smooth1990

Before I get into depth about what Is going on I want to give you guys the list of mods on the car so you know what I am working with! It is a 1992 with a built 383 and a D1SC procharger. It has a full MSD ignition system. MSD OPTI- MSD BOX-MSD COIL and good wires. All brand new!




Anyway::::

Ok so I am stumped. I have been fighting a 5000 rpm ** stumble** misfire**with this car since I went to the track last week. I got 2 good pulls and after that the car would fall on its face at 5k. almost like it was hitting the rev limiter. So I brought the car home, change the plugs the next day and took it out for a test run. It seemed good to go. I drove it for 20 minutes and made a few hard pulls. It reved to 6500 no problem ! yay- fixed! so I thought.

The next day I took the car to a city about 45 minutes from me and it started acting up again as soon as I got there. same thing. 5,000 rpm miss. drove it all the way home and it never got any better.

The next day I changed the fuel filter ( 3rd one in 2 months) and went over all the plug wires. Everything looked great! So I took the car out and drove it for awhile and no issues, Pulled all the way to 6500 rpm. Finally-fixed! haha NOT!

I drove the car back to the neighboring town again today and as soon as I got there (45 minutes straight driving) I noticed the stumble starting in 1st gear! so I hit some back roads and no doubt it was right back to the same thing! 5000 rpm missfire!

So this time I had the factory GM coil mounted in the car and I had the bypass plug for the MSD box! So I pulled over to the side of the road and hooked up the factory gm coil. its mounted so it took 2 minutes. I then took the car back out on the road and still the misfire was there. so I pulled over and put the bypass plug in to bypass the msd box. Now I am running soley on the factory ignition with the MSD optispark. Pulled back out on the road and got on it- dang car wouldnt pull past 2500 RPM! the stumble was terrible! dang it!!

So I pulled back over and took the bypass plug off and ran the car back on the msd box so I could atleast drive it home. When I got back to the edge of town (25 minutes of easy driving) I came to a stop sign. So I decided to get on it again. It pulled to 6000 rpm no problem!! What the heck? so I slowed down and made another pull through 1st and second. at the top of second there was a slight hesitation but it pulled to redline. Ok- One last pull from low speed 1st gear. this time it broke up at 5k again! It seriously feels like I hit the rev limiter each time! I thinnk I have it definatley nailed down to ignitiopn and heat related. I do have a fuel pressure gauge and it does not fall off when this happens and I dont see anything out of the ordinary in my Air Fuel Gauge!

So two things- the stock 60,000 mile ICM or the relativaly new MSD opti spark that has never been wet.. ever

From what I know ICMs can get hot and show signs of failure up top of the power band.

If it OPTI is going bad- its usually a constant misfire up top of the power band, not just when it gets hot???

Im not certain on that last statement!!! I am really looking for some help here! what do you guys think???

I know for 100% sure if I take the car out right now it will run perfect- for 45 minutes or so- before it falls on its face. This is crazy frustrating!

HELP! lol

smooth1990

Come on guys! Somebody has to have some sort of idea!!!

LTxDave

I read somewhere of taking a can of air duster (like for keyboards) and cooling off the ICM when the symptoms occur as a 'test' for a failing ICM. I haven't tried it, but from what I recall, the normal failure of the ICM begins as a problem at higher operating temps.

eutu1984

You may have to data log and then post over here https://www.corvetteforum.com/forums...n-and-tune-82/

smooth1990

Thats a good idea! I will try that tomorrow. if it pulls good after cooling it down then I know that is the issue. I did take the ICM to orilies and tested it over and over on their machine for 40 minutes. It didnt fail at all. not one test.


My datalog doesnt show anything at all. everything looks normal besides the ignition curve. when it starts failing it will break up the curve a bit.

Deakins

Can you get it on a good video with sound by chance? Without being able to hear/feel what's going on it's hard to say much. If it runs out of timing it will lay down on you, if it has fuel issues it will lay down on you. Do you have the ability to monitor those parameters and log them?

smooth1990

Timing and fuel are good. None of that changes at all. In my data logs I can see timing advance. Not the issue. I think the icm is heating up and failing.

383vett

Hey Buddy. I'm not sure if this pertains to your situation, but I had a similar problem earlier this year. Mine however was not intermittent and repeatable during every run. During a run, my motor would not rev over 6600 rpm. I needed 6800 through the lights. I had a 7000 rev limiter chip on my msd 6al unit. The motor would break up just before the mph stripe just as if it were on a rev limiter. I changed the chip to 7200 and no change. I changed plugs, wires, coil, distributor, ignition module, checked valve springs, changed to a msd digital 6al. Still same thing. Finally for the heck of it, I raised the rev limit to 7500 rpm and the motor now runs clean through the quarter. I recently broke an input shaft and the data recorder showed a max rpm of 7000 which was perfect. I don't have an explaination and I know you swapped the msd to stock, but maybe start playing with rev limiters. Another thing that comes to mind is the ignition coil wire. I'm not sure if your setup has a separate wire going to the coil, but thie coil wire gets 8x the impulses of a regular plug wire and can get beqt up pretty quickly. I know if this wire is not in good shape, ignition misses can result. Good luck. Willie

cudamax

Throw a toggle switch on the knock sensor wire. Put it up in the cabin somewhere even hanging. Next time it acts up. throw the switch and disable them and see if it pulls hard and goes away.
IMO You have to high of a C/R for the levels of boost your forcing down the throat of that motor but its right on the line because only after driving it for a while and things get good and hot {not water temp} it acts up. Still. How long was it O.K. with the MSD? Were ya good for a while? and now its doing that shirt again? or you just started driving it on longer runs and noticed it?

Aardwolf

You checked the plugs which was good but not the wires. Swap out the wires also.

cudamax

You running 18 to 22 spark plug gap? Maybe you need 10 to 1 A/F to cool things off more. Plugs get real hot under boost and a 10 to 1 cools them and the crowns of the pistons when ur pulling WOT with high boost. Also those high per modules help a lot. The dwel and output is determined by the module. Some of those can get almost 8 amp pull on the coil.
Even the Northstar motors have a little more added in and they aren't running any boost. They set A/F at 11.5 ;1 to 12.0;1 at WOT then if it sees I think more that 5 sec there, then it drops it to 10;1 to kool shirt at a little at a small cost of power to kool things. 11;1 even 10.7;1 will make more power but 10;1 might kool things more and allow it to rev

smooth1990

I have gone over everything. Today I pulled the opti spark. What do you guys make of this? Ive never saw anything like it!!!!

DanielRicany

Looks like copper corrosion. Did it get wet?

smooth1990

Nope. never been wet and the car runs fine until it starts misfiring after 40-45 minutes of driving

cudamax

Moisture. Check the vents

smooth1990

Vents are good to go. this thing doesnt get driven in the rain nor does it get washed at a car wash. never been wet at all.

cudamax

Technical service bulletins were created by General Motors to make technicians aware that the vent on the base plate could become clogged and accelerate the moisture buildup and corrosion inside the distributor. Not only would this damage the cap and rotor but it could also affect the ignition module.

The module connections can turn green and even disintegrated if the problem is not addressed. The technical service bulletin informed technicians that they needed to check that the vent screen and the base of the distributor was clean and allowing moisture to be vented out.

Just saying. Moisture is what causes that. Shrit can all that and replace or clean and scrape and put a small film of good old Vaseline and seal the cap. Even high humid days. Check the module connections for that stuff and it looks like you found ur problem

smooth1990

Can I clean this up and reuse it or should I toss it and buy a new one?

cudamax

When the bronze oxides, the corrosion is a blue-green color. The copper in the alloy turns green (patina) the iron turns red (rust), and the lead turns a grayish white. The corrosion can be accelerated by the presence of high voltage and the resultant ozone generated in the cap.

cudamax

Sure you can clean it up with a tooth brush and baking soda. Dry it off real good. Seal it with a film of something Gel and put a little gel it the cap towers and go for a Blast. And keep checking it