Miss/Hesitation/Engine Stalling - PLEASE HELP!

[oquinn24]

About six months ago I bought a 1990 300zx N/A. It had a few small problems but ran okay, and I got it as a project car. I since have painted it, swapped the interior, and have been working on the motor ever since.

Here's the issue I'm having:

Up to this point the car was running on 5 cylinders, and had a high idle. The O2 sensor went out on the side with the dead cylinder and I ended up driving with it unplugged for a while until finally it began bucking and would not respond to the pedal but about 15% of the time. Of course I replaced the O2 and got it running. We tried to track down the dead cylinder by:

- checking injector connections
- ensuring spark (which was good)
- setting tps (just to be sure)
- power balance tests
- ohmed injector (12.9)

Finally we plugged in another injector and it was not firing.
We ended up replacing the ECU and just like that it ran as smooth as can be. (With a high idle)

Then, as the days go on, a miss started.
(Still working on high idle)

- fixed all vacuum leaks we could locate
- checked MAF by changing it with a good one (no change)
- changed tps
- cleaned coolant temp sensor

Honestly I can't figure this out with all the research I've done, and I'm in need of things to start testing. If anyone has any idea, help would be much appreciated. If I'm missing anything, let me know. Thanks!

[z.Leinbach]

Try the replacing the ptu with the phase 2 one and or all the coils. Just so you're sure it's not them

[oquinn24]

Try the replacing the ptu with the phase 2 one and or all the coils. Just so you're sure it's not them

I did indeed see that the PTU is a phase 2, and also noted that while the connector was clean, the wires pinned into back had corrosion. I'm going to bypass the connector and wire them together Friday. I have ohmed the injectors and confirmed the one for that cylinder has spark by cranking it with the spark plug out and watching it.so you think the PTU is causing it? I'll also be checking the CAS Friday. Thank you!

[z.Leinbach]

Also have you replaced the fuel filter and made sure you were getting adequate fuel and air flow? And maybe tune if you can

[oquinn24]

Also have you replaced the fuel filter and made sure you were getting adequate fuel and air flow? And maybe tune if you can

I have not yet replaced the fuel filter, nor have I checked anything past the injectors to be sure they're firing. Another good recommendation, thanks!

[z.Leinbach]

Anytime bro. I gave my car a red bottle of heat and she started to run alot better too.

[oquinn24]

Anytime bro. I gave my car a red bottle of heat and she started to run alot better too.

A red bottle of heat?

[z.Leinbach]

It's a fuel dryer. Takes all the moisture out of the fuel and lines, red bottle is also a fuel injector cleaner

[maik3r]

Maybe try taking the coil pack from one of the cylinders that works and put it in the bad cylinder. I wouldn't think its your CAS as it would prevent your car from cranking all together if it were bad. If the CAS is out communication halts before cylinder one is fired therein not starting at all, let alone reaching cylinder 5 in the firing sequence. Z1 sells an upgrade to the PTU harness, it could definitely be the culprit. If you're breathing, drinking, sparking and banging but still have a hesitation issue id look at the fuel pump control unit. It may be failing or failed and not sending proper voltage to you fuel pump and causing it to hesitate or lag.

Best of luck

[z.Leinbach]

I noticed in the general section earlier that someone was having a hesitation problem and one had replaced the fuel pump, and relay, and he had seen dramatically improved performance