mcm95403

Recently I've been trying to troubleshoot the engine miss I've had ever since doing my ignition upgrade. In doing so I've been trying to test the existing sensors rather than just replacing the whole array of them.

This past week I ended up replacing the TPS sensor (which tested perfect) and the car ran noticeably better. Last night I replaced the MAP sensor (thanks to RockAuto.com it was only $21) and again the car runs significantly better. The miss is still there, but I'm pretty convinced at this point that it's only ignition related as you can watch the tach bounce several hundred RPM's when it's missing under acceleration.

I already have a new engine temp sensor and a new IAT sensor and I'll try to get those in later this afternoon. I've ordered an ignition terminal block (only $16 on Amazon instead of $70 at Mid America), so when that gets here I'll swap it in too. Worse comes to worst, I'll swap out the computer since it could have been damaged by the cap carnage and (suspected) HEI coil arcing inside the distributor to the ignition system.

Previous threads for reference:

http://forums.corvetteforum.com/c4-s...ermittent.html

http://forums.corvetteforum.com/c4-g...in-please.html

65Z01

I assume it's the '91 in reference:
-are there any trouble codes generated?
-do you have a scanner with which to monitor engine performance during acceleration?

mcm95403

Yes, it's the 91. No codes at all and no scanner to monitor it while driving. My best friend does have a code scanner. I could try that to see if it will give a real-time feed.

jfb

Pull the underhood light fuse and at night, engine idling, look for sparks along the plug wires and at the plugs. Replace missing or damaged plug wire looms and dress the wires to not touch metal. Measure the plug wires, they should measure close to 5000 ohms per foot. Look inside the distributor cap for carbon tracks. Look at the rotor, at the hemispherical dimple in the middle for carbon tracks. Ign coil and spark module inside the distributor aren't expensive and while I do not like helter skelter parts replacement, but when they aren't expensive, I don't mind doing this.

mcm95403

Thanks, but I've already been through all of that. No arcing - you can handle the wires bare while it's running and no shock. All the ignition components are new except for the "terminal block" which should be here by the end of this week. If that doesn't do it I'm seriously looking at swapping out the computer. The only other thing I can think is that the arcing coil from the broken cap may have damaged the distributor pickup.

When it's dead cold it runs quite well, but once it gets warm it's missing most of the time - and it stinks of high HC at idle. Tonight I even tried unplugging the injectors one at a time while it's running and each time it dropped RPM, then picked up again when I plugged it back in.

Frizlefrak

Pull the ignition module and have it tested. If it tests out fine, ensure it has a nice thin coating of ignition module grease (NOT dilectric grease). Wipe both the module and the mounting surface clean, and put on a thin layer.

I'm betting your problem goes away.

jfb

Those are symptoms of a defective spark module when it gets hot. Also, the spark coil doesn't have enough energy to harm the pickup coil, not to mention it is so low in the distributor and so well shielded that the coil couldn't generate a spark that long.

mcm95403

The original one tested good, but I replaced it with an Accel unit anyways. It runs better but still isn't right.

VetteMed

They can test OK but still be defective when installed.

I had bad results with an Accel ignition module on a TPI IROC years ago... put a stock module back in and it ran great.

mcm95403

I'll keep that in mind. It did run better with the Accel unit in it. Yesterday I pull the coil and made sure to grease up the rubber isolator well. That seems to have helped a lot but it's still not right.

Frizlefrak

I'd try a known good module, and double check that layer of grease. I had the exact symptoms on the Cadillac 4.9 I had in my Fiero. Ran fine cold, like crap warm. Ran every diagnosis I could think of ( I DON'T throw parts at problems). Finally narrowed it down to a lack of grease under the module. A nice thin film of module grease, and it ran like new.

redvetttes

We used the module greese (heat sink compond) on a lot of high power SCR's
thse sercet to that is a very,very thin coat,its olny thing to fill in nicks in the mounting of the SCR
shop name for the SCR, we call them hockey pucks

mcm95403

Thanks for the replies guys. I can pull the module and grease it again. I used a very thin film, but enough to do the job. I also have some nice silver compound for CPU's that I could try as well. I got the new terminal block harness yesterday but it's the wrong part. I'll see if I can locate the correct one tomorrow and then re-grease the module when I change out that block.