90' Intermittent power - Spark problem

[Great White]

From what I gather, you say that the spark on #5 looks/sounds like it's more frequent than any of the adjacent cylinders at any given RPM, and provides no change to RPM when unhooked?
That coupled with the carbon build up is the only thing that really narrows this down to cylinder #5...
This sounds to me like cross-triggering either between coils, or plug wires, or within the #5/8 coil. If the #5 wire or coil has compromised insulation, it may be intermittently shorting to ground as well. This would all explain why it's so sporadic/intermittent. If this issue was occurring before you replaced all these parts, it may be an issue in the primary wiring before the coils.

If in fact this was occurring only on #5 before the parts were replaced, I wonder if there's a mechanical issue on cyl. 5 like a dead lifter, burned/bent valve, or wiped lobe.

Is the plug on 5 tan, black, or covered in raw fuel?

[JerryRig]

Originally Posted by Great White

From what I gather, you say that the spark on #5 looks/sounds like it's more frequent than any of the adjacent cylinders at any given RPM, and provides no change to RPM when unhooked?
That coupled with the carbon build up is the only thing that really narrows this down to cylinder #5...
This sounds to me like cross-triggering either between coils, or plug wires, or within the #5/8 coil. If the #5 wire or coil has compromised insulation, it may be intermittently shorting to ground as well. This would all explain why it's so sporadic/intermittent. If this issue was occurring before you replaced all these parts, it may be an issue in the primary wiring before the coils.

If in fact this was occurring only on #5 before the parts were replaced, I wonder if there's a mechanical issue on cyl. 5 like a dead lifter, burned/bent valve, or wiped lobe.

Is the plug on 5 tan, black, or covered in raw fuel?

Yes, no RPM change when unhooked.

I just put on new NGK plug wires and still have the #5 issue.

You think primary wiring? As in a piched wire or loose plug somewhere?

All cylinders look like they're burning great minus #5 which looks like it's burning uneven...and darker.

After talking with Marc he was thinking it couldn't be a valve, valve/spring because the problem comes at random times. I borescoped the #5 cylinder and compared to other cylinders the valves looked closed...enough, nothing abnormal I could spit.
All that to say it could still be one of those problems, I was trying not to take the Cam cover off unless I had to.

[JerryRig]

I think I'm making some progress. It's been right in front of my face for awhile now, I think it's the Camshaft sensor. I had got a Code 31 in the past.

I unplugged the sensor and It seems to run great now...I ran it for 30 miles like this. Idles smooth no abnormal sounds and goes through the gears cleanly.

I cleaned the leads up a bit and zip-tied connectors together(connection seems to be a tad loose)

I just took it out for another drive with it plugged in. No SES light for at least 15 miles, runs smooth like it should.

I might be in the market for the rare Cam Sensor.

[DRM500RUBYZR-1]

Perseverance, attention to detail and a plan pays off again!

Well done!


Marty

[JerryRig]

Well, I was wrong. Come to find out the Camshaft Sensor was only part of the problem. I did get a new Camshaft Sensor installed, and the code went away; BUT the main problem came back and I called Marc to get a recommendation on what to do next. this was about 3 weeks ago and It came down to sending the ECM in to get checked out, he ran It in his own car and was able to produce the same problem.

The problem was, per Marc, "The ECM is not driving a fuel injector on the left bank."

He sent the ECM out for repair, and I got a call today saying he had received it back, and he ran It in his car and It ran flawlessly!

I'll probably get the ECM back by the end of next week and be able to test It out.

[WARP TEN]

Originally Posted by JerryRig

Well, I was wrong. Come to find out the Camshaft Sensor was only part of the problem. I did get a new Camshaft Sensor installed, and the code went away; BUT the main problem came back and I called Marc to get a recommendation on what to do next. this was about 3 weeks ago and It came down to sending the ECM in to get checked out, he ran It in his own car and was able to produce the same problem.

The problem was, per Marc, "The ECM is not driving a fuel injector on the left bank."

He sent the ECM out for repair, and I got a call today saying he had received it back, and he ran It in his car and It ran flawlessly!

I'll probably get the ECM back by the end of next week and be able to test It out.

Great to hear that he sorted it out for you. You can't go wrong sending a problem to Marc for a solution. --Bob