell you how to test the coils without pulling the plenum .. do you have a scan tool to ck the operation of some of the other sensors ?if not see if a buddy has one .it could save you alot of time and money .

[tomtom72]

Cold idle = really bad miss? Then I would check the IAC, temp signal to the ECM from the discrete sensor, also IAT & TPS sensors. I'd look at the "cold start" sensors and what signal they should be sending to the ECM.

Warm miss = I'd still look at the TPS and coolant temp sensor & IAT and see that the IAC is at the correct # of counts. You said the ignition system is good? Also, the rebuilt injectors are an outside possibility, very outside.

Fuel pressure = I have a sense that if 45 psi is the best reading you got then the pumps are checking okay, but they're at the lower end of the range the FSM has for good pumps. I doubt that this is the reason for the miss. Fuel filter is good, right?

You are sure the MAP hose and MAP are good? The MAP & hose can become flooded with the blow by oil and that causes the sensor to give bad signals to the ECM. Also, make sure that the vacuum hose is the correct type, a hard line and not standard soft rubber.

If you have a scanner you can look at all these parameters from cold start thru warm up to normal operating temp and compare your readings to the FSM's numbers. You didn't mention a SES light, so I figure that there are no DTC's?

Oh forgot this. When you put the plenum back on you are sure that none of the injector harness wires got pinched. Another member had this issue & it drove him crazy as it yielded a miss. It's kind of cramped under there as you know!

:cheers:
Tom

[XfireZ51]

When was the last time fuel filter was replaced? Fuel pump "socks"?

[EWest]

Noticed something today that I had not noticed before, vacuum pump cycles on briefly about every 15 seconds with key in ACC position. Looks like I do in fact have some sort of vacuum leak, but would that cause this severe of a miss and stumble?

Fuel filter replaced recently, don't know about the fuel pump "socks".

Forgot about this, but the only time I get a SES light is if it backfires. It has been a while, so would it still be stored?

I'll check the MAP & hose for oil.

Don't know anyone with a scanner to check the other sensors, what type do I need?

[tomtom72]

Bingo on the vac pump cycling every 15 or so....that is a slight vac leak somewhere. Now discon the hose at the right water outlet tube and cap the fitting and try the key on engine off and see if the pump stops running after she pulls a vacuum. At the end going under the plenum hook up a vac tester and pull a vacuum and see what happens. Any vac leak adds more air/leans out the A/F ratio, so yes that could cause a miss.

The intermittent SES light will have set a DTC if it stayed lit long enough and the DTC will be stored as a History DTC in the CCM.

Call out to the WAZOO, as The Captain has a tech1A and that will be the best choice to see what is going on.

Okay on the filter, and the socks are proly the original ones. IDK what to say about the socks. Mine were clean after 17 yrs, but the original pumps were down on output so I changed them out. The butt-dyno could tell the difference between old OE pumps & new Bosch pumps. Just saying.....:o



:cheers:
Tom

[gunman0]

You can see if there is a bad check valve for the secondary system by pulling the vacuum line to the middle of the driver's side of the plenum. Put your finger over it and see if the vac pump stops running every 15 seconds. If so, you can just throw another check valve there as a quick fix until the next time you pull the plenum.

I don't think a secondary vacuum leak would cause the low rpm miss like you describe. Also the fact that it picks up power over 3000 rpm seems to negate that as being a major problem.

[scottfab]

Forget all effort into the secondary vacuum for now. Cycling every 15 min is not perfect but not going to be a big issue yet. When it stays on, then worry about it. Once you've found your miss then you can come back.

Yes do get a scanner. Don' t waste time with any "short two pins" and "watch the led flash" and "count the flashes".

Spend time on the 45 psi fuel issue. It may not be the source of miss but is low and will cause hard starts (long start time) and shows the pumps are tired. I would not settle for less than 50 psi. How many seconds does the engine crank before starting?

Check the codes and go from there. However, checking the MAP hose is easy and has been known to cause issues like you are describing.

[EWest]

I have an update on my woes, and was looking for a few opinions. I finally got frustrated enough, combined with a lack of time to tinker recently, to take my Z to a local Corvette shop who has a guy that is familiar with the lt5 and has a good electrical guy.

At first they found a cracked spark plug at #6, which made me do a celebration dance in anticipation of it being fixed...but not so fast.

She still stumbles and sputters under load, but does not miss at idle.

He replaced the coils, the miss came back and same problem. SO he isolated the problem to cylinders 4 & 7. He thinks it is the DIS, but "thinks" is not enough for me to drop that kind of coin on a new one. He's in the process of checking all of the wiring around the coils, module, and ecm and so far everything has ohmed out ok.

Any thoughts?

[scottfab]

She still stumbles and sputters under load, but does not miss at idle.


It would be nice to know if this same behavior is exhibited with the power key off. And what does the fuel pressure drop to when under load? I'm concerned that all the effort is going to spark.

[Kevin]

I have an update on my woes, and was looking for a few opinions. I finally got frustrated enough, combined with a lack of time to tinker recently, to take my Z to a local Corvette shop who has a guy that is familiar with the lt5 and has a good electrical guy.

At first they found a cracked spark plug at #6, which made me do a celebration dance in anticipation of it being fixed...but not so fast.

She still stumbles and sputters under load, but does not miss at idle.

He replaced the coils, the miss came back and same problem. SO he isolated the problem to cylinders 4 & 7. He thinks it is the DIS, but "thinks" is not enough for me to drop that kind of coin on a new one. He's in the process of checking all of the wiring around the coils, module, and ecm and so far everything has ohmed out ok.

Any thoughts?

sounds like he's throwing parts at it. did you check the injectors? fuel pumps?

[EWest]

It would be nice to know if this same behavior is exhibited with the power key off. And what does the fuel pressure drop to when under load? I'm concerned that all the effort is going to spark.

Power key off = same behavior. I never got a chance to check the fuel pressure under load, but would that cause a problem in 2 specific cylinders, and only under 3000RPM?

[EWest]

sounds like he's throwing parts at it. did you check the injectors? fuel pumps?


Injectors tested out fine, fuel pumps test ok, but like I said I have not had the chance to test them under load.

[XfireZ51]

4-7 are on the same coil pack. If this guy knew much about LT-5s he'd know not to suspect the DIS module.

[scottfab]

Power key off = same behavior. I never got a chance to check the fuel pressure under load, but would that cause a problem in 2 specific cylinders, and only under 3000RPM?

absolutely. If you get to a low pressure situation one cyl is going to go lean before the rest. The "miss" has to start somewhere.
I assume the wires were checked out? Length on a few are the same so they can be swapped. AND if one plug was cracked I'd not rule out more damage to others. These aren't those funky Bosh plugs are they? I've seen multiple problems with those.

[Kevin]

Injectors tested out fine, fuel pumps test ok, but like I said I have not had the chance to test them under load.

do you know what the injectors tested at?

[scottfab]

And I assume you've cleared any codes. But did any get reset?

[GSJoe]

You're getting lots of good advice here from people with lots more mechanical "savvy" than I have, but I need to suggest something you may have already done, and if not, you must do it whether or not it's the cause of your miss issue. Let me explain. Last year my young friend Matt who owns a '91 experienced a similar miss issue. We did just about everything--plugs, coils, wires...tons of other checks. We chased our tails in vain for months. In August at Carlisle, however, Matt talked to the smartest ZR-1 guy who exists in mho, Marc Haibeck. He gave Matt the trick we implemented when we returned home to Charlotte, and the problem went away. Here's what you must do if you haven't done so already. First, take out those old plugs--I don't care if they're new--and throw them away. Get yourself a brand new set of AC plugs of the right heat range and install them. Next--and here's the trick Marc Haibeck gave Matt--get yourself a 3oz can of Permatex Dielectric Grease (item #81153) at any Advance or AutoZone parts store. (I'm sure other versions of dielectric grease will work, but this one dispenses easily from a narrow nozzle because the can is pressurized.) Put the nozzle into each spark plug boot AND COMPLETELY FILL IT with the grease before putting the boot on the new plug and do the same with the remaining seven. OK, why does it work? According to Mark, the extremely high intensity discharge from that ignition system causes air trapped in the spark plug boot to ionize. When it does, the spark then takes an easier path down the outside of the plug rather than through it. If you look at your existing plugs you'll probably see several with a very fine burn line on the insulator where the spark has been tracking. That's why you must replace the plugs again when you do this. As I said, if you've already completely filled those boots with dielectric grease and the problem still exists, I apologize for the verbosity of this post. If you did not, however, do so. It's a must, and even if your problem does not go away it will prevent another common and similar one from occurring. I sincerely hope this helps you.

[Funracer]

You're getting lots of good advice here from people with lots more mechanical "savvy" than I have, but I need to suggest something you may have already done, and if not, you must do it whether or not it's the cause of your miss issue. Let me explain. Last year my young friend Matt who owns a '91 experienced a similar miss issue. We did just about everything--plugs, coils, wires...tons of other checks. We chased our tails in vain for months. In August at Carlisle, however, Matt talked to the smartest ZR-1 guy who exists in mho, Marc Haibeck. He gave Matt the trick we implemented when we returned home to Charlotte, and the problem went away. Here's what you must do if you haven't done so already. First, take out those old plugs--I don't care if they're new--and throw them away. Get yourself a brand new set of AC plugs of the right heat range and install them. Next--and here's the trick Marc Haibeck gave Matt--get yourself a 3oz can of Permatex Dielectric Grease (item #81153) at any Advance or AutoZone parts store. (I'm sure other versions of dielectric grease will work, but this one dispenses easily from a narrow nozzle because the can is pressurized.) Put the nozzle into each spark plug boot AND COMPLETELY FILL IT with the grease before putting the boot on the new plug and do the same with the remaining seven. OK, why does it work? According to Mark, the extremely high intensity discharge from that ignition system causes air trapped in the spark plug boot to ionize. When it does, the spark then takes an easier path down the outside of the plug rather than through it. If you look at your existing plugs you'll probably see several with a very fine burn line on the insulator where the spark has been tracking. That's why you must replace the plugs again when you do this. As I said, if you've already completely filled those boots with dielectric grease and the problem still exists, I apologize for the verbosity of this post. If you did not, however, do so. It's a must, and even if your problem does not go away it will prevent another common and similar one from occurring. I sincerely hope this helps you.

Never heard of this before but I too am chasing a stumble and am willing to try this to see if it helps. Anyone else tried it or know if it works?

[scottfab]

Never heard of this before but I too am chasing a stumble and am willing to try this to see if it helps. Anyone else tried it or know if it works?

I've never heard of this causing a stumble either.
A simple one to try is cleaning the IAC and the tunnel it sits in.
Two screws and you can inspect it for carbon build up.

[Pete]

Just a couple of questions.

What spark plugs you using? hope not any of those pricey crappy plugs just regular resistor plugs work just fine.

Where did you gap them? .035 is best.

Did you buy new injectors or rebuilds?

Injectors can Ohm out good and still be bad.

Did this start after replacing injectors,if so borrow a good set and replace them.

Pete

[Starman]

It will be plugs or injectors. Injectors must have 14 ohms, any less and they are on their way out - replace the entire set when needed - trust me it will save a lot of frustration down the road.

The guys are also right about the plugs, use only the correct AC plugs with the stock coils - fancy, multiprong plugs can cause misses. Always seal your plug boots with dielectic grease.

Good luck

[EWest]

I have an update! After trying many of the tests and fixes suggested, cylinders 7 & 4 were still out which obviously took precedent over the stumble. Finally after 4 plenum pulls and countless hours testing everything I could think of, I decided that I would go to a specialist not far from me. From August until late February the only thing I did was a weekly warm up and an occasional sputter around the block.

I was away most of February so the battery completely died at one point. On February 22nd I charged it and fired the engine up, but this time something sounded...different, and by different I meant ALIVE! A quick check of the spark situation revealed that all 8 cylinders were firing! I was dumbfounded.

A test drive revealed there was no miss or stumble and the ability to boil rubber was back. I drive it about twice a week and it is still running well (with the exception of this vacuum leak I have to chase down).

Any thoughts?

Is it possible that after the battery completely died, something reset?

[scottfab]

Yah, that's pretty weird. I've heard of the SES light coming on which indirectly signaled a failing battery but not this.
Unless by clearing all learned block/trim setting and running
off the factory defaults it's masking the failing injector or O2 sensor?
dunno.
Keep an eye on it. Should the stumble come back then disconnect the battery again to see if it disappears again.