# Car runs fine when cold, then dies when warm



## 85-ZEE (Mar 6, 2009)

I am the original owner of an 85 Turbo, CA Emissions, 5-speed, digital. I love this car, but for the last month, it’s been inoperable. The car starts and runs fine when cold, until it warms up, about 10 minutes, then suddenly dies. When I try to start it up again, it kicks over, then either immediately dies again, or sometimes sputters. I can slowly rev it up, but you can hear that something’s wrong. The engine stumbles, backfires, vacuum goes way down, etc. I can keep it going at about 1500 rpms, however, it dies quickly back at idle.

I have checked/replaced the following:

All vaccum lines. I have all new silicon tubing. No leaks. Perfect seals throughout the intake assembly.

ECU Codes are all fine, but even so, I have checked/replaced many components: Crank Angle Sensor, Air Flow Meter, fuel regulator & temp sensor, CHTS, Ignition Coil and power transistor & Throttle Valve switch. I ran the AAC test according to the FSM and either the test is wrong or I’m doing something wrong, since it failed. I checked voltage going to the AAC, and that was fine, and I checked continuity on the valve. I double-checked the continuity from the AAC to the ECU harness. Even though everything checked out, I still replaced the entire AAC assembly, since I know that this is a critical component. No change, so I am confident the AAC is working. When I disconnect the AAC connector when the car is running, it stumbles and dies, so I know it’s working. 

I double-checked the EFI relay, even leaving it connected to a power source for 15 minutes to see if the relay faltered, but it’s good. I pulled the Air Regulator and it closed like it should when hooked up to 12 volts. I thought I had the problem solved when the Oxygen sensor failed the diagnostic, and I know this is a critical component. I replaced it with a new Bosch (fun getting it off!), test runs fine, but the problem remained the same.

I hooked up a fuel pressure gauge and it always has a healthy 30 to 40 psi. Pressure goes up when vacuum hose is disconnected from the FPR, as it should. Also, the timing is spot-on. I have rewired all suspect electrical wires and replaced connectors that have cracked. I know it shouldn’t make a difference, but I even rewired the Fuel Injectors back to the way it was before the recall.

So, by now I figured that maybe I have a faulty ECU. Steve Berman (300zxpartsforyou) had some tested ECUs for my car for year. I replaced it and same problem. I looked carefully at a ECU chart that was posted, and found that the stock ECU was listed as an ’84, even though the manufacture date on my car is 1/85. So I asked Steve if he had a working ’84 ECU, and he did. I tried that, and it still did the exact same thing. Very disappointing, but at least I have, what appears to be, 3 working ECUs.

I am open to anybody’s suggestions, and will be forever grateful if someone comes up with something that leads to a solution. I figure that since the car runs so well when it’s cold, I don’t need to check for compression, but perhaps when things heat up, something may be expanding. Do you think I should run a compression check anyway? I would have to run the engine until it’s hot, then quickly remove all plugs and test before it cools down.

Thank-you for whatever assistance you can provide. I hope no one takes offense if I post to a couple of the different forums. Whichever forum helps me figure this out will get me as an ongoing contributor (I hope I will be worthy!)


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## nkdmolerat16 (Mar 10, 2009)

i have the same issue that is still unresolved. so far i have replaced maf, tps, ignition coil, power transistor distrubitor cap and rotor. but there is also the possibility of ects or o2 sensor. so i would check all that to start with. and you might still want to check compression just beause but like i said mine is doing the same thing and my compression is just fine. hope that helps


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## 85-ZEE (Mar 6, 2009)

Good luck to you too! I already replaced and tested the Exhaust gas sensor and (I think you meant) the ECCS, twice! Do you also get normal codes?


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## nkdmolerat16 (Mar 10, 2009)

what codes are you getting?


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## 85-ZEE (Mar 6, 2009)

In an 85, normal codes are 14,23,24 & 31 until you do more tests.


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## nkdmolerat16 (Mar 10, 2009)

interesting. well i would just test all the sensors. i have a feeling im just a moron and didnt check the plugs (duh) so im replacing those soon and getting fuel injectors put back together. also see if your car has gone through the fuel injector service campaign just call you local dealership and have your vin. because that also might be the problem. hope that helps


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## 85-ZEE (Mar 6, 2009)

I think I had that done about 20 years ago! Good luck!


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## reddzx (Jun 22, 2006)

I've heard of a problem like this and it turned out to be the coil was cracked and worked until it got warm. Check your coil.


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## 85-ZEE (Mar 6, 2009)

Already tried a new coil & pwer transistor.

I just downloaded the 89 FSM and it has a bunch more tests that are relevent to my '85. I'm also going to run a compression test this weekend.


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## Knightstorm (Mar 21, 2009)

I had the same problem in my 87 Turbo. Replaced the distributer, CHTS and sub-harness, ECU, cap, rotor, plug wires, plugs, power transistor, MAF, maf-connector, IAA, Coil, and all vaccum lines. I even tried cleaning all grounds, all contacts, and running extra ground wire for better grounding. Then one day I tried wiggling my ECU connectors and slightly-but-firmly pushed the still plugged-in connectors outward while the car was starting to run rough. Suddenly the car ran fine. Now I have a couple big pencil-erasers wedged into the space between the ecu connectors and the lower-wall to push against them and make better contact with the ecu pins. It's a temporary fix till I can get new connectors. 

--It now runs fine all the time.


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## dave_g (Mar 21, 2009)

it seems to me it could be something simple as the automatic choke sticking
it would explain why the car dies when warmed up.


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## 85-ZEE (Mar 6, 2009)

Dave, that would be the Air Regulator, and I've already checked that.

Knightstorm, good suggestion - I'll do some wigglin' at the ECU.


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## reddzx (Jun 22, 2006)

I don't think the EFI has a choke


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## rogoman (Dec 16, 2004)

Here are some possibilities:
- Sticking EGR valve.
- Hot pipe leak.
- Leaking blowoff valve. Not sure if your motor has one.
- Faulty MAF.


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## John'sAltima (Mar 23, 2009)

You can check the EGR by connecting a vaccum pump to it, pump it up and you should notice a drop in RPM and seeming as though it may die. Release the vaccum you pulled and it should return to normal. If so, EGR is fine.
Also, I would'nt wast the time or money on O2 sensors as Ive never seen one actually kill a motor before, cause it to run very rich or lean...yes but not kill it. In which case shooting some carb cleaner into the intake while running will give you a fair idea. Just a little shot, not much. An increase in RPM indicates a lean mixture and a decrease indicates a rich mixture.
Most often than not on problems arising when it gets hot, I usually find electrical problems. A weak connection may have circuit when it's cold but as it gets hot, expansion occurs and breaks circuit.
Good luck!


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## 85-ZEE (Mar 6, 2009)

Thanks for the tips, but I’m pretty sure that the injectors stop working when the engine gets warm. I can hear them all clicking away when cold, but can’t hear them after warm up. Still getting a spark. Compression was not great, but I don’t think this should cause the engine to run great when cold and the quit suddenly when it warms up. If I leave it for 15 minutes or so, it will start up again, but only run for a couple minutes, since it warms up that much quicker.

I think it’s electrical also, but I am at my wit’s end trying to figure this out. I am now checking the same systems 5 and 6 times now. I have extensively checked out the EGR, MAF and all electrical to the ECU. Same with the CHTS, TPS, AAC, Oxygen sensor & EFI relay. I have swapped CAS, coil & power transistor. I have checked ground. I have I have wiggled so many connectors and cables, that I think I’m starting to cause more harm than good. As I said, above, I have 3 ECUs, all for my model, but they all do the same thing. Ignition wires are new and tested, and well seated. Distributor rotor & cap are new. I’ve double checked timing and whether the timing belt could have slipped.

Can someone give me some good ideas or techniques to use to check out electrical outside of the FSM? Rogoman, what is a “hot pipe leak”? Any more suggestions would be greatly appreciated.


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## Knightstorm (Mar 21, 2009)

A "hot pipe" leak, as I've always heard variations of it referenced, is a leak anywhere in your exhaust system (correct me if I'm wrong). If such a leak exists, it could be from your exhaust manifold to anywhere on back in your exhaust system. However (if I'm to understand the reference correctly) that would not explain your problems, nor likely make your car run that rough or die-out.

There is a posibility that the contact points on your connectors are oxidized or corroded. If you feel secure in your ability to work on it yourself. You might examine the connectors closely in good light (and consult the manual) to ensure you know the proper way to disconnect them without breaking anything. Then, with the car and ignition turned off and battery disconnected, gently and carefully disconnect suspect connectors briefly and look to see if the metal is shiny and looks clean, or if they are dirty, corroded or greenish tinged. This seems to be a common problem in our old Z31s. What many people do to help this is to buy some electronic parts cleaner and spray the points off, then blow them out and let them dry thoroughly before reconnecting. In severe cases, some have gone as far as getting a tiny, ultra thin, very fine sanding file and lightly filing any connectors till they shine. However, in doing this you must be super-careful, because you're taking the risk of breaking the connector or bending the contact to the point of creating a problem! The connectors in these old cars can be extremely fragile. So you are taking a big risk if you mess with them!

You could also use a multimeter and check grounds, per the Factory Service Manuals instructions. Then, if you see exposed wire that may be shorting, you can try applying a good liquid-electrical-tape to the exposed area. You might also check and clean any ground wires/points where they go to the body or engine.

If the contact points in some connectors are your problem, you might be able to find connectors in better condition at a local auto-graveyard. But again, be very careful that you know what you're doing before you start snipping wires. 

If in any doubt about attempting anything mentioned above, hire a professional!!!!!!!!! Don't try to do it yourself.


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## 85-ZEE (Mar 6, 2009)

I am looking at electrical problems extensively, but not coming up with anything.

I have already replaced or repaired many suspect or damaged connectors & wires. I am wiggling wires & connectors everywhere along the EFI harness, all the way to the ECU. I wiggle them when the engine is cold and running fine to see if I can make it quit, and then I wiggle them after the engine is sputtering and ready to die to see if I can hear an improvement. All fusible links are in great shape. Engine grounds (at least the ones that I know about) look fine. I have I even rewired the injectors back to factory (before the injector campaign), even though that shouldn’t make a difference. 

So what would be some more good tests to try related to the injectors quitting when the engine reaches operating temperature?


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## genuts (Apr 13, 2005)

*What was your outcome*

:ballsid you fix your Z?


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## 85-ZEE (Mar 6, 2009)

No, not the underlining problem. After changing the EFI harness, which didn't make a difference, I disconnected the CHTS & installed a 5K variable resistor across the input leads. It sits in the ashtray and I can now dial-in a working resistance so the engine behaves itself. The car is now drivable. I might want to add a perminent resistor in series with the CHTS to make it automatic, but I fear I will still need the manual override in certain conditions. Thanks for asking!


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## Titus01 (Jul 23, 2009)

That is intense.
Going so far as to having to do such a thing.
I guess when Z's want to die they really make it happen.
Good to know it's drivable, though.


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## 85-ZEE (Mar 6, 2009)

Sorry I didn't post earlier. A Nissan service manager I've know for 30 years remembered that they used to install a 500 ohm resister in-line with the CHTS, to cure similar problems. This tells the ECU that the engine is running a little cooler. I did this and it solved the problem. I not sure if it ever goes into a closed loop, but I do know that it runs fine now. It starts well, the idle comes down to range when warm, and there's lots of power. Mileage is the same as before. The car, in fact, runs great!


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## Backinblak (Jul 16, 2015)

Location of resistor you installed for the CHTS? Thanks ,,,,


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