1 (edited by Goofracer72 2021-05-07 01:41 PM)

Topic: Z31 300ZX Running issue..

Ok...so not Lemons related.  But I know theres gotta be at least one Z31 300ZX guru in here..or at least a nissan guru(Im looking at you Sasha). Ive raced against a few of them. So I have a customers 1987 300ZX automatic here at work thats been sitting.  Has an issue where once the car gets hot, it randomly starts idling rough, wont rev, and when you try to rev, it backfires through the intake and would usually start running worse and worse till it stalled.  Has codes 12 Air Flow Meter, and 41 Fuel Temp Sensor(Broken sensor..which as i understand doesnt really do squat).  Started diagnosing the MAF and voltage readings out of the sensor seemed slightly questionable.  So replaced the MAF, cleaned and adjusted any questionable pins in the connector, cleared codes and started from started over.  Car ran for almost and hour in the bay...and would rev freely without issue(cant roadtest the car at the moment).  Then randomly one of the times i went to rev it....got the back fire and wouldnt rev.  And of course would start running like crap.  And the code comes back.  Voltage outputs and inputs as well as grounds at the MAF are normal.  Everything between the MAF and the ECU ohms out correctly.  Checked all the basics.  Has good fuel pressure...in fact the pressure goes up when it starts running shitty...telling me the injectors are cutting off fuel.  Has good spark...courtesy of a new coil.  Started questing the throttle position sensor.  Then discovered its a switch and a sensor all in one.  Switch seems to be working properly..but dont get a 5 volt reference to the sensor portion.  But i find conflicting information on all repair programs at work and online as to whether the tps sensor is even controlled by the ECU or the trans controller.  Nor can i find where in the ECU plug the wire would go to.  One page will the say the switch controls the trans and the sensor controls the fuel....another will say its the opposite, another will say it uses both for fuel and the trans.  Found a diagram for a car one year newer that told me what pin on the ECU went to the TPS....but that pin doesnt have a wire going to it.   Im going out of my mind with this thing.  Someone please point me in the right direction.  Or am I completely stupid and is that fuel temp sensor is causing all of this?

Re: Z31 300ZX Running issue..

(I'm _NOT_ a mechanic, so this may be completely off topic - apologies if so; however:  )

tl;dr: vacuum leak?

If you had said, "Ford, starts ok when cold", and the rest of the issues you mentioned, and thought 'maybe the ECU is bad', I'd say this sounds very similar to a problem I heard about - from a mechanic.

He used those "mechanic's terms of encouragement" for hours (he also cleaned the grounds. He found one bad sensor - I _think_ it was the MAF that didn't have the correct reference voltage, maybe the TPS).  After that, he called an auto computer specialist (he never calls for help).

The specialist verified the bad sensor; and then asked if he had 'smoked' the engine.

Turns out: Vacuum leak.

Cracked plastic something, and two rotten hoses. Replaced sensor and those other things. "Running better than in the last 50k miles". Not the computer. Diagnosed by the computer specialist, who didn't replace the computer.

Just a thought.

Re: Z31 300ZX Running issue..

As I recall, the TPS on the z31 triggers the ecu/eccs in a few ways:
1. Senses rapid throttle application to quickly enrich the a/f mixture (like an accelerator pump on a carb).
2. Kills fuel over 2k RPM if the throttle is closed (coasting).
3. Goes open-loop mode at 3/4 throttle.
4. Might possibly affect ignition timing at idle when the throttle is closed.

None of these really explain your issues.

I would suspect the CAS in the distributor.  Bad CAS (or possibly a corroded CAS connector) usually show up at first with a hot engine, especially if run for a while and let heat soak.  You might notice the tachometer going a little nutty when the misfire starts because the eccs is losing track of crank angles and RPMs.

Other than that I would check the main ignition solenoid that powers the ECCS and most of the engine electronics.  Could be that it got corroded when the car sat for a while and is barely working.

My 2 cents.

Re: Z31 300ZX Running issue..

I smoked the intake system and found no vacuum leaks.  I was kinda thinking the CAS may be suspect....but again..it has spark.  Strong spark at that.  From what I've read, a weak or bad CAS should also cause weak or no spark shouldn't it?  I'll check the connector for corrosion when I get into work in the morning.  Can't go by the tach...most of the gauges don't work on this turd.

5 (edited by zRacer70 2021-05-10 05:41 PM)

Re: Z31 300ZX Running issue..

I don't think a malfunctioning CAS would affect the strength of the spark because that is generated by the coil triggered by the ECCS.  But I believe it could cause spark and injector timing to jump all over the place.  This is a really common failure on the 82-83 s130 280zxt CAS, which is almost identical to the z31 300zx.  The fix for the s130 is to use a z31 CAS which is more reliable, but not indestructible.

You should be able to hook up any old tach to the negative terminal on the coil.  Or maybe a timing light to check timing before and after the misfire starts.  And replace the plug wires if they are old crappy 7mm house brand crap.  Probably not the main issue, but for some reason they wear out quickly on these engines.  Maybe it's the hot spark that does it.

Also check the 2 ECCS grounds on the engine.  I can't remember where they are (usually on the intake), but they are the only grounds for the ECCS.

Of course the eccs could also be failing, but that's a lot harder to troubleshoot.