300zxClub.com (Nissan 300ZX Forums)

Wanted to see if anyone here had suggestions. I posted the link to a thread I made about my issue in another forum. Thank you.

93
na
2+2
manual

 

Here's the post so you don't have to go though the link.

There are bountiful threads on similar issues that I've read over the months but I can't seem to find the solution to my issue. I hope I can fix this thing and record the solution here for someone else to reference in the future.

My Z car stumbles at idle and low rpm. Early on, the car would have to get to full running temperature for the problem to occur. As time went on, the issue got progressively worse. Now, after running for only a couple minutes, the car will begin to stumble at idle and low rpm. I discovered my #6 spark plug wasn't firing when I pulled and spun my cas.

After discovering the non firing plug, I cleaned the harness grounds at the back of the plenum. Following this, the car ran amazing for a few miles at full temperature but the issue returned and it's as bad as it was before.

Detailed description of the stumble: The stumble is intermittent at idle and low rpm. The car will fire on all cylinders momentarily and #6 will cut out and come back on again. As the car gets to running temperature, #6 will cut out more consistently. The video I took of the car running is useless. You can't effectively here the issue.

The car has the correct ngk plugs

Here is a summary of my diagnostics.

Pulled codes: 55 all good

Cleaned electrical connections: all coil packs, all injectors, mas, ptu, tps, and every other connector I could find.

Ohm'd all injectors: all were good

Screwdriver test: all injectors click

Checked for spark and fuel: pulled the cas off and spun it. The only cylinder not sparking is #6. Also, switched the coil packs around but it didn't make a difference. Used known working spark plugs.

Cleaned grounds: two grounds at the back of the plenum

Checked voltage: Every coil pack connector and injector reads good.

PTU: Switched in a known working ptu and subharness. Still no spark.

PTU relocated: no difference

Compression: good

Currently I'm following my harness down the line to see if I can find anything funky. I want to try cleaning any grounds coming off the ecu but I'm having some trouble finding those. Any other suggestions or anything that comes to mind?

Thank you.

 

Any suggestions at all? Can anyone point me in the direction to someone who might be good with the electrical in these cars?

 

The next thing down the line from the PTU is the ECU..

Borrow an ECU and see if the Problem comes back..

If the Borrowed ECU has the same issues, then it must be a Wireharness issue..

Cap

EDIT:.. here is a '93 NA ECU for an Automatic that will work.. ( Even on a Manual )..
http://www.ebay.com/itm/90-91-92-93-...sories&vxp=mtr

 

Good call. I haven't tried that yet. Thank you.

 

I bought a known working ecu and installed it. Following the installation the car idled fine. When I would rev the engine there was some obvious hesitation but the car still idled fine with no misses.

I took the car around the block and as soon as the car was under load, I could feel the engine start to miss again like it has been. After the test run around the block, the car began missing at idle just like it has been doing.

So, changed out the ecu and it didn't make a difference. So that tells me something's definitely wrong in the wiring harness.

Anybody have anything that comes to mind?

 

Go back and verify.. You have missed something..

Cylinder Balance Test to isolate which one it is..

Spark Jump test to tell health of the Coil Pack..

Then I'd go after an Injector..

Cap

 

I agree with Cap, I just had pretty much the same issue and it turned out to be an injector. I'm pretty sure the service manual states that you can unplug the coil or injector connectors one at a time to see if there is any change (it should bog down). My injector ohm'd fine but wasn't closing causing the cylinder to flood.

 

You may want to consider all new wiring harnesses. I was having the same problem with my '90 n/a. We changed 5 of 6 injectors. One had been changed with a reman'd right before I bought it. Word of advice, don't go with reman'd injectors. I replaced that one after only a year because of problems. But I digress. After chasing the stumbling/misfire issues with only temporary results, my son did some in depth research. With the age of the car and the heat in the engine bay the wiring gets brittle. It can break or the connectors become corroded without being obvious. When we changed out both harnesses, the issue resolved.