86 300ZX NA Starts then Dies, wont idle. HELP!

[spaceviolence]

Hello all,

A little preface: I am new here, and somewhat new to Z's but by no means new to cars. I have the FSM and have **** near read the entire
thing, I've googled and forumed to no avail, as nobody's problems seem to fit my problem exactly. I've troubleshooted many components and circuits
and fixed ones that were wrong, which I will list later, I've worked on this problem for a couple days and even worked on it in my dreams with no
luck haha.

So I've only had this car for a few months, and in that time it has been awesome, great, reliable, runs like a beast, always starts right up (With
the exception of that occasional vapor lock in the rails), still does but wont stay running. Drove the car the other morning to get breakfast, ran
great, no problems, nothing weird. Exited the cafe and got in and started my car and it started right up but died about 3 seconds later. Was not
successful in getting it to run correctly so drove it home frustratingly running like crap. Will not stay running, runs more like normal in high
RPMs but even then if accelerator pedal held down at a fixed level it will still eventually die. The only way to keep the engine is to keep moving
the throttle. This problem exists regardless of whether cold or hot starting. Disconnect MAF and engine will idle (too low) and run, but will not
rev above 2.5K (duh lol) and dies once throttle is release (will not hold idle.) When engine ran well, all devices seemed to be working correctly,

Idle up did work fine, any time lights or A/C came on engine would idle up correctly.

On to progress.

Checked for vacuum leaks, large ASCD vacuum hose had hole burnt in it from EGR valve tube and was leaking, replaced.
Vacuum line from canister to watercock was brittle and broken, replaced.
Have done a listen check on injectors and all seem to be clicking, will do OHM test and update later.
Checked battery connections, seem legit, grounded to frame and starter.
Checked EVERY fuse, all were fine, anti theft system fuse was always removed and I'd like to keep it that way.
Checked FPR, wasn't squirting any gas so seems fine. *shrug*
Ran a fuel pressure gauge T between fuel filter and fuel rail, got 40 PSI primed and ~36 PSI when running. Stays there.
Fuel pump primes perfectly and does run while engine is running. Don't think that's the problem as I've got pressure all the way until and after it dies, fuel pump runs for a couple seconds after engine is turned off as it should as well.
Checked air cleaner. Nice K&N lifetime filter. Seems barely used.
I should add I checked every electrical connector I could get to. I know corrosion is usually a problem on these cars but only maybe one had any corrosion, the rest are pretty spotless and shiny. This car HAD to have been garage kept as everything is in great shape. (Minus some minor bumper and fender/ mild frame stuff underneath due to it being driven off the road into some deep shrubs as previously stated.

So it seems I have ruled out fuel delivery (unless injectors suck) besides, seems that the exhaust is crazy rich at times and lean as **** others. Even when car was running smoothly, It would have a bit of trouble on warm up for about 30 seconds or so, accompanied by missfires upon pressing throttle, is worse the further down and more quickly you move the throttle (would eventually catch up and accelerate to correct engine speed if you held pedal in
place) and then it seems air/fuel ratio would even out nicely, after that, idled great. Not anymore though.

Spark plugs are black, but despite all the carbon the look fairly new, the electrodes are not worn in any way and the wires seem to be in decent
shape. This car runs rich methinks, at least on warm up. Previous owner ran it off the road into some hefty shrubs and let it sit for about 3
years (live in the desert, not many trees) and probably cracked the CAT/Exhaust pipe to muffler/flex pip/whatever so its got an exhaust leak and
probably needs a new O2 sensor on as well. (Light was on when I got the car, due to its age I just diconnected the light as recommended in the
manual)

Ran diagnostics. Got codes. Fixed most problems. Got:

(13) CHTS code, checked harnesses, sensor and circuit and all seems well, resistance is at 4.3 kOhms approximately when cold seems correct for a
cold early spring morning here in the desert (40-50 degress outside), afraid to run car long enough to warm it up to check hot resistance, after I
reconnected all connectors, code went away.
(41) Fuel Temp Sensor: Wire and harness were both misshapen and broken, installed new connector, code went away.
(31) Load switch: turned on lights, A/C, code goes away.
(23) Throttle valve switch/Idle switch. Adjusted TPS, code went away.
(12) MAF code, did not test at this point.

Reset codes, retested and got: Load switch (31) (duh) and (12) AFM.

Checked all wires from wiring harness to computer.
Checked ground resistance for both ground wires, close enough to zero,
Checked incoming 12V to MAF, comes on with EFI relay,
Checked signal voltage at ECU with engine running as long as it would, when engine not running voltage is maybe 1.4 ish, when running its well over 2.0 but not past 3.0, could not check on rev or higher RPMs as engine kept dying.
Checked EFI relay and all associated sensors/switches/solenoids. All are getting power when EFI relay engaged with key ON.
Cleared codes, retested, all codes gone but code 12 for Air flow meter.
BUT WHY if everything seems correct?? (Or am I missing something?)

Cleared codes again, reperformed diagnostics, held throttle at fixed levels to try to get engine to run normally. At 1100 engine seems to run
smooth with no noticeable missing (although engine may just be revolving too quickly to actually tell) however at 1000 engine runs rough and
labored, missing it seems and wanting to die, but wont if i hold it at 900-1000, still really wants to die though. Seems as though the longer held
at a fixed throttle position, the RPM will decrease slowly and steadily until engine dies. Code 12 still shows up. Not immediately, but sure enough, always comes back, even though AFM seems to be within spec.

Results: Still stuck with code 12: AFM.

Things I haven't checked yet:

CAS (is getting voltage but havent checked actual sensor yet)
Dizzy/Ignition
Injector impedance
Cylinder compression (don't have compression gauge, will rent one this week.)
Timing Check (don't have timing light, will rent one, will rent one this week.)

I do plan to do these in the next couple days and will include results below.


Questions:
For AFM testing, FSM says with 12v on D&E you should get 1.6V at B & Ground +/- .5V I get about 1.4V so that seems right.
FSM says blowing air thru should get 2-4V. I get 2-3 with engine at idle (Didnt remove AFM, did tests from harness @ AFM and @ ECU harnesses.
Seems right, right?
This being the case, is it possible my ECU had suddenly gone bad as I ate breakfast that day or does anyone have ideas?
Could my timing have jumped?
Are CHTS as they go out, intermittent/work sometimes/not othertimes?
Could my AFM give decent readings and still be bad, fluctuations I can't see/my multimeter maybe to slow?
Could this just be a bad ECU?

Follow up: Sorry for the length of my post, I hope you all will appreciate my detail rather than become frustrated by it, as it has been opposite for me in my experience. (Never enough details given to help fix problems in forum posts) Also thanks in advance for anyone who takes the time to read and respond. Also, don't grill me or troll me, if you think I've missed something or am wrong, just lemme know, I'll appreciate the knowledge. As you can tell I've really taken the due time to do the correct research and troubleshooting as well as taking the time to write this in a cohesive, coherent and professional manner. Thanks.

[spaceviolence]

I adjusted the idle screw on the Idle up solenoid valve, backed it out enough that the engine would actually stay running (incorrect but i wanted to warm up the engine), was able to take it for a short drive, at first, smelled very rich. During the drive, if I give it any throttle that puts it under a load, it misses horribly and bogs down. Let off the gas and it idles crappily at about 500 RPM. Still has power like it used to in but struggles with more throttle. I guess I should say its still got torque, but any load other than normal driving, and it misses. NOW I'm thinking back to the fuel system, but i've got good pressure, I guess I should OHM the injectors next.

I should add that I tested for spark on #1 and all seems well, blue spark most of the time. Took off dizzy cap, rotor spins, doesnt look horrible, I do get spark

[spaceviolence]

Unplugged AFM, went for a drive, idles correctly, has plenty of power, but obviously not above 2K RPM.

The self cleaning circuit doesnt seem to be working. only gets up to .04 - .05 V after engine is turned off. Is that correct? Thought it was supposed to be 12V. Either way, tested AFM signal voltage at ECU harness and read 2.4 @ 1.1K RPM, 2.8 @ 2K, got it up to almost 4V with a large rev. Seems correct to me, maybe i'm wrong? ECU starts reading voltage around 2V up to 7V if i'm correct, and also correct me if i'm wrong but voltage should be within 1-5 V?

Also, CHTS is weird. At first, with engine cold just now, couldn't get a resistance reading on any of my ranges on my multimeter. After engine warmed up, got ~0.70 kohm, which seems to correlate to ~122 F. Tested continuity from CHTS harness ground to frame and nothing, also to ground in the AFM harness, since they intersect, and still nothing. No continuity. Does anyone know if the sensor is grounded thru the head? if not, my results make perfect sense, which i'm leaning toward since it is a 2-wire type.

[evildky]

Wow, you have certainly given us a lot to think about!

Vapor lock should not be an issue, this is a high pressure return type fuel system the fuel lines are not obscenely close to the exhaust or any other heat source, if you are truly experiencing vapor lock something is wrong with your fuel lines!

The fact that the car drives fine up to 2500 with the maf unplugged makes me think you have a bad maf, the fact that you have a load circuit code makes me think ECU.

The cas is in the distributor, it sends out 2 signals, one every degree and one every 60 degrees, both 5v square wave. The ignitor is known to go wonky after 30 years but I doubt thats the issue here, I would however check my chassis and engine grounds, bad grounds can cause all sorts of weird issues. The CHTS measures resistance between it's poles no to earth.

[spaceviolence]

Wow, you have certainly given us a lot to think about!

Vapor lock should not be an issue, this is a high pressure return type fuel system the fuel lines are not obscenely close to the exhaust or any other heat source, if you are truly experiencing vapor lock something is wrong with your fuel lines!

The fact that the car drives fine up to 2500 with the maf unplugged makes me think you have a bad maf, the fact that you have a load circuit code makes me think ECU.

The cas is in the distributor, it sends out 2 signals, one every degree and one every 60 degrees, both 5v square wave. The ignitor is known to go wonky after 30 years but I doubt thats the issue here, I would however check my chassis and engine grounds, bad grounds can cause all sorts of weird issues. The CHTS measures resistance between it's poles no to earth.

Thank you so much for taking the time to reply!

I think I'm wrong on the vapor lock, with your description, doesnt seem to have been my problem. I was describing situations that I had when I first bought the car after it'd been sitting for about 3 years and every so often, if the fuel pump ran too many times (key on ON) it wouldnt start. dunno if not enough fuel or too much in those moments. hasnt happened anymore so i think that problem is gone.


I'm kinda thinking the same route as you. MAF cause of the idle situation, ECU but for different reasons. (that load signal code went away with turning on the AC or turning steering wheel or turning lights on. So my only code now is 12 for AFM. The reason i think the ECU might be bad is because I've tested this AFM every way possible and it all seems pretty close to in spec. I don't know how far out of whack the voltages/ohms have to be to really confuse the computer, but everything seems about right with the AFM, did the OHM test across all pins and all but one were exactly what the FSM says and the one that wasnt was only a tenth or two of an Ohm off. The other reason I'm thinking ECU is because there is no voltage from the Cleaning Cycle that should be turned on by the ECU and the wires appear to be fine and have continuity.

I will do a test on the CAS but from forums topics dealing with that, seems to be a different symptom than that of what i'm getting here.

The ignitor/power transistor I think is OK since I'm getting spark, although I'm aware it could still be flaky. Someone had suggested freeze spray but as my problem is NOT temperature dependent I'm leaning away from ignitor as well.

This ALSO doesn't seem to be a compression problem, as I'm fairly familiar with what that feels like, but who knows, I guess it all depends where a cylinder is losing compression. Either way I want to do a compression check and a timing check today. Do you think my timing could have somehow jumped?

I will hit the junk yard and grab an AFM today if they've still got one. Failing that, I will grab an ECU. Failing that, I guess my options are MAF cleaner and the aforementioned tests.

Also thanks for the info on the CHTS, that clears all that up for me.

[spaceviolence]

Just wanted to follow up in case anyone else has a similar problem, its documented here. Fixed. AFM was the culprit, even though all tests came out correct. Replaced it and all is well.