1994 5.7 - bad miss at idle please help

hi ,

i have a 1994 chevy 4x4 5.7 engine , it has a miss or stuble at idle , every couple of seconds it will miss or stumble and it almost feels like a plug cutting out . i have put 3 sets of plugs and wires on it and same thing . i have tried everything and the miss is still there . i have done this -

new fuel pump - napa
new fuel filter - napa gold
new wires - napa belden prem
new plugs - autolite 25
new fuel injectors - advanced auto - cp sorenson
rebuilt throttle body
tried a used distributer from a good running engine
new dist module
iac valve
map sensor
coolent temp sensor
tried another computer
new 02 sensor

when the truck is at idle it will have this miss , it used to idle good untill it warmed up and then it started to miss when it went into closed loop now it does it cold or hot . it does take of good as i can grab 2nd gear with no hesitation and it seems to do pretty good at 50-60 you really can feel the miss but it still seem's the truck is lacking something as far as power . but you really notice th emiss or stuble at idle . i took of the egr valve and cleaned it but i don't know if it is ok or not . you can unhook it and it does not affect the running of it any and i can unhook the egr solinoid and it will kick a light but does not change miss . i have tried the time at 0 degress and i have timed at 35 degrees and the miss is still there . could the new fuel pump be weak or could this be egr releated or distributer related . i would appericate any help . it does not have any cat converter or muffler . the fumes comong from the truck are strong and they are white as if it is not burning all the gas , also has real bad gas milage . please help thanks

Your problem sounds very similar to issue that I have with my 90 GMC K1500. I recently pulled the ECM and measured the resistance of the Quad Driver outputs based on the Factory Service Manual. The resistance is support to be 50K ohms or higher. I am not sure if a bad solenoid or relay is to blame or if the ECM is just bad. I need to figure that out next weekend.

My plan is to replace the ECM. In my case, the miss is more noticable in closed loop. If I force open loop (disconnect 02 sensor).

If this fixes my problem, I will post back but it might be a couple of weeks before I swap in a new ECM.

//2000CAYukon

First off, I would junk those autolite plugs in a gm and put in some ac delco plugs. I had a miss in my '94, did a tune up, it had autolites in it, changed to ac and has ran fine ever since. The other thing that you need to look at is the egr valve and egr valve control solenoid. If the egr valve is stuck open it can cause a miss, or if the solenoid is bad and allowing it to pull full vacuum it can cause it to open the egr valve to early.

In my case:

AC Plugs, MSD Wires (all wires under 3K ohms)
New Timing Chain (it was worn)
Rebuilt Dist (new shaft, pickup coil, cap and rotor)
EGR was replaced at 100K (now 118K), EGR solenoid was replaced since it did not hold a vacuum.
Fuel Pressure is 13 PSI
Intake Manifold gaskets done when the chain was done
Compression Check all cylinder 170 +- 5 PSI
New IAC
New MAP
Rebuilt TBI (new injectors and fuel pressure regulator)

If I unplug the 02 sensor, I don't feel the miss. The warmer outside, the more it seems to happen. If the engine is shut down for an hour or so, the idle is nice until I drive it for a while.

Once I push the gas, it runs fine. It is only warm idle in closed loop.

I am concerned that my quad driver resistance is not what the book says it should be. I wish I had a good working ECM to try.

Again, my symptoms are the same as silver100.

Any ideas?

Thanks,
2000CAYukon

2000cayukon ,

i did try another ecm but it was used so i am not for sure that it could be bad also as it ran the same no fix . the computer in my truck now is a reman from advanced auto but i don't know how old it is . i put a timing light on my fuel injectors and they seem to be working okay . i just have a miss at idle and feels sorta like a plug cutting out as it will cutout every couple of seconds . i did find out that my egr is bad so i ordered a new one so maybe that will help . but the miss really stands out in when the truck warms up . i can advance the time and it will help a little , but the miss is still there . i also tried ac delco plugs and it did not help either , also 2000 ca yukon it could even be in the e-prom in the ecm going bad maybe . if i find out anything i will be sure to post . also i would really appericate any input as i would like to fix this truck as it has beat me so far . thanks silver100

Silver100,
When you tried the other dist, did you also swap out the coil? I did replace the stock coil at one point and that seemed to help.

Let me know how the new EGR works for you.

This is driving me nuts. Other than the miss at idle, it runs very well.

Good luck,
2000CAYukon

2000CAYukon have you checked the coolant temp sensor? Your O2 sensor doesn't come in to play until the engine reaches a certain, the ecm is told that by the coolant temp sensor.

2000CAYukon have you checked the coolant temp sensor? Your O2 sensor doesn't come in to play until the engine reaches a certain, the ecm is told that by the coolant temp sensor.

Yes. I installed a new one when I did the intake manifold gaskets. I also have WinALDL which shows the temp at 195 (which matches the tstat).

Exhaust back pressure is good. Less than 1 PSI at idle. Around 3 PSI under full acceleration.

I have checked (and rechecked) for any vacuum leaks. It runs good otherwise. That is why I suspect ECM but would like to be sure beforw throwing more parts at it.

Thanks,
2000CAYukon

2000cayukon,

i put a new coil on it from advanced auto , gp sorenson brand , but it may be bad as i have heard alot about their electric parts . i put the egr valve on and set the time back to zero . still has a miss though it seems to be not as bad but still their . i am going to try to change plugs to ac delco or any other brand your guy's recommend and i also thought i would try to put on a set of 8 mm wires and a better coils such as msd or hypertech , what do you guy's think . it seems as the miss is in the fire to me but i could be wrong . i appericate all the input from evryone . thanks silver100 . i will post as soon as i fix the problem and i appericate any help . thanks

I think you do a compression test on all of the cylinders. You may have internal problems. If they all check good (125 to 150 psi), at least you know that the engine is sound and have an external problem.
Another thing you can do is try to narrow the miss down to a specific cylinder by doning a balance test. Run the engine at idle and pull off ignition wires one by one. If the cylinder is firing the idle will drop, if not the idle will not drop. Its a little harder to see on a v8 than a 4 cyl but you can still do it.
Also, get some AC Delco plugs in that beast.

This morning I decided to recheck the timing. When I disconnected the EST, the engine started to surge real bad (I disconnected it while the engine was running). I shut it down and restarted it. The idle was still surging but not as bad. As I watched the timing, it would jump from 0 to 8 degrees advanced and then back down to zero. This is with a brand new timing chain (double roller). I advanced the timing a little and the idle smoothed out some. WTF is going on???

I reset the timing to zero but I may play with changing the timing this weekend when I have some more time. I did replace the balanacer a few years ago when the rubber on the original one went bad. Perhaps the balancer is off causing the timing to be off.

This is absolutely driving me nuts!!!

Silver100, does your timing jumping around like mine?

//2000CAYukon

2000cayukon ,

i have played with the timing some and if i set it at 35 degrees it seems to make the miss not so noticable as it does at 0 degrees . i have not checked it to see if the timing is jumping that could be possible . i can take it to the local garage and they set the timing and it runs good for a few day's and goes right back into the same thing . i don't know where they are setting the time but it seems to help for a few day's .what could cause the time to move or jump , i am not for sure . also i noticed that my truck stays lean on exhaust all the time on the scan tool but the 02 sensor seems to be working as it should with the mv moving as they should . i am running out of idea's . i am going to try to unhook the 02 sensor and see if the miss goes away . maybe i have a shorted wire , any ideas . thanks silver100

2000cayukon ,

i have played with the timing some and if i set it at 35 degrees it seems to make the miss not so noticable as it does at 0 degrees . i have not checked it to see if the timing is jumping that could be possible . i can take it to the local garage and they set the timing and it runs good for a few day's and goes right back into the same thing . i don't know where they are setting the time but it seems to help for a few day's .what could cause the time to move or jump , i am not for sure . also i noticed that my truck stays lean on exhaust all the time on the scan tool but the 02 sensor seems to be working as it should with the mv moving as they should . i am running out of idea's . i am going to try to unhook the 02 sensor and see if the miss goes away . maybe i have a shorted wire , any ideas . thanks silver100

Tonight, I set my timing at 5 BTDC instead of zero. It seems to be idling better but the timing is still jumping around. If I disconnect the EST wire, the timing jumps +2 degree every 3-5 seconds. If the EST is connected, it seems to jump +10 degrees every 3-5 seconds. I also disconnected the ESC and it still jumps around. I am starting to wonder if the ECM is bad causing the timing to jump. Originally, I thought the jumping was my timing chain. When I replaced it, the chain was very loose so I assumed that was it. When I retimed the engine, I did not notice the jumping around. The problem only happens when the engine is fully warmed up.

If the EST is disconnected and the ESC is disconnected, what could cause the timing to move? I replaced the shaft and pick up coil and the shaft did not have any play in it (unless it does when it is fully warmed up???). If I remove the cap and try to move the rotor, it seems pretty solid. I guess the dist gear is OK.

I sure hope we both can get this figured out. This is driving me to drink:grinyes:

//2000CAYukon

2000cayukon,

i am going to try to unhook my o2 sensor this weekend and see if it takes the miss out , if so i am going to start checking the wires that feed the 02 sensor . you could possibley have a shorted wire that feed 's teh 02 . need to check wires from 02 to ecm . i seen on the scan tool that my o2 goes into closed loop within a few seconds . i thought it should at least warm up first . it does go from 0.1 to 9.0 mv as the hayens manuel say's it should . but the sensor does not stay in open loop but for a few seconds . my miss seems to be getting worse as i can tell it at higher speeds now . it still has the feel of a plug breaking down and i am going to put ac delco in it to see what happens . also i am wondering about grounds to the ecm as we'll . i am going to check that also . will post if i find anything . thanks :banghead: