mcm95403

I'm working on diagnosing an intermittent misfire in my 91. 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.

Nathan Plemons

An ignition module can go bad and they can get flaky with temperature. Unfortunately I don't know of any way to prove if it works or not short of replacing it. Perhaps some of the TPI guys will chime in with more advice.

mcm95403

They were both seated correctly. I think the snap was the sound of the underside of the cap cracking.

Cliff Harris

I saw a suggestion a while back on testing ignition modules. They said to run through the tests about 5 times so that it warms up. That will show if it's got a problem at higher temperatures.

mcm95403

We tested it 3 times back to back to warm it up, but it still wasn't anywhere near engine temp. I think I'm going to go ahead and pick up a new module.

mike100

You might consider just getting an entire replacement distributor from a wrecking yard. My friend had a high mile 89 Camaro where the magnetic reluctor and pick-up itself went south. These distributors are all similar (do you have the small one?)- just find one out of a similar GM car and get it for parts/diagnostics- especially since you damaged the one you have now. For $40 you could just have a whole replacement- I think it would be a good move at this point.

The other possibility that comes to mind is you are getting into early head gasket failure stages. Didn't you mention that the intake manifold gasket was leaking coolant out of the corner? I wouldn't put it past somebody to just keep driving the car like that and maybe getting a little hot.

mcm95403

I replaced the module today with an Accel unit and the car runs better, although it's still not right.

I still need to replace the fuel filter. Hopefully that will take care of the rest of it.

mike100

I would buy a fuel pressure gauge instead...or borrow one. tape it to your windshield check pressure under accel.

mcm95403

http://forums.corvetteforum.com/c4-g...g-sensors.html

jfb

Unplug the underhood light fuse and at night look for sparks along each plug wire and at the plugs. Replace missing or damaged plug wire looms and dress the plug wires so they do not touch metal. Measure each plug wire, stock wires are about 5000 ohms per foot. I do not know what Taylor wires measure. Throw out the MSD coil and install a GM coil, there is no advantage whatsoever with an MSD coil!!!!!!!! The stock GM ignition system is high energy and very reliable, MSD coils are not as reliable. Testing spark modules will only find gross module faults and you can test it continuously for days and it won't come up to the temp it gets with a warm engine. A common failure for high time modules is to put out weak spark when hot and work fine after cooling down. Engines that, "miss", have a spark problem, you need to find out where.
Oh, also look inside the distributor cap for carbon tracks, look at the hemispherical dimple in the middle of the rotor for carbon tracks.