Hi Everyone,
I have a 91 Corvette with an odd problem. I recently did an ignition upgrade (cap/rotor/Taylors plugs) and had the cap break during install. Since then I've had a misfire condition. It gets worse when hot, and isn't there at all when dead cold. So far I've:

replaced the broken cap and rotor the next day, but there was an hour of drive time - this helped a lot

replaced my injectors with a set of Bosch III's from FIC - this helped a lot
also fixed some vacuum leaks at the same time

replaced the o2 sensor - not much improvement

since doing the injectors I've been able to re-add the wires - more improvement

replaced the igniton module in the distributor - again, helped more, but still missing

Today I replaced the fuel filter and did the throttle body bypass - again, it's running better, but still has the miss - although not nearly as much as before. I also pulled the AIS motor, cleaned it and the housing well - it idles much better now.

I tested the MAP sensor today and it appears to be good, but my questions concerning that and also the TPS is: can a digital VM really show an intermittent/worn contact area in these sensors? It seems that the meter doesn't respond quickly enough to show what is really going on.

I'm almost to the point where I think I should replace the MAP, TPS and IAT sensors just because they're all 20 years old. The car has no codes. I'm also wondering if it's possible that the broken cap could have let the MSD coil arc to the internal wiring and damaged the computer and/or sensors. What do you guys think?

Prior to the ignition "upgrade" the car didn't show any signs of the misfire. Mileage per the dash was pretty steady at 23-24 @65 on a fairly level road. Seems like a lot of the Vette guys with similar cars get about 28 so I'm thinking that something electrical is still screwy.

Also, with all these fixes, the car pulls quite well at full throttle, although you can still feel a miss as the RPM's climb.

Pics of the cap damage:

 

Yes those sensors can be intermittent but still "test" "good" with a DMM. For that reason, I keep an old Simpson 260 around.

Keep in mind also, the ECM is about as slow as that DMM, and won't respond to millisecond hits on the sensors. Any intermittent has to last for a fairly extended period of time to affect its operation

However, they CANNOT cause a "misfire". That condition is when ONE cyl runs differenty from the rest. They will affect ALL CYLS EQUALLY at ALL TIMES that they are malfuctioning; so you end up with a wandering idle, no power under load, and similar "runs like crap" problems, but NOT a "misfire".

For a condition where some cyls seem to behave differently from others, look at things related to individual cyls; injectors, spark plugs, plug wires, valve train.

No the dist cap failure didn't allow spark to go to any such place. It will arc right straight to the plate the cap sits on.

Inspect your work replacing the cap and rotor CAREFULLY. Make sure the coil ground wire (or strap) is hooked up; make sure it's not arcing to the rotor screws; that kind of thing.

One curious thing I've had happen once before, was a "hair" of a spark plug wire, at the factory-crimped end, was not crimped into the connector barrel; instead, it was sticking out the back side of the boot about ¼", along the wire itself, and arcing to the header. Took FOREVER AND A HALF to find it. Finally one night I opened the hood when it was real dark, and voilà! there it was. I could see the arc. You might want to try popping the hood in total darkness and looking for stray arcs.

 

Thanks for the reply and info. I've also tried a different set of wires with no appreciable change. The other thing I need to look at is fuel pressure as it was on the low side (35-37 psi) when I checked it the first time. Now that I have the new filter in place I'll check it again.

 

See if you can narrow down which cylinder is misfiring. Then you can swap pieces connected to that cylinder (injector/plug/wire) around to see what's what.

 

Rather than changing a computer out I decided to head down to Race Street Rally to have them scope the car. That hour of diagnostic time was worth it when we found the problem - one of my brand new spark plugs was bad internally. Swapped it out and the car runs perfect!

I don't regret changing out all the sensors that I did though as every one of them was original 20 year old stuff and for the most part each one improved the way the car runs.

Woohoo, now I can move forward on other things.