Engine miss at idle and under load - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 07-09-2009, 01:33 PM   #1
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Default Engine miss at idle and under load

95 Bonneville SSEI

Here is the background story. The car had run very well to the point where I became negligent with tuneups, etc. I purchased an Actron scan tool and began troubleshooting. Since then I have replaced plugs and wires, O2 sensor, catalytic converter,Boost control solenoid, repaired upper EGR tube, oil change, coil packs, tranny flush, tranny cooler lines, oversized tranny cooler, water pump and harmonic balancer, 180 deg drilled thermostat, replaced TPS sensor,replaced PCV and O ring, cleaned throttle body, cleaned connector plates and grounds to ICM.

The problem- I have a problem with the transmission dropping out of OD after it heats up. (P0740 TCC Failure) which I am not trying to address in this post. I haven't resolved the issue yet but decided to take the car out of town for the weekend. The workaround that I read about was to use cruise control to prevent the overdrive issue. It worked fine for the first 100 miles then I began to get a serious miss when accelerating. I stopped at a parts store and found that it was missing under load and slightly at idle. The Actron said it was missing at the # 4 cylinder. I swapped out the # 4 plug and wire and the ICM and none of those made a difference. I had to drive it like that 100 miles to get home and switch cars. With the hood raised it sounds as if there is a rattle or ping coming from the supercharger whenever it misses at idle. I installed the new coil packs a week ago and the car ran very strong but there still seemed to be a very slight miss at idle but nothing like what I am experiencing now. I need to check the resistance on the coil packs just to be sure that one of the new ones isn't defective but I thought I would post this and hopefully someone can help steer me in the right direction. BTW, all of the above were needed except possibly the coil packs but since they were the original ones I decided to go ahead and replace them. I went with the cheapos. Probably a mistake. Unfortunately I threw the original coil packs away. Any thoughts?
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Old 07-09-2009, 02:30 PM   #2
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I tested the coils and the primaries on the coil servicing cylinder # 4 is reading .1 -.2 ohms. The others are reading about .8 so I am off to the parts store to exchange the coil.
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Old 07-09-2009, 02:44 PM   #3
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Does it sound like this?


If so, remove the SC belt and do this test.


If you find play, then do this.
https://www.gmforum.com/t279064/

Also check to make sure your #4 spark plug wire is not touching the O2 sensor wire. This can cause problems.
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Old 07-09-2009, 04:18 PM   #4
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No, it doesn't sound like that. I checked the 02 sensor wire and it does touch. I have been fighting with that since I changed the O2 sensor. I just tied it back with a piece of string to keep it out of the way but it still missed. I unplugged the O2 sensor and cleaned the contacts since I had kind of overdid it with the dielectric grease. Blow dried the plug and went for a drive. The O2 threw a few codes but finally settled in once all of the moisture was gone. I got the new coil but it still tested at between .4 and .5 compared to .7 and .8 so I switched positions of two of the coils and it made no difference. Now I am only getting the p0300 code and it still shows a misfire on cylinder # 4. I am going to pull that plug and inspect it once the engine cools down since I did drive 100 miles with it missing. I also notice that it seems to run smooth for maybe a little less than a minute then the engine speed changes as if something is kicking in and it starts to miss. It misses mostly at idle. If I accelerate hard it seems to smooth out or maybe I just can't tell but it has plenty of torque and I could smoke tires if I wanted to.
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Old 07-09-2009, 04:25 PM   #5
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If you changed coils and still show a miss on #4 then that leaves the plug wire, the plug, and then injectors. You can switch the injectors and see if the problem follows that injector. If the coil was bad it'* more likely you would have problems with the other plug on that coil. Pull those plugs and inspect. I would say pick up a new set of plugs and change them all. I've seen bad plugs come right out of the pack. When it'* dark try taking a water bottle and misting the coils, wires, and plugs. Then turn off the lights while the engine is running. This will show any ground problems with a blue flash. The P0300 code is a random misfire, generally from a rough idle. Misfires can also be caused by a vacuum leak. Spray carb cleaner around the vac lines and listen for a change in RPM. Don't forget to spray around the EGR tubes and around the LIM gaskets. any unmetered air entering the system can cause a rough idle.
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Old 07-09-2009, 04:44 PM   #6
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Will do, I sprayed carb cleaner around the LIM and EGR as well as all of the vacuum lines that I could find. I have changed them all out. I have the actron cp9145 and I pulled the live readings. That is where it says that it is missing on cylinder # 4. I may not get to the injectors until tomorrow because I don't have a garage but I will try all of the above. A couple of weeks ago I sprayed the wires, ect. with water and couldn't see a spark (it may have not been dark enough where I was parked). It hadn't been missing but I was trying to rule out as much as I could to deal with the TCC issue. I took it for a drive and it started missing but eventually cleared up That is when I cleaned up the ICM plates and started looking at the coils, plugs etc. It only started this latest wrinkle when I was on the highway so the injector does seem suspect. Also, after I had cleaned the O2 sensor plug one of the codes that it threw was p0171 sensor signal lean but that code went away after I cleared it and drove for a while. I will keep you posted.

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Old 07-09-2009, 04:49 PM   #7
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When this happens more with moisture it tends to be a electrical leak. A cracked coil or bad plug wire, even cracked porcelain on the plugs.
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Old 07-09-2009, 04:52 PM   #8
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Ok, I guess I will start with a fresh set of plugs.
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Old 07-09-2009, 05:56 PM   #9
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Dan, I noticed that your plug wires are running across the top of your supercharger. Mine ar running underneath the snout. Any chance they could be grounding out? I did reroute the # 4 wire when I was testing it but no change. Also, where can I get a new set of wire looms? Mine are shot.
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Old 07-09-2009, 06:02 PM   #10
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Mine now go under the snout. Anything i possible when it comes to grounding. You should be able to get plug wires at autozone or advanced, or what ever parts store you have local.
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