Klondike

As you can see from the title, I'm having dificulty with my ignition and suspect the coil control module. I had one tested as bad at Advance Auto a while back so I bought one of theirs. After a couple of weeks I'm back to RANDOM MISFIRES again. Took it and the original back and had them both tested as bad. THe parts guy took a new one off the shelf and it tested bad as well! So, I don't have a lot of confidence in their tester mainly because all the other modules they test have dedicated plugs and they tested mine with aligator clips....
So do any of you guys know of any auto parts places that have a reliable test procedure for these LTx ignition modules? I don't want to just throw another one at this thing without knowing if it is good or bad.
I've already changed the Opti, wires, plugs, O2's & injectors, so no advice on those other things needed. Just need to find somebody that has good tester & how to operate it.

Lichen

If the ICM is bad, you won't get a random misfire, your car will just die. You should get a code 42. Maybe the coil itself is bad.

rick lambert

Have you tried AutoZone?

LD85

AZ has tested mine good and bad and got the correct result

Klondike

Tried Auto Zone. Their tester couldn't test LTx modules.
Tried NAPA = Same story
Advanced Auto = Everything tests bad, even new ones off the shelf Not a confidence inspiring test rig.
Been calling shops around the San Antonio area and not finding anything.
I buy another one if I needed it, but I don't know if either one of these things are bad or if the coil I tested is bad at load & heat under actual running conditions. Maybe I should start building test rigs

Klondike

I've never owned a car that NOBODY could test simple parts on before! I guess I'm back to shotgunning this bastard with my wallet again. I wonder if it will ever run before I run out of money. This thing has been broken off and on, longer than it has run. My wife loves this thing or I'd pull it down for parts. It's worth more in pieces than if it ran anyway. I'm starting to get a case of C4 red a$$.

rementis

LOL, I feel for ya man, that's tough. Good luck!

pcolt94

As I make my living in the world of electronics from commercial the high tech military, I can tell you the testing parts or modules is not a certainty in life. Duplicating exact operating conditions sometimes not possible. Testing is great but never is a 100% confidence factor. If a part tests bad, it probably is. If it tests good there is always room for a margin or error.

Back the fixing your car. The ICM is basically just an amplifier that amplifies a 5 volt square wave to about 70 volts at various frequencies. It also works in a hot environment for electronics. Most of us don’t like to through parts at the car but for an item like this that is relatively inexpensive and real easy to replace, substitution is the only real valid way to determine if the original part is a problem. Even if the original ICM is good, you will know exactly where you are and can move forward with confidence.

New parts can be bad also but in this situation you might consider getting a GM part. I just don’t know how good the after market parts are in this case.

Besides, having a spare ICM is not a bad thing.

Having said all this stuff, have you looked at the engine running in the dark for any arking when revving the engine.

SunCr

Assuming your Vette is popping a 300 code, I'd be more inclined to suspect dirty or plugged injectors; if only because GM addressed that issue with a Service Bulletin and a special 200K warranty (sorry CA cars only). You might just try having them cleaned.

daytonaer

Regarding the parts store testing an ignition module, check the setup carefully. I have some experience with one of those test rigs and if they are not hooked up properly will result in failing scores. I was in the same position, testing new ignition modules as bad until the polarity of one of the wires was noticed to be hooked up backwards, and after that was corrected, passing scores were rewarded for all the new parts.

They can be confusing charts or setups, its not necessarily the operators fault, but check that over carefully. And good luck fixing it! BTW, I have seen MSD "performance" ignition control modules which were about the same price as aftermarket replacements, not sure if they make them for the LT1 but worth looking into if you need to replace it.

Klondike

I've replaced everything except the coil. I'm doing that next just because I can't say EVERYTHING yet.
2 new ignition modules
MSD opti distributor
MSD 8MM WIRE SET
NGK TR55IX Iridium plugs (don't remember now what the gap was set at, .045 I think, but I did check them ALL before I put them in)
4 new bosch O2 sensors
8 new Bosch III injectors from injection connection (thanks Jon)
New fuel filter & checked pressure under full throttle.
I'm very careful with my work and do a checklist to make sure every bolt is tight and every wire is seated. I really don't feel like redoing this whole project again, but I've got the same P0300 as I had before I changed everything except the coil. If the coil doesn't do it, I guess it's time for an excorcist.

pcolt94

When does the miss-fire happen. Is it a stumble or cough, or just like a miss. Does it happen under acceleration and/or at and idle. Is it temperature related when it happens. Has any symptoms changed since you started this project.

Just some suggestions that might make you think about something.

Vacuum leak or EGR leak.
Plugged CAT
Fuel related, tank, contamination, fuel filter
Loose connection or corroded/bad contact on PCM, ICM, etc.
Have you carefully inspected for anything arking in the dark. Rev the engine and look everywhere.

Klondike

The original Opti failed and the rest of that stuff looked like worn out and original, so I changed all that stuff too. At first it wouldn't run much past idle and it turned out to be the O2's got toasted from the old Opti. Out came the old and in went 4 brand new Bosh O2's. That helped but it still wouldn't rev past 5000rpm. It ran pretty smooth and responsive under that and got great mileage. Then I drove it over to where I work (12miles) to pick up some stuff I needed over the weekend, and it got me to the front gate and suddenly the check eng. light came on and it started missing like every other plug wire was off. I managed to get it back out on the srteet and up to 45mph where it cleared up and got me home. I put the code scanner on it, hopping to finally see what the low rev problem was, only to have the reappearance of the same old P0300 random misfire display again. I can't find anything wrong with any of the parts I put on and the only thing I didn't change is the coil (which tested OK)! Although it sounds wastful and stupid, I'm going to change the coil too, just because it is the only thing I didn't touch.

Klondike

Finally found an O'Rilleys that had a tester for ignition modules that worked. It still had funky and hard to use little aligator clips instead of a dedicated plug for the module, but at least they could test them. Both the original and the replacement I had bought, both tested as GOOD. The reason I had replaced it is, it had tested bad on advanced Auto's tester. I trusted the tester and blindly bought a new one without testing the new one first. Later I took both of them back and they both tested bad along with another new one from the shelf! What a way to sell new parts huh. Today on a different tester at O'Rilleys they both test OK. So I'm down to the coil because nobody can do a real "under load & heat" test on a coil. I can understand that, and a new one will probably not cost but a small fortune for an LT4, so I'm going to the dealer where I can get a club discount on Vette parts and lay my money down. So; We'll see what happens when I reassemble that garage constipating slug in a couple of days.

ukpete

Hi, did you eventually fix your problem?