Z Pal

My 1985 non-turbo 300zx has an idle problem which has been going on for some time and seems to be getting worse. It idle's rough at idle, runs fine at higher RPM's. I changed the plugs and wires and put in NGK plugs, gapped to specs. The battery is one year old and has clean corrosion free connections. I replaced the starter 10 months ago. I cleaned the TPS and connectors with electronic cleaner and that seemed to make a big difference but gradually the idle problem came back. I took the hoses off the MAF and looked in there, it seemed to be pristine so I didn't do anything to it. I cleaned the ECU connectors with electronic cleaner. I took it to a garage where the mechanic hooked it up to a diagnostic analyzer and said the number 6 cylinder was misfiring. He checked the no. 6 fuel injector and said it was good. The compression was 170 lbs. across all cylinders. It ran good for a little while after he changed the cap and rotor but then the idle problem returned. Now, an idle problem like this happened years ago, I took it to the Nissan dealership and they changed the distributor. That cured the problem. I'm wondering what you guys think. Should I go ahead and change the distributor or should I look somewhere else?

86tcoupe

when ever I had a rough idle but runs ok full throttle
I start with a leak down or a cold ( completely ,has not been run in 12 hours like ) compressin test
the 170 psi sounds as it was done hot which distorts the readings.
it can save a lot of $$ needless electric repairs.
You might have a hyrdo lifter parcially clogged not able to bleed at low RPM
inexpensive to replace if you do it

Z Pal

Thanks for the tip, I'll have to buy a leak down tester and do the test.

86tcoupe

A good quality compression guage works as well or better leakdown testors are $$$ and take a bit of training to get down

Z Pal

I know, I just bought a leak down tester at Napa for $130. It has 2 gauges: one PSI readings 0-100, the other a percentage of pressure loss gauge. I have an air compressor so I should be good to go.

ch33z3x

Have you checked fuel pressure/ changed the fuel filter?

Z Pal

No, that's another gauge I have to buy. The fuel filter was changed last year. I'm waiting for good weather to do the leak down test.

86tcoupe

you can likely get your $130 back testing other cars for people . it is $$$ to have done in a shop

Z Pal

I decided to follow the FSM procedures for error codes. I was getting an error code of 23 (throttle valve switch(idle switch) circuit). I then followed the Electronic Control System Inspection (EF & EC-62) for the throttle valve switch. I checked the signal for the ECU. It was ok so I replaced the ECU as directed. Problem solved, the rough idle, stalling, hesitation, stumbling problem disappeared immediately when I started it up. I replaced the ECU with a re-manufactured unit from Cardone (72-3033). They sell 8 models so it's important to get the right one. The car originally came with California emissions because it was sold in California but I'm in Montana so I opted for the Excl. Calif. emissions computer. I bought it from Car Part Kings on Amazon for $272.28 + $7.50 shipping. The core exchange is $70.

https://www.cardone.com/find-parts/v...00ZX&year=1985