Occasional severe misfire @ startup & after heavy throttle

**seldomseen72** · 10-10-2011, 11:50 AM

Hello everyone! I have "lurked" around the forum for some time now and learned a lot. However, in all my searching over the past few months I have been unable to find the answers to a problem I am having. Any assistance would be greatly appreciated.

So, I bought (stole might be more appropriate) a 99 Trans Am back in May. LS1 of course with an auto. Car has about 135k on it and a factory WS6 hood with Ram Air lower box and SLP lid. It had been sitting for some time (maybe 1.5 to 2 years). After some fresh gas, battery charge and some new fluids and filters she came right to life. Occasionally, it would have a severe misfire upon startup. Maybe 1 in 10 times of starting it. Usually it would come out of it on its own but definitely would clear right up and purr like a kitten if you warmed it up, shut it off and then fired it back up. Then it might be fine for a day, week or month. No real rhyme or reason. All different temps, times of day, fuel levels, etc. By severe, I mean completely dropping a cylinder and of course setting off the SES light. Then I noticed it started doing the same thing every time I would get pretty heavy with the loud pedal. Again, it might eventually come out of it on its own but almost always clear right up if you shut it off and restarted. First course of action was all new plugs and MSD wires, took the TB and MAF off and cleaned them all real well. No change. Always throws a cylinder 4 misfire so I went with a new coil. No change. I have checked all wires and plugs. None are damaged or cracked and all are seated good. It's the intermittent thing that gets me. This car will drive awesome majority of the time but every once in a while it will completely drop cylinder 4 when you start it up cold or get a little dominant with the foot feed. Starting to pull my hair out. Injector???? Would cause it all the time I would think. Fuel pressure??? Why just cylinder 4? O2's????? Again, why just cyl 4? I have searched and searched to no avail. Any thoughts? Many thanks in advance.

**35th-ANV-SS** · 10-10-2011, 12:28 PM

Have you done a compression test?

**seldomseen72** · 10-10-2011, 12:40 PM

No I haven't. Guess anything is possible but seems like bad compression would cause the problem to be all the time and not every once in awhile. Could be wrong though. Thanks for the input.

**smokestack** · 10-10-2011, 01:40 PM

Had a issue like that on my truck once. Turned out the injector wire had a small slit in it. Still not sure if it would short itself or moisture would cause it. But anyhow. I chased it for months as well. But once i fixed the wire its been fine for a year now.
I know you said you checked the wires. I had as well. I didnt see it until i unplugged the wires and twisted and turned them so to say

**Knight** · 10-10-2011, 01:51 PM

Just because a plug looks good, doesn't mean it is. I've had plugs that won't fire under compression before. Might be worth doing the general tuneup stuff - plugs, wires, clean the MAF, inspect the seals and gaskets, check the wires for the O2s, ect..

**cammed goat** · 10-10-2011, 02:41 PM

Do a comp test and check the wire harness for cyl. 4 and get back to us. Sounds like maybe an exposed wire arcing off that harness. Spray some brakleen or starting fluid on the coil harness and check for any sparks jumping from that location.

**Cutlass** · 10-10-2011, 03:13 PM

You could try swapping #4 injector with another one and see if the problem follows the injector. Also, what brand plugs did you install?

**seldomseen72** · 10-11-2011, 04:06 AM

Thanks guys for all the input! I certainly have some things to try. Might be a couple weeks before I can get it all completed due to some work related travel but I will keep everyone updated. Thanks again for all the ideas and if anyone thinks of anything else, by all means throw it out there.

@cutlass, Apparently it is too early in the morning because I seem to be doubting myself. However, I am 99% sure I went with Ac Delco Iridiums. Not sure of the number but I know it was between those and the NGKs that eveybody seemed to recommend for the LS1 on all the forums and places I have researched.

Thanks again everyone.

**01SsRacing** · 10-11-2011, 04:47 AM

I have a 2001 camaro ss it only has 35k miles on it and I have this problem sometimes it only on start up its very random it doesnt sound like it happens as much us you make your car sound like it happen. But with it happens to my car it feels bad my whole car shakes and the volts gauge goes crazy so im not sure whats causing this, I think it happen if you hope in the car and turn the key all the way to start fast if you take your time and and turn the key far enough to turn all the electronics on then turn the car over I feel like that helps. also I have the slp lid and a slp smooth bellow and the bellow came with a senser that plugs into the lid and then the factory one plugs into it. does your car have this becasue I removed it and put the factory one back in and havent had the problem since

**35th-ANV-SS** · 10-11-2011, 05:33 AM

Sounds like maybe you had a vacuum leak 01SsRacing.

**seldomseen72** · 10-11-2011, 09:29 AM

01SsRacing, My problem used to be very occasional. It stills is ever once in a while but certainly seems to be more frequent. Maybe, I have just gotten better at learing what causes it. LOL! I also tried the get in, turn the key to "on" and let it sit for a a bit (maybe 10 or 15 seconds) and then try to start it. I sort of thought that helped also but think it may have just been a coincidence.?.?.? My factory air temp sensor is mounted in the hole for it on the SLP lid. Had this problem before the new plugs, wires, and lid. Same deal either way. Thanks for the input.

**cammed goat** · 10-11-2011, 09:42 AM

Originally Posted by 35th-ANV-SS

Sounds like maybe you had a vacuum leak 01SsRacing.

Ditto on that.

**Cutlass** · 10-12-2011, 03:07 PM

Originally Posted by seldomseen72

@cutlass, Apparently it is too early in the morning because I seem to be doubting myself. However, I am 99% sure I went with Ac Delco Iridiums. Not sure of the number but I know it was between those and the NGKs that eveybody seemed to recommend for the LS1 on all the forums and places I have researched.

Good, you did use good quality plugs so that should be a problem unless you damaged them on install or didn't get a plug wire snapped on all the way. I just wanted to make sure you weren't using some oddball plug.

**01SsRacing** · 10-12-2011, 04:04 PM

Originally Posted by 35th-ANV-SS

Sounds like maybe you had a vacuum leak 01SsRacing.

Vacuum leak?

**35th-ANV-SS** · 10-12-2011, 04:16 PM

Originally Posted by 01SsRacing

Vacuum leak?

If your car was shaking and it quit when you put the stock lid back on I would suspect that you probably had a loose connection either in your lid, or where it attached to the bellow.

**2002ssslp** · 10-12-2011, 04:28 PM

Try swaping the coil and wire with another cylinder to see if the problem moves to another cylinder. If so, replace the coil and put in new wires and plugs. Don't forget the anti-seeze on the plugs.

**Ramairetransam** · 10-13-2011, 04:42 AM

disconnect your maf and see how it misifres or at all , i had the small hiccup misfire problem and it was so slight only seen on my aeroforce .

**01SsRacing** · 10-13-2011, 05:40 AM

Originally Posted by 35th-ANV-SS

If your car was shaking and it quit when you put the stock lid back on I would suspect that you probably had a loose connection either in your lid, or where it attached to the bellow.

Its still the SLP lid the only thing that ive changed is that I took out the aftermarket sensor and pluged the factory one back in and since then I havent had the problem

**cammed goat** · 10-14-2011, 03:47 PM

Which sensor? MAF sensor? Our cars DO NOT like aftermarket MAFs. Unless you are running a larger intake, larger heads, larger cam or forced induction, stick with the factory MAF.