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Discussion Starter #1
I've done alot of searching on this forum for this problem and it seems like its happened to a lot of people over the past couple of years and I have found no one that has come back to the forum and told us if they fixed that exact problem yet or not so I'm posting to find out if anyone has found out what the problem is.

The problem is only happening in the morning or after the car has totaly cooled down.. She starts hard (but mines not too hard to start) and then kind of doesnt like to accelerate for about 2-3 mins then as soon as the temp gauge gets to the bottom of the first bar its fine all day long runs great Again only having this problem in the mornings or after sitting for a while and totally cooling down?? Anyone fix this same problem yet?
Please help I've tried alot of things
 

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bdijanni - I am posting a clip from another post that I made earlier today. As mentioned below I have not tried this. I too have done a lot of research and come to no conclusive solution ot this problem which I also suffer from.

"I have been told before that 84-85 NA's (like mine) sometimes suffer from a rough-idle when the engine is cold.

The rough idling/acceleration is obvious in my 85 n/a until the engine warms up. I believe the fuel mixture is lean. I have replaced the CHTS and CHTS subharness and verified (by running the ECU codes) that it works to no avail.

Slicktop from z31.com suggests replacing the ECU with an 87 or later ECU. Apparently they have upgraded fuel curves and "CHTS bypass" if the CHTS wire in the main wiring harness has corodded. I have not gotten to test this theory yet.

He also suggested fooling the CHTS by using the FPS's signal to trick the current ECU into thinking that the engine is colder then is currently being reported. If you go this route a 3-way switch is recommended to switch the signal back to normal once operating temperature is reached so as to not run a little rich. He strongly recommends the replacement ECU method however."

I have not had the money to try the ECU method and although I have the wire I need I have not yet tried the FPS method.

Hopefully that is some helpful input!



Post Edited (Sep 12, 2:30pm)
 

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yep, what Stu said. It's either the CHTS, CHTS subharness, or you've got some chronic ECU problems that updating your ECU would help.
 

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Discussion Starter #4
Yes that might help where But I replace my fuel pressure sensor.
And still the same problem. I read in posts from over a year ago that people had tried replacing the CHTS and they still have the same problem.
So I am totally confused Let me tell you something more confusing about the problem alot of Z people are faced with I have an 88 N/A so the ecu method probly wont work for me any way but I appresate all help. We have to figure out what is causing these cars to act like this in the morning only..
 

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Hopefully it will be solved one day! I've heard a lot of other suggested fixes in the course of researching the cause of this problem. There is an article here on zcar.com that suggests it's an ignition issue but I think the individual was suffering from a different problem as even he admited the problem was not totally gone.

http://www.zcar.com/forums/read.php?f=2&i=74402&t=74402
 

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Ive got this same problem with my Z; a guy I take my Z too build engines himself so he knows a bit and suggested to me that it might be the coolant sensor. I haven't got this fixed in mine yet; I am planning to later this week so I will update ya'll on wether or not that fixed the problem.
 

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Discussion Starter #7
The coolant sensor? I don't think thats the problem this is going to be some kind of cold start sensor or something like that, But thanks for the input
 

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Im no mechanic but the coolant sensor does have something to do with it. I googled the following info which may help explain it's operations better.

The engine/powertrain management system uses a number of temperature inputs to enhance the performance of the engine, control emissions, and optimize efficiency.

The most common applications are the:
· coolant temperature sensor
· intake air temperature sensor
· transmission oil temperature sensor
· cylinder head temperature sensor

The coolant sensor is a very important player in the computer control system. The coolant sensor signals affect the fuel mixture, timing, idle speed, cooling fans, evaporative purge control, the exhaust gas recirculation (EGR) valve and the torque converter clutch control (TCC).
 

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ya i wouldent rule out the coolant temp sensor or the head temp sensor

one of my buddies 5.0 mustangs had a simular problam just rant like crap when cold but was good after warm up

he replaced coolant temp sensor and it fixed the problem
 

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The Z31 ECU cares little about what the coolant temp sensor has to say. The CHTS is the ECU's primary engine temp sensor input.

S130 and Z32 ECUs relied on coolant temp sensors. For some reason the Z31 sensed the cylinder head metal temp.
 

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u might want to check 4 moisture in dist cap , ad clean the contacts. i have had a similar problem and that did work for me
 

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Discussion Starter #12
I have brand new Cap, Rotar, Wires, Plugs--fuel filter all that... Its not the dist cap.. But thanks Mr T knows what he is talking about But Mr. T in the last year I have read that people have tried changing out the CHTS and are still having the same problem
 

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I replaced the CHTS and it wasn't the resolution for me. I haven't look into the distributor cap/ignition stuff yet but I have heard a few people say that they were having this kind of problem and it solved it for them. I don't think it's the salvation however.
 

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For a problem that has apparently been going on for some years, even rumours of new cars off the showroom floor doing the bad start, I would have thought it would have been all sorted out by now.
I have not chased it up myself since as far as I can tell the problem did not affect Australian market cars so is not much relevent to me.

For a problem as pervasive as this I would have expected a factory TSB to be floating around.

If the problem is caused by bad mapping in the ECU itself, a substitute ECU from another year should pretty much fix things as long as the O2 sensor used is electrically compatible. There were variations.
If the ECU mapping is indeed the culprit, somewhere along the way someone should have noticed a trend in the ECU part number, or even the unit serial number beside the Nissan or JECS part numbers.

Going by the available PDF FSMs, the "fail safe" systems were built into 85+ ECUs and covered both CHTS and MAF system failures. The fail-safe is what limits engine revs to 2000 when a bad MAF is detected. The 84 PDF FSM does not describe a fail-safe facility.
If the ECU decides to invoke the fail-safe facility, it would be reasonable to assume that an error code for the failed system would be stored.
Exactly how far out the signals really are to invoke either fail-safe system is not described in the FSMs. A fair assumption would be open-circuit and short-circuit, at least.

If the problem is dubious harness wiring between the subharness and the ECU then that should be easy enough to test with a reasonable multimeter. Just bridge the CHTS connector at the engine harness and measure the total resistance of the wiring at the ECU harness connector/s. A few ohms (at most) would be ok.
Note that it would take a lot of resistance to make much difference to the perceived temp as the temp vs resistance CHTS specs published in the FSM have quite a broad range. The tolerances did tighten up a bit by the 88 PDF FSM.
Also note that a fault causing increased resistance in the CHTS circuit makes the ECU think the engine is cooler than it is. As CHTS temp drops, the resistance rises.
 

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I have an 85 z31 and this problem hasn't always occured; it only started fairly recently so I doubt it could be the ECU but I dont know much about that stuff. I might also add that my car is an Australian model Z. From what I've been told it will be a process of elimination in testing all of the sensors to determine which is faulty as not many things effect the cool idle.



Post Edited (Sep 13, 4:11pm)
 

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Have the same problem with my '84 n/a, GLL model with digital dash. Thought it would solve the problem 4 years ago when I had the recall injectors done! Ran exactly the same. I took it back and had the dealer (West Valley Nissan, Glendale, Az) do their computer analysis on the vehicle (which I have had for 21 years). They stated there was an open circuit on the chts, and the Dist. Cap was inadequate because it was arching, the wires were not Nissan (Bosch, just like the injectors), and the the throttle valve switch needed to be cleaned and adjusted. Their bill for this proposed solution was $1,200!

I have since replaced (on my own), the chts, distributor cap, rotor, wires, egr valve, egr solenoid, capcitor, idle up solinoid valve, throttle body assembly, MAF(remanufactured), (Fuel Pressure regulator was in incluced with the injector recall and they did throw away a regulator I had bought 3 months before the recall for $75. Always ask for old parts,) I also repaced the sensor regulator control module.

I am sure I have missed some components that were replaced; however,
the results are the same! Nissan must know something about this problem an unfortenately has not considered the interests of those that try to keep their vehicles running. Remember when they had the campaign to restore your old Z's when they disconnitued the 300ZX?

That was also an expensive proposition.

Bottom Line, I have replaced everything except the ECU, and speed sensor; Didn't mention that I have also repaced the distributor.

Although the temp sensor is not in the wiring diagram to have any input on the ecu's operation, I wonder why the car runs great for about 30 secs when the Idle up solenoid is working, then goes to ****, (Possibly the Air Bypass Valve (Which I have gone through three trying to solve the problem)(Could it be wireing harness?)

In any case, if you find a solution, please advise me, I have dealt with this for 7 years and will not pay more than the value of the car to have this problem resloved

Good luck, sorry for dancing around your problem!
 

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Mister_T - thank you for your post - it shed some light on some testing methods for the theory that I presented early and you provided some information that I was previously unaware of.

Hopefully one of these days we will find a plausable solution and get an article published on zcar.com or z31.com.
 

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It's almost all there in the FSMs to read.
The 88 PDF FSM in particular has some fault finding info that is more detailed than in previous editions. Use it in conjunction with the FSM for your particular year.

Those folks whose cars use a separate external Fuel Pressure Pressure Control Module (FPRCM) might want to disconnect that module while testing a bad cold start/run problem. The FPRCM listens in to the output of the CHTS. A fault in the FPRCM, or the wiring harness to it, could influence the CHTS output adversely. The FPRCM should only really come into play during particular hot start conditions so its disconnection should not affect overall engine operation.

I also suggest creating a test subharness inline with the CHTS circuit to enable measurement of CHTS output voltage while the motor is running. A high impedence voltmeter should be used. This would establish whether some other circuit is influencing the CHTS output. A sudden variation in CHTS output during or after start may give a hint of problems to follow up.

Of course, you would be wise to first verify that the CHTS resistance is within spec according to the temp-vs-resistance graphs in the FSMs, and that the CHTS receives a steady regulated 5V input. This is best done when the engine is stone cold as it can reasonably assumed that the CHTS temp is consistent with the rest of the engine. A contact thermometer would adequate in this situation to establish the temp.
Folks with infra-red ("non-contact") thermometers would be served equally well as long as the emissivity of the measured surface is considered.
I suggest stone cold measurement since there is no FSM spec as to what temp the CHTS should actually see in a warmed up engine. CHTS temp may or may not correspond with coolant temp. Take some readings and let us know.
 
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