jshuberg

Hey, I've got an '84 US 928S, with Euro cams, MSDS headers, Y-pipe, and Borla cat back exhaust. After having the exhaust work done, a problem has risen to the surface that I believe has existed for awhile. It's now bothersome enough to look into.

When the engine is cold, I have no power between around 1200 and 2000 RPM. It's actually occasionally stalled on me when giving it gas when cold. After a few minutes when the temp comes up it runs a little rough in that same band, but I do have power. If I set the ignition timing to around normal, the problem is really bad. When I advance to 35-37* it's not quite so bad. When idling, it also occasionally misses even when warm. Discoloration has developed on the headers that would tend to imply that she's running lean. I've had her up on the dyno recently though, and the A/F mix is right where it should be. I've verified the exhaust is tight, and there is sufficient backpressure so I don't believe the new exhaust is the cause of the problem.

All of the plugs, wires, injectors, cap & rotor, fuel lines, mass air sensor, etc have been replaced. The fact it only happens in that one RPM band, and is significantly worse when cold is what is confusing me. It's possible that one of the newly replaced components could be bad, but it's more likely that it's something I haven't replaced yet. The green wire is old, but shows no discoloration or signs of problems.

My guess is that it's either the green wire, or possibly the fuel brain. Is there a way for me to test these other than replacing them, or does anyone have another idea of what my problem could be?

Thanks much!

robot808

jshuberg, I had a similar problem with my L-jet. IIRC, the fix was the temp sensor.

kelanel

Doubt you have the same problem as I did when I received my car, but we have a very similar setup.

The O2 sensor in my case was moved back to the cat and spliced with extra wire to compensate for the distance with the headers, however they didnt tie down the wires and it wound up fusing to the y-pipe, grounding the signal. one day after checking the timing and seeing it way off, we set the timing back to stock and the car ran terribly and would miss-fire. it would stutter specifically at those rpm ranges and then go nuts. pushing the throttle hard enough would start open loop mode and the engine would run with good fuel trim causing burnout. later noticed the o2 wiring and re-spliced (now zip-tied) with a new sensor, running much better.

Again, likely not your cause, but fuel trim issue seems to be the problem. unplugging your o2 sensor would tell you if that was your issue though. wideband sensor would help diagnose.

jshuberg

Thanks guys. I've tested the temp II sensor with a multimeter in the past, and it looked good, but I agree it's worth another look since it is 30 years old and the problem is definitely temp related.

I'll also try disconnecting the O2 sensor to see if it changes the symptoms. Seeing as how I just had some exhaust work done, it's possible there may be an intermittent open or short that was caused by flexing the old wiring to the sensor. It also would make sense that something like this could be temperature sensitive, and change as the exhaust heats up. Actually, this does seem pretty likely, since the problem did get noticeably worse after the exhaust work was done.

If anyone else has idea on things to check or rule out, let me know

Thanks Again!

Lizard928

It likely could be dirty contacts in the barn door.

jshuberg

Thanks, I'll check that as well. It was replaced a couple years ago, but it's possible that the connections might still be corroded.

I tested the temp II sensor again, and it looked good. I've decided to replace it anyway since it's cheap, does tend to cause problems and is 30 years old. There's a remote possibility that it's the cause of the problem, and that I just didn't see it with my meter when testing, but chances are replacing it will just end up being preventative maintenance.

I disconnected the O2 sensor, and it didn't seem to effect my problem at all. Strangely, the car did seem to run the smallest amount better overall with the sensor disconnected, but it could have been just in my head. The roughness in the 1200-2000 RPM range was definitely still there. Would this indicate a bad O2 sensor, from the fact that disconnecting it either had no effect on the engine running, or possibly a very slight improvement? What is the expected behavior when disconnecting a properly working O2 sensor? I would have expected a change in idle or something, but it was exactly the same.

Still trying to figure this out...