1990 300zx NA stalls when idling

[Koma Wolf]

So here's my problem; Car starts great, runs well for about 15 seconds, idle lowers then it starts stumbling and stalls. If I give it gas it will stay running. I can drive it and it runs great while accelerating, it has problems at idle and steady speeds.

I recently had the EGR valve and Timing Belt service done. Timing was dead on at 15, I was still having trouble with the EGR, so I did a delete. I deleted the EGR and PCV system, as well as the PRVR solenoid. I replaced the fuel rails while I was in there, I also rewired all the fuel injector harness'. I have properly re-routed the vacuum lines (Triple checked this). Fuel Injectors are new, Fuel dampener, Pressure Regulator, and filter are all new too. Cleaned the MAF, IACV, AAC, and replaced the FICD (connector was broken). IACV, AAC, and FICD all pass ohm test, have new connectors, and have been cleaned. Throttle bodies have been cleaned and open/shut properly.

Now my timing is at 25, could this be the cause? I'm going to run a vacuum test later to test for leaks. I will also post a video showing the start up, and read outs from the consult.

Thanks in advance

[centralcoaster33]

With timing off so far, I'd think it was the IACV or a vacuum leak. A vacuum pump would be useful in your diagnoses (good idea). Additionally, TPS, MAF and O2 sensors could contribute or cause similar issues. Have you checked the voltage of the TPS and verified your MAF and O2 sensors aren't throwing codes? You've got a lot of variables to keep track of. You've pretty much touched everything that can be a culprit. Maybe work in a methodical approach. Probably re-doing and re-checking the stuff you've already messed with.

Maybe your car is frustrated because it's polluting your environment too much now. PCV was like the first emissions part and is still one of the most useful for reducing pollution and prolonging your oil life. EGR and PCV delete certainly aren't solutions to a poor idle situation. Okay, I digress.

I think you may have made too many changes at once to know what is causing your issues. After you verify no vacuum issues with a pump, then I suggest putting back the PVRV, the EGR and the PCV and make sure your idle and timing are correct. Then you can delete them one at a time. See which one causes poor idle and timing issues. The potential for vacuum leak by doing the EGR delete incorrectly and or the issue with fuel pressure from the PVRV delete seem like things to investigate. Maybe redo those first to help with your diagnoses.

Welcome to NICO Club! Some of our 300zx owners know a ton of stuff, so hopefully they'll see this thread and lead you on the right path.

[Koma Wolf]

Thanks for the reply! Here's an update; car is running now, idle and timing are set. It runs really rough though..

I forgot to mention that I did replace the o2 sensors, and spark plugs (NGK)

ECU reads 55, no errors.

I hooked the PRVR back up, only change was it took longer to start up lol.

EGR and PCV are gone, I removed these without an engine pull, so some of the metal piping is cut up.

MAF is good, tried checking the TPS, I don't think I'm doing it correctly, but I will be getting conzult software soon. When checking the TPS, I noticed the plug from the harness only had 2 wires, yet the TPS has 3 terminals, should there be 3 wires from the harness? I'm going to look for a wiring diagram..

Checked all injectors JIC, all are working.

I have new ignition coils on the way, they all ohmed out at 1.0-1.2 should be 0.7 hoping this will help the misfires I'm getting.

[centralcoaster33]

Okay, it was good to check what you did. Look closer at your TPS. Do you have a pigtail and a plug? If so, one is for WOT/ idle and that is the two wire, the other is the potentiometer for the TPS and has 3 wires. Here is a link to a wiring diagram:
http://www.nicoclub.com/service-manual? ... 90/Circuit
and another to the chapters of the FSM:
http://www.nicoclub.com/FSM/300zx/1990/

I think you should check all your vacuum lines. The EGR and PCV deletes both affect vacuum. I had the worst vacuum leak on my 240sx and it was all from the hose that goes from the PCV Valve, to the lower intake piping. There was a sort of mini manifold in there that routed the crank gasses back into the cylinders. If that's not plugged up properly, it's a huge leak. As far as the EGR stuff, those are typically smaller hoses and would have smaller effect on the vacuum. If you messed up the block of plates on each end, those would be much bigger vacuum and exhaust leaks. What about the EGR/ manifold temp sensor. Is that deleted too and plugged maybe?

[Koma Wolf]

Awesome, thanks for the wiring diagrams.

Looks like my TPS plug is good. Just weird to have 3 pins going into a plug with only 2 terminals.. Still need to test the TPS itself though.

I replaced all vacuum lines minus the AIV ones that go behind firewall. I clamped all new (larger) vacuum lines, even threw some clamps on lines that previously did not have them.. I ran a vacuum test got 16-17psi on both sides of the plenum, stays steady at idle, when throttle is opened it dips a few psi then ramps up. Can't hear any hissing, checked block off plates, those are good, checked bung in exhaust mani, that's good too. I think vacuum system is fine. I used all new OEM gaskets on intake manifold, throttle bodies and aac; tightened bolts to torque spec in FSM. For the block off plates and bung I used permatex aviation form a gasket... I replaced the O-rings on the balance tube as well.

EGR temp sensor is gone, I have a resistor on the way to stop the CEL from popping up (comes on every other start up)

[red coupe89]

Awesome, thanks for the wiring diagrams.

Looks like my TPS plug is good. Just weird to have 3 pins going into a plug with only 2 terminals.. Still need to test the TPS itself though.

I replaced all vacuum lines minus the AIV ones that go behind firewall. I clamped all new (larger) vacuum lines, even threw some clamps on lines that previously did not have them.. I ran a vacuum test got 16-17psi on both sides of the plenum, stays steady at idle, when throttle is opened it dips a few psi then ramps up. Can't hear any hissing, checked block off plates, those are good, checked bung in exhaust mani, that's good too. I think vacuum system is fine. I used all new OEM gaskets on intake manifold, throttle bodies and aac; tightened bolts to torque spec in FSM. For the block off plates and bung I used permatex aviation form a gasket... I replaced the O-rings on the balance tube as well.

EGR temp sensor is gone, I have a resistor on the way to stop the CEL from popping up (comes on every other start up)

im in this same boat. ive changed all the same pieces as you, injectors, harness, hoses, egr delete, pcv delete and my car wont idle. my a/f is 21.0 dead lean on my wideband, can not figure out where to look.

[Koma Wolf]

UPDATE: I installed new Ignition Coils, and adjusted the TPS to .45V and she's running great.

@red coupe89- I would ohm test the Ignition Coils, should be around .7 ohms, mine were all at 1.1 or 1.2, check your TPS voltage as well, should be .45V (timmz32tt has a video on the TPS adjustment on his youtube channel). Also, how did you route your vacuum lines after the deletes? You will need to run a new vacuum line from the carbon canister to the upper fuel rail (I found this on a vacuum diagram, was not in any of the EGR delete guides). Are you working with a TT or a NA?

[red coupe89]

@Koma wolf: i used the guide from tt.net.

http://www.twinturbo.net/nissan/300zx/f ... ypass.html

its a removal and bypass. THANKS! ill have to ohm them all out tonight and see what i turn up with. ill report back!

[Koma Wolf]

Ah I see, you deleted the Carbon Canister, well that takes care of that lol. I didn't see a step to cap off the upper fuel rail that the carbon canister used to attach to. You may have already done that, but that's one thing to check.. Also, what do the vacuum lines on the passenger side look like? Did you delete the AIV too? If so, I saw one write up that said to re-route a line, when in fact it should be capped off (Line going from pass. side upper fuel rail).

Hopefully it's just your ignition coils, such an easy fix! FYI I got some Hitachi ones from RockAuto for $50/ea, everywhere else is about $100/ea.

[red coupe89]

Welp.....still no luck. All the coils checked out I got 4 at .7 ohms and 2 at .8. Couldn't dial in the tps because it won't stay running long enough to get warm, then once warm won't stay running. Funny thing is all I changed this off season was the cams and injectors, when I drove it in car was pretty much 100%. Don't know where else to look. Sprayed all points of upper to lower intake, all attaching vacuum hoses, tb, iacv, etc. it seems like I have a dead miss at idle?!?!?!?!?!?

[centralcoaster33]

Is it possible that cam lobes aren't in proper timing position? Or did your car run fine after the cams and injectors for a while, then something else happened?

[Koma Wolf]

Bummer.. Since you've been removing stuff, perhaps a vacuum line got switched, or one that should be capped off is hooked up or vice versa.. I know I ran in to that lol. I'd revisit the vacuum routing/capping.

Maybe adjusting the idle to be higher might let it run, this worked for me.



I know you just did the injectors, but have you tested them to see if all are good and pumping fuel? I'd check the ohms on those, should be 10-14 ohms also, do the screwdriver test.

Since it was 100% before the changes, it's safe to assume the problem lies somewhere within the changes made. Could be something as simple as injector or ignition coil plugs getting mixed up, double check that.

Here's a wiring diagram for the injectors and ignition coils, check the colors to see they're going to the correct cylinders



For the symptoms you have, FSM says to check the following;
-Injectors
-Spark Plugs
-Fuel Pressure
-Air Regulator
-AAC
-Do a power balance test

[red coupe89]

Is it possible that cam lobes aren't in proper timing position? Or did your car run fine after the cams and injectors for a while, then something else happened?

when i first fired it up after the cams/injectors it ran decent and got it timed to 15 btdc all my marks are lined up. also when i hit the accelerator my a/f goes to and almost perfect 14.8-14.9. once it returns to idle is when the really lean a/f 21.0 and miss fire appear.

@koma wolf: my idle screw is about to fall out i will have to back track some of the wiring for maybe a pinched or split wire, but as i eluded to when i hit the throttle im almost perfect on both ignition and a/f.
im going to pull the plugs again and see whats up, coils checked out 100%. i did the ol screwdriver to the top of the cap on the injectors and all seem to be firing correctly. after that i guess its

-Injectors
-Spark Plugs
-Fuel Pressure
-Air Regulator
-AAC
-Do a power balance test

[centralcoaster33]

Have you thought to start over with the idle adjustment on the IACV? I mean, if it's nearly unscrewed, maybe it's past the point where it should be. I only say this because it's happened to me. So, you screw it all the way back in and start over with slowly unscrewing it until it holds idle near the proper RPMs... while checking timing, possibly twisting the distributor as needed. Also, I know I have a 240 and not a 300 zx, but there is like a tiny screw on the TB that adjusts the idle location of the butterfly and that would have an effect on idle air as well. They're not supposed to be touched after being set by factory, but sometimes, someone messes with it and it might need a slight adjustment. Have you looked there?

[red coupe89]

Have you thought to start over with the idle adjustment on the IACV? I mean, if it's nearly unscrewed, maybe it's past the point where it should be. I only say this because it's happened to me. So, you screw it all the way back in and start over with slowly unscrewing it until it holds idle near the proper RPMs... while checking timing, possibly twisting the distributor as needed. Also, I know I have a 240 and not a 300 zx, but there is like a tiny screw on the TB that adjusts the idle location of the butterfly and that would have an effect on idle air as well. They're not supposed to be touched after being set by factory, but sometimes, someone messes with it and it might need a slight adjustment. Have you looked there?

Thanks, I have check the timing and it seems to still be dead set. I'm on my way to the car now so I'll try re setting the iacv. If that doesn't work I'll be pulling the manifold and "starting over". The tb's have never been touched so I can't see that being an issue but at this point I have no f'ing clue!

[red coupe89]

welp, ive narrowed it down to either the iacv or the coolant temp sensor. had it running well until the car started to get warm. in which case the car started to run super lean and began searching for idle. going to try to change the iacv with it in the car, dont know how well its going to go.