300ZX/LS3 persistent P2135 problem

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Apr 22, 2022 | 10:03 PM

momosc

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Teching In

Joined: May 2018

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300ZX/LS3 persistent P2135 problem

Hi knowledgeable car folks:
I really need some help here. These last couple of years my freshly built 1991 Nissan 300ZX/LS3 (see specs below) has developed a nasty, and potentially very dangerous habit of going into limp mode unexpectedly.

The code is always the same: P2135. It used to be quite intermittent, like going several hundred miles, then failing two or three times in a row. Now it’s happening much more often (except of course if I have tech in the car with me!)

I’ve gotten a lot of support locally, and the folks at PSI are great, but still we’ve been unable to find the actual cause of the problem so we can fix it. I bought a $500 Autel scanner and do have some live data, and enhanced freeze data after a failure, but still no smoking gun.

I love this car; it’s beautiful and big fun to drive, but I’m afraid to drive it much for fear of being rear-ended or causing an accident behind me.

I really need a solid course of action here. Can anyone help?

Thanks, Mo

What we have done so far:

· Replaced aftermarket TB with OEM

· Replaced gas pedal with OEM

· Replaced PCM (The car likes this new PSI one a LOT. Runs smoother, quieter, doesn’t stall off load like it did before. The first one was not done by PSI)

· Went through the throttle actuation wiring and found it to be in-spec.

Engine & Related:

· Brand new GMPP Crate LS3

· 6.2 Liter

· Factory GMPP hot cam (480HP)

· OEM gold plate throttle body

· OEM gas pedal

· Aftermarket Sheet metal intake

· K&N C6 Corvette cold air kit and filter

PCM:

· E38 Programmed for my car by Performance Systems Integration (PSI) (Have spec sheet)

· 2008 Corvette

Wiring Harness:

· Custom built for this application by PSI

Last edited by momosc; Apr 26, 2022 at 07:11 PM.

Apr 26, 2022 | 07:12 PM

sbwrench

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My shop does V8 miata swaps for a living and I run into this a lot and there are lots of things to look for. First, there are several gas pedals that will plug in but there are two different wiring configurations. Usually you will get no response right off the bat and your throttle position on the scanner will read backwards. 83% no pedal and like 8% full throttle, so backwards. Not likely but worth checking.
The biggest one I find is the 6 wire pedal harness bundled and or running parallel with the main harness. It is very susceptible to rf interference. Isolate it by itself.
Make sure the speed the pcm is seeing and the speed on your speedo are close. In other words dont use a convertor box to correct a gross difference between what the pcm is seeing and actual road speed.
Highly suggest getting a local reputable tuner to go over your tune and make sure all the various tables are talking to each other and clean up the maf curve and anything else. There is as much a difference between what you had and PSI and between PSI and custom tune. Not knocking PSI. They are the only company I use for harnesses. They are awesome and Gary, on their tech line, has bailed me out more than once! Its just a fact .
I have seen back feeding issues through both fan circuits and a/c circuits when the proper diodes weren't in place. I had a car with a bad fan and I was using a power probe to test the circuit. When I energized the power circuit I could heard the throttle plate clacking open and closed. Made the fans a stand alone set up and no more REP issues. Hope any of this helps.

Last edited by sbwrench; Apr 26, 2022 at 07:15 PM. Reason: grammar and sentence structure

Apr 27, 2022 | 06:25 PM

momosc

Thread Starter

Teching In

Joined: May 2018

Posts: 5

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Solid info borne of experience. Thank you so much. I have confirmed that the gas pedal reads properly. More throttle opening = higher %. Later I'll make sure that the road speed is a close match for what the pcm is seeing.

Although I'm a lay person and don't know exactly what I'm seeing, I did look at the wiring as it come from the pcm, and think I identified the wires going to the gas pedal. There were 7 wires in the loom, two of which were purple, and one of the purple wires split off by itself above the gas pedal. Anyway they do seem bundled together, at least for a while. The harness I'm using is this one:
https://www.psiconversion.com/produc.../HAR-1037.html
It's PSI's one for Gen 4 w/manual trans.

I do have a great tuner, and I am in line to bring the car there for diagnostics and custom tune, but of course it's their busiest time of year, so it may take a minute.

Last question (for now :-) As I drive and record live data, can you suggest some things to keep an eye on?

Thanks very much,
Mo

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