500rpm Fluctuating Idle And dies

Slicktopz32NA · Post by **Slicktopz32NA** » Fri Aug 21, 2020 5:09 pm

Hi, recently engine swapped my NA 1991 300zx, after getting the car running and driving I went out for a drive and had a great time, on my second day on my way home it started to bog and sound like a misfire but didn’t get any check engine lights or warning, after parking and checking all connections I assumed it was PTU related as there was no noticeable difference in cylinder 6 when I unplugged the boot connector, but all others had a difference. After switching to the new style PTU and sub harness I had from the previous engine the misfire went away but then I noticed the fluctuating idle and overall rough running nature of the engine. All injectors pass the sound test, no codes from ECU, cleaned MAF, 12v at all ignition connectors, fuel pressure from filter at idle is 37-38 fluctuating, plenum vacuum pressure is 19-20 at warm idle fluctuating, smells like it’s running rich but wasn’t able to get any response from O2 sensor test at the ECU. But both O2 sensors ohm and Volt test appropriately. This engine has 3000-5000 miles on it, long story short the guy I bought the car from had it from a guy that bought a Z brand new in the 90s and got in a wreck and kept the engine and sat on it for 20+ years. All new hoses and full emissions delete as well as new fuel filter and air filters. PCVs are still in and sound good and all vacuum ports are plugged. Both egr ports are plugged and the car drove like a dream and had tons of power all the way through up until the last night on the ride home. AC and cruise control are deleted as well as AIV, carbon canister, PRVR, and relocated the small fuse box by battery to inside the passenger front wheel well. I’ll post videos so you guys can hear the idle fluctuation but I’m gonna test ignition with extra plug and extra boot and see if the sparks are solid. Also reset TPS TO 8.75 and haven’t touched IACV as it idles at 850-750 for small instances before fluctuating

Z Nation · Post by **Z Nation** » Fri Sep 11, 2020 1:47 pm

Damn. I have very similar problems with my 1990 300ZX 2+2 NA automatic just yesterday. New plugs, boots, fuel filter, coils, very well maintained and stock except for the BB Exhaust. Tapped the gas to get around a truck on the road, no big deal but almost immediately bogging down, running rough. Pulled over right away. Almost like it's starving for gas. Car almost stalls backing up and going forward at around 1,000 rpm. There were no error codes pop up, no unusual mechanical sounds when it happened. Will be letting my mechanic take a look at it. Will post what we find.

Post by **sns00** » Sun Sep 13, 2020 9:09 pm

I'll strongly suggest to check the CAS circuit to include the sensor itself & the FPCU.
Use the FSM, almost all the knowledge is in there. I say almost because components can fail in ways that are not obvious and dont trip a code.

For the CAS, remove the CAS and back probe the green wire terminals, one at the time with the negative lead to a good ground. Turn the ignition to on, do not start the engine; as you spin the rotor shaft slowly, you should see between 5-0vdc.
If you find anything other than those readings, check the CAS circuit power, ground and signal output; continuity should be 1 ohm or less at the circuit.

For the FPCM, back probe the 18 AWG black/pink wire; with ignition on/engine off 0vdc; during warm up 1-2vdc.

All this assuming you have a good battery holding charge, etc, etc, etc. Also wiggle test the ECU connector (F1) and wiring harness at the engine bay with the ignition on/engine off, have an assistant sit inside and keep an eye for changes; then with the engine on, BE CAREFUL, do the same.

Report back.

Slicktopz32NA wrote: ↑
Fri Aug 21, 2020 5:09 pm
Hi, recently engine swapped my NA 1991 300zx, after getting the car running and driving I went out for a drive and had a great time, on my second day on my way home it started to bog and sound like a misfire but didn’t get any check engine lights or warning, after parking and checking all connections I assumed it was PTU related as there was no noticeable difference in cylinder 6 when I unplugged the boot connector, but all others had a difference. After switching to the new style PTU and sub harness I had from the previous engine the misfire went away but then I noticed the fluctuating idle and overall rough running nature of the engine. All injectors pass the sound test, no codes from ECU, cleaned MAF, 12v at all ignition connectors, fuel pressure from filter at idle is 37-38 fluctuating, plenum vacuum pressure is 19-20 at warm idle fluctuating, smells like it’s running rich but wasn’t able to get any response from O2 sensor test at the ECU. But both O2 sensors ohm and Volt test appropriately. This engine has 3000-5000 miles on it, long story short the guy I bought the car from had it from a guy that bought a Z brand new in the 90s and got in a wreck and kept the engine and sat on it for 20+ years. All new hoses and full emissions delete as well as new fuel filter and air filters. PCVs are still in and sound good and all vacuum ports are plugged. Both egr ports are plugged and the car drove like a dream and had tons of power all the way through up until the last night on the ride home. AC and cruise control are deleted as well as AIV, carbon canister, PRVR, and relocated the small fuse box by battery to inside the passenger front wheel well. I’ll post videos so you guys can hear the idle fluctuation but I’m gonna test ignition with extra plug and extra boot and see if the sparks are solid. Also reset TPS TO 8.75 and haven’t touched IACV as it idles at 850-750 for small instances before fluctuating

Post by **sns00** » Sun Sep 13, 2020 9:18 pm

Your mechanic knows about these cars? If not, I'll suggest finding someone that is knowledgeable, preferably that owns or has owned one.
I take mine for inspection to the dealer, every time is always the same old timer, never a "hot shot" tech kid.

Z Nation wrote: ↑
Fri Sep 11, 2020 1:47 pm
Damn. I have very similar problems with my 1990 300ZX 2+2 NA automatic just yesterday. New plugs, boots, fuel filter, coils, very well maintained and stock except for the BB Exhaust. Tapped the gas to get around a truck on the road, no big deal but almost immediately bogging down, running rough. Pulled over right away. Almost like it's starving for gas. Car almost stalls backing up and going forward at around 1,000 rpm. There were no error codes pop up, no unusual mechanical sounds when it happened. Will be letting my mechanic take a look at it. Will post what we find.

Z Nation · Post by **Z Nation** » Mon Sep 14, 2020 6:34 am

Thanks everyone for the good advice and tips. My mechanic is "old school" and I will be bringing my car to his shop. Of course I live in a rural area where nearest city is 1 hour away. People find him through referrals. So busy, never advertises. Audi, BMW, Subaru,Triumph are all brought to him. Will post updates on what we find.

500rpm Fluctuating Idle And dies

500rpm Fluctuating Idle And dies

Re: 500rpm Fluctuating Idle And dies

Re: 500rpm Fluctuating Idle And dies

Re: 500rpm Fluctuating Idle And dies

Re: 500rpm Fluctuating Idle And dies

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