Misfire has me puzzled?

[RL2000]

I have a '91 Chevy K1500 pickup (the 4x4 full size model of it's day) with the 4.3L 6cyl motor.

It has a misfire on the 6th cylinder that has me stumped. I tagged it down to the ignition system. It has a distributor, so I replaced the cap and rotor (just to be sure, as the old one was corroded).

Basically, there's just no juice coming out of the distributor for the 6th cylinder wire. It now has a new cap and rotor and is still not giving juice. The spark plug wire is also new for that cylinder.

I'd say the coil, pickup sensor (in the bottom of the distributor), etc., but the rest of the cylinders work like a charm. It's just this one cylinder.

How did I end up at this diagnoses? (a) Having the TBI system, I ruled out the idea of a clogged fuel injector on a single cylinder. (b) I ran compression check on all the cylinders and they all test out around 180psi. (c) When I pulled out the old plug from this particular cylinder, it was black, whereas the other plugs were normal. I also hooked up a spark tester (like a spark plug, but clamps on the engine block) and found no spark coming from the distrib for that cylinder. I'm baffled!

Any ideas? What should I be looking at next?

The only other things I can think of include:
- Ignition control module
- Electronic spark control module
- Engine control computer (ECU)

I guess I should ask... which one of the above 3 parts actually control the spark signals, thus skipping the 6th cylinder?

Any advise or insight would be greatly appreciated.

Thanks!

P.S. Maybe I should add that one of the spark plug wires had arched and therefore was replaced... that is the cylinder that is misfiring. Perhaps the arching damaged something? But how? Since the high energy part of the distributor is separate from all the sensitive stuff... I don't see how it could shoot back into the system and damage anything.

[2000CAYukon]

Could be the pickup coil or a bad magnet on the shaft. I would replace the dist since it is usually more cost effective to replace it that to just replace pickup coil and the magnet.

//2000CAYukon

[RL2000]

For some reason I got her to spark... now my only question is why this particular cylinder keeps turning the plug black after running it a little bit. The other cylinders don't do this, and I'm curious as to why.

Being TBI, if it were a fuel issue, I'd think it would be the same on all the cylinders. Compression checks out flawless on all cylinders, so it's not a worn valve or rings (I'm thinking).

I'm baffled!

If all plugs are firing good, is there anything else that would cause just one cylinder to keep turning the plug black in a TBI system with good compression?

[2000CAYukon]

Valve seals?? Does it have the blue smoke at startup?

//2000CAYukon

[RL2000]

Originally Posted by 2000CAYukon

Valve seals?? Does it have the blue smoke at startup?

//2000CAYukon

No blue smoke that I can recall, but then again I haven't had somebody else start it while I watch the exhaust.

I think you're right... with only 95k miles, and I know the guy took very good care of the truck, I can't see just one valve guide being worn and the rest are fine. Being that compression is up to par, I'm assuming that the rings should be just fine. Everybody else I've talked to confirms that good compression pretty much eliminates ring problems.

I think I'll buy a valve cover gasket and grab a loaner tool to do the valve springs... one thing I really like about this truck is how easy it is to get to the valve covers.

Thanks for your input!