I'm working on diagnosing an intermittent misfire in my 91 Corvette. It started when I replaced all the ignition components a couple of weeks ago:

New plugs (Autolite double platinum)
New Taylor wires
Blue Streak cap & rotor
MSD coil

During assembly there was a "snap" sound that was unusual. The car immediately ran rough but it was the end of the day and I had to go home. Drove back down to my friends house the next day (30 miles) and when I removed the cap it was carnage. The cap had shattered under half of the coil. Molten/ground up plastic everywhere, molten plastic on the rotor. The coil contact was ground up and the spring connecting it to the coil was twisted sideways. I'm not sure how all that happened as I had assembled the coil and cap on the bench and all was right.
At this point I've replaced the injectors and fixed several vacuum leaks (loose bolts). I've verified that the MAP sensor isn't defective, and neither is the TPS. The miss isn't there when dead cold, but after the engine has come up to operating temp and been there for a while the miss is quite obvious.

Here is my idea/question: I'm thinking that the coil may have arced to the HEI module inside the distributor ansd damaged it. It does test good on a bench tester, but I'm thnking that when it gets up to engine temps is where it starts to go intermittent. Can an HEI module do that or is it one of those "works or not" parts?

I'm trying to avoid throwing parts at it to fix it.

PS, no word on fuel pressure yet. I'll check that tomorrow when I pick up my fuel pressure tester from storage.

 

About a 99% chance you broke something while installing it. Most likely something wasn't aligned correctly when you tightened it down.

 

Mechanical carnage from an arcing coil........ hmmmmm....... very unlikely. Maybe it's possible but I think I agree with five7. Possibly you got the cap on crooked or the rotor wasn't seated all the way.

 

I'm assuming all of this 'carnage' was repaired with new parts and the miss is still there.

The ICM will generally either work or not work. The 'not work' part can intermittently occur, usually when hot. I've not experienced, nor heard of, an intermittent miss associated with the ICM.

Because this miss occurs when hot, I'd suspect a bad component that is causing a weaker spark when hot. The pickup coil is a possible suspect and very inexpensive to replace. Before changing it I'd check shaft play, side to side, on the distributor. The shaft should have no discernible 'wobble' (another possible culprit).

Sounds like the rotor wasn't pressed all of the way down leading to the carnage?

 

I'd suspect the coil before the module.
If you still have the old coil, throw it in and see if it still misses when hot.

 

I picked up a new Accel ICM this morning and will swap it in shortly. I've already tried the coil, different wires, etc. all with no difference. I'll post a pic of the cap and rotor carnage shortly.

 

Granted, this is with an old HEI setup, not a TPI-style distributor, but I had exactly that problem. I had an itnermittent miss at high RPMs. It would get better or worse sometimes. But I could go WOT and as it cracked past 4500-5000 it would start to stutter and pop. It finally got so bad I started replacing parts. Put a new cap and rotor and coil on it and it was even worse, started doing it at 3000+ RPMs. It would even miss at idle. You could see the tach jumping all over the place every second or two. Car was drivable other than that. A little while after that it stranded me. Repalced the ICM and all the aforementioned issues went away completely.

The issues turned up again a few months later, and I was scratching my head thinking the problem might have been something else, but shortly after that the car left me stranded again. Replaced THAT ICM (both were Accel modules, btw) with a parts store stock replacement one, and it's been fine for a couple of years since.

I dont put anything past the ICM anymore when it comes to ignition stuff. Those things are a constant thorn in my side.

 

Well there you go. Can't say I haven't heard of intermittent failure of the ICM now.

thanks

 

Update:

Pics of the carnage:







I've since replaced the ICM with an Accel unit and the miss is still there. I did the TPS yesterday but haven't driven it yet since I'm also doing a few other things (suspension related) right now. I've ordered MAP an IAT sensors and they should be here in the next day or so. I also ordered a "terminal block" but that is still about a week away.

I did the throttle body bypass and cleaned the IAC setup - that helped it pull better and the idle control is a lot better now.

I'm really leaning towards the problem being either MAP, TPS or both, but who knows. Weird thing is, no codes and the car runs almost perfect at full throttle. It also will (occasionally) run perfectly normal, but only for a few seconds then it's back to missing.

Fuel pressure was at 35-37 psi with vacuum on while running, 41 after shutoff. I've since chnaged the fuel filter as well.

I hate to just start throwing parts at it, but since everything I've tested so far checks out I don't know what else to do at this point.

I'm starting to wonder if the computer is bad even though there are no codes.

 

Yesterday I pulled the cap/coil and noticed there was a lot of wear on the button and a lot of carbon dust in the center of the rotor - too much for less than 1K miles of driving. I swapped it for a new one and greased the rubber isolator up really well. It doesn't stink at idle like it did before but the part-throttle miss is still there. The miss at idle is almost gone now.

 

It looks like something under the coil (or the coil itself) is caving in the center of the cap.

I've never heard of greasing the isolator. I recall putting a small dab of dielectric grease on the carbon button in the early days of HEI (circa 1974) to keep the rotor contact from burning up.

 

Agreed.

I have seen pickup coils crack in such a way that they look fine but caused the spark to break up. When you take them out they come out in pieces.

Just my

 

Obviously, it's impossible to know how those parts were assembled originally after the carnage has already happened but let me post up the PROPER assembly order, just in case there is any confusion.....

Rotor on first, obviously. The next parts you can do either on the bench as an assembly and then put the whole thing on the car or you can build it up right on the car.....

The cap. Then the "button" with the spring pointing UP towards the coil, then the rubber isolator donut (smear of dielectric grease on both sides if you like) then the coil, then the coil cap/cover.

The whole thing including the coil will kinda flop around in there until you tighten down the coil retaining screws and "sandwich" all the parts together pernanently.

 

That sure looks like the rotor rubbing on the cap. Exactly why it would do that and still run - not sure. Perhaps something under the coil when you tightened the coil down.

 

How old is your O2 sensor?

 

I've replaced the o2 sensor a couple of weeks ago. I've also now replaced the TPS, MAP, IAT, cooalnt temp sensor, ICM, cap, rotor, wires and injectors. For the most part, the problem is minimal, but most of the time at idle it is rough and belches out hydrocarbons like an old LS6. The first inch or so of throttle is smooth, but as soon as you pass that point it's VERY rough. Full throttle is almost normal, but you can sometimes see the tach needle bouncing as it revs up.

The cap/rotor/coil install was done right the first time - don't know why it broke like that unless it had a hairline crack to start with.

Tonight driving home the SYS light on the dash came on every 2-3 minutes. I'm beginning to think it needs a new computer.

The pickup coil requires the distributor to be removed and torn down on the Vette. It's not the old style HEI pickup - this one is round and goes all the way around the shaft. More fun.