93 Z28 M6 Backfire problems

[scbird1]

Well im back here for some imput as to what might be my problem. I have replaced so many parts im going broke on this car. Seems like no shops will even look at it only because it speed density and not Maf controlled. A little history on the car, it has 157k on the clock but was completly rebuilt at 140k. The only things are original are some of the sensors have not been replaced. It sounds like something electrical as its a miss fire that only pops up at times. It will break up around 4500 rpm evry time but idles perfect only to pop once about every 4 min. It has a new opti spark, plugs, wires, fuel filter, fuel pressure regulator, purge valve and has 42 psi fuel pressure. Cat is new along with new O2 sensors. The coil and ignition module and original. I have no codes and it only flashes 12 all day long.
Mods, Lt4 hot cam kit, headers, 3:73 gears and tune.
Any isight to what my problem could be will help me from going crazy
Here is a pic of the beast. Thanks, Mark

[Joe 1320]

By any chance did you re-use the metal plug wire boot covers?

[scbird1]

No, but at one time I bought a new one for #7 as my headers roasted it. I have the shortie plug in there now and have about 3/8 from the header pipe and even checked it at night with all the lights out. Its all good there. I went to advance auto and had them check out my coil and ICM. Coil was weak and the ICM tested bad but I dont have much faith in these kinds parts stores when it come to testing parts as he might have hooked it up wrong but well see when it gets here tomarrow. It seems to be more of a exhaust backfire. I will let you guys know what fixes it. Thanks, Mark

[Joe 1320]

Be sure to check the alignment of the pins in the coil driver module and it's harness connector. The connections must be spot on and not loose. Any intermittant contacts will give you the symptoms you describe. If the module tested bad, that's a possible cause too. Just be real sure those contacts are tight. It's also a good idea to seal the weatherpack connector with some dielectric grease for added insurance.

[scbird1]

Just got done installing the new ICM and coil. I have the same problem. Its 95F here and under the hood it must be 200F but i will go back and check everything again. My opti is a year old and is a GM replacement. It cant be that? Or could it? Thanks

[Joe 1320]

Sure could.

there could be lots of issues that would cause this. A bad plug/plug wire, corrosion in the opti itself, corrosion in the harness connections, slightly fouled injector, bad cam lobe, low comression in a cylinder, the list goes on and on. If you cannot find the issue, I would suggest a repair shop that has a scanner with enhanced parameters that will allow a pinpoint of what cylinder(s) are dropping. So yes, even a "new" opti could have problems. When you say a new opti, was that a genuine GM part or a cap and rotor form some other source?

[scbird1]

The opti is a GM part. Any parts I buy I like to make shure they are delco remy or GM made in USA. I checked every wire to every sensor, computer,coil you name it. I tried removing my chip and same thing. Im going to run some injector cleaner though it and drive it as-is as long as the air-fuel meter shows good, it should be alright.

[scbird1]

Still having problems and now some new ones. When I go to start the car from cold the tack is jumping around and the car seems like its only hitting on a 3 cylinders. It will finaly start to run somewhat normal after 1 min. Im going to replace the wires to a better brand I hope(Tailor spiro pro`s) and some new plugs( AC Delco 41-906) The car has been sitting for awhile as the PCM was sent off to be rebuilt to rule out problems with that. No change in the way it runs. Im going to do a compression test as well as long as the headers and plugs are out. I have even removed my chip from PCM for less to see if it was causing problems. I might add that it has been to more than a few shops and they cant find anything wrong. It still chugs after starting it from warm for the first couple min of running and then it seems like it clears out. It still sounds like its hitting a rev limiter at about 3800 to 4000 K. The funny part is the motor is very strong like roast the tires in second and part way though third strong. The tires are 275/40s. Im going to put the car up for sale soon but I need to get this problem fixed. Only things original are the injectors and fuel pump and they might be next. Im still thinking it has something to do with the ignition system or wireing due to the tack jumping around when its cold now. Any thoughts? Thanks, Mark

[Joe 1320]

Mark,

This may be a wild goose chase, but I've seen the same symptoms from wiring harness issues. I've seen the breakup problem due to intermittant coil driver power. all connections weatherpack connections must be tight or as the load and rpms increase, the resistance jumps and the misfires show themselves.

Was heat sink paste used on the backside of the coil driver module when it was mounted? have you removed the module and had it tested at an auto parts store? I've seen new modules last a day, some tested bad right out of the box. Just because a part is new doesn't rule out the fact that it could be marginal.

Are you certain the plug wires are 100%?

If a mechanic says they can't find anything wrong, find a different mechanic.

What you need is to put the car on a diagnostic machine that will show each cylinder event. Any misfires will show up and each combustion event in each cylinder can be documeted. I hate to say it, but a dyno tune facility or the stealership is about the only place that is going to have the needed equipment to do a complete diagnostic.

[scbird1]

Thanks for the reply and a goose chase is what ive been on for months now. I have tried to check every wire under the hood. Car was untuched when I got it untill the motor was pulled. I did the rebuild and im always carefull with wires. I replaced the coil and what I thought was the electronic ignition module right next to the coil. I used the heat sink compond that was supplied with it. How can I make sure that the connection on the coil is perfect? No way to tell when you couple the connector. It just clicks in and I make sure i strait on when installing. Im installing ne plugs and wire right now but the old ones are less than a year but they were Beldons 7MM. Im going with 8MM over the tops of the valve covers this time. Here is a pic of the coil. Thanks again Joe.

[scbird1]

Well after 2 days of fighting with the exhaust, starter and plugs and wires its running great again. Yeah the starter had a loose connection from the solinoid to the starter. Its only 5 months old. I took everything apart again and did a compression test on all cylinders and they were all 200 psi so the motor is still sound. Looked at all the lead wires again and after alot of testing found nothing. The plug wires were just a little crappy as their under the car but this new set from Tailor is great. The plugs did not look bad but something changed because it does not backfire or break up now. When they run great its a good feeling. Thanks guys for the help. Mark