NA running rich, stumbling at idle

[Jeff B]

So my 1990 NA has been stumbling some lately. Listening at the tailpipe, the drivers side seemed to be running fine but the passenger side was running rough and rich.
I check and there are no codes. Do the cylinder balance test and they all seem the same (removing any of them makes it run worse)
So, I replaced the passenger side O2 sensor (the drivers side was bad and I had purchased a pair, so I went ahead and swapped that). No change. I had plugs ready to go in, so I swapped those out. Drivers side all plugs look good, passenger side all cylinders running rich. Swap plugs, no change. Battery terminals were a little loose, so I replaced those. It SEEMED to get a LITTLE better, but I think that might have just been in my head.
Seems to run much better in the higher RPMs, dont know if its actually running that much better, or if the random stumble has less of an effect due to RPMs. At 3000 it seems OK, at 3500 its pretty good and over 4000 it seems to get a lot better. Putting it in a higher gear (so that it takes longer to get out of the low RPMs) really highlights the issue.

Any Ideas? CAM timing? do these cars ever jump timing? I need to get the 120K service done anyway, so that might be a good place to start.

[float_6969]

Have the fuel injectors been replaced on this engine? They had a common problem with the injectors failing. Or maybe it was the lower o-ring failing. Either way, it sounds like an injector problem to me.

[Jeff B]

I am back to thinking it is electrical.
on the way to work today it exhibited the issues of a weak electrical connection (every time I hit the brakes the radio shut off)
on the way home it was running rough, and once I got on the highway it started bucking so bad I had to take it out of gear. As soon as I did it died (the engine and all electrical)
10 or so seconds later I had power again, started right up and ran fine the rest of the way. radio no longer died when hitting brakes either. Got home and all connections at battery are tight. Gonna try and trace the battery cables all the way down and see if I can find something.

[float_6969]

Bad ground. Download the FSM from the FAQ, and look through the wiring diagrams and see if you can find all of them. On my S14, when I first bought it and was driving it home, the CEL came on, then like 5 min from home it started running terrible. I checked the codes when I got home and had an array of failures. I knew that wasn't right, so I just cleaned every engine ground I could find, cleared the codes, and fired it up. It ran perfect. The car had sat for a long time and the vibrations from driving had deteriorated the connections.

[300ZXttZMAN]

Have the fuel injectors been replaced on this engine? They had a common problem with the injectors failing. Or maybe it was the lower o-ring failing. Either way, it sounds like an injector problem to me.

Very common problems for the early style injectors.

[Jeff B]

well this car is gonna be the one that drives me insane........
traced all the grounds and they seemed fine. the wire that goes to the fusebox was loose on the positive battery connector. all fixed now but still runs rough.
except, its both banks now, and both running rich.
I swear one side had plugs that indicated rich and one side looked fine before.

so I remove the vacuum line from the fpr and there was no change.

[float_6969]

removing the vac line from the FPR won't make much of a difference at idle. It would actually make it run MORE rich. I'm not a Z32 guy, so IDK what the specific common problem with the injectors are. Have you done any research on that?

[InitialForce]

How are your ignition module wires and the module itself?

When I relocated mine I accidentally pulled one of the wires out of the plug just far enough to make a bad connection and I had one bank with misfires and weak ignition signal. Check those first. Rich doesn't necessarily mean to much fuel. Could be weak ignition. Do you have any noticeable loss of power?

[Z31toZ32]

your injector is fizzing out man. verify it by doing this: http://www.twinturbo.net/nissan/300zx/F ... ctors.html. after you verify it is out of spec, you could replace the one that is failing for a quick fix, but generally once one starts going the rest follow suit, so you might as well replace the group if you are planning to keep the car.