Sebastian Bauco

Hi everyone,

First of all, thanks to everyone that contributes to this forum; I've been browsing old threads for weeks now, and it has been immensely helpful.

So to the problem:

A few weeks ago, I purchased an '86 NA Auto Z31 in California while I'm here for the summer. I have no prior experience with the Z31, so diagnosing this car's problems has been quite the experience for me. The car runs and drives, and did so when I purchased it. Right off the bat, however, I noticed a pretty significant exhaust smell in the cabin; when I bought the car, I assumed this was caused by some sort of exhaust leak, but I was soon proven otherwise.

My primary mistake here is that I bought the car without an updated smog test; the car had been smogged the previous year, and I assumed that I could do so again without significant issue. However, when I brought the car in to get smogged, the test showed HUGE amounts of CO and HC - something like 10x over the legal limit, if I remember correctly. NOx was borderline, but not obscene. The conclusion here is that the car appears to be running extremely rich, which makes sense given the fact that I basically smell unburnt fuel in the cabin all the time.

I started off by checking all the ECU codes, but I quickly realized that the car was not running an 86 ECU - rather, it was fitted with an A18-A03M87 for an 88 NA Auto Cal emissions Z31. It seems like this ECU is fairly similar to the 86 ECU, except for a few key differences:
-Injector impedance
-Presence of an EGT sensor (the 86 model does not have this sensor)
-O2 Sensor Type (88 ECU expects Zirconium, but my engine has Titania)

I figured that at least one of these problems was contributing to my extremely rich condition. So, I picked up an 86 ECU at a salvage yard and hooked it up, only to find that on this ECU, the car would barely start at all! On some occasions, perhaps moreso on hot days, I could get the car to briefly start and then die out after "bouncing" between 100-1000 rpm for a couple minutes. Applying throttle did nothing but cause the engine to sputter and die out faster. On some days, especially with the engine cold, it would not start at all on this new ECU - I would just get endless cranking with a couple sputters here and there. On one occasion, I could get it to start and rev properly for a brief moment, after sputtering for a minute or so. I went to check codes. I had codes 23, 31, and 41, so I went about making sure that my TVS switch and fuel temp sensors were both wired up and functioning; turns out the wiring for both was corroded and/or disconnected entirely, so I fixed both and got both codes to clear; at that point, I could get the ECU to report only a code 31 or a code 44. Still, no start! I figured that maybe I picked up a bad ECU, so I bought another one, only to find that the same thing happens.

At this point, I'm stumped. I've tried running the car with the exhaust gas sensor unplugged or the AFM unplugged, with no difference in the starting issue. I've done the electrical diagnosis suggested in the FSM for the air regulator, EGR control valve solenoid, CHTS, crank angle sensor, and idle-up solenoid, all with no significantly anomalous results. I was convinced that there was some sensor-related issue that was causing only the incorrect ECU to work, but at this point I don't know what else to test.

When I do run it on the 88 ECU, the cooling fan runs constantly; I'm not sure why this is happening, since the CHTS reads the correct resistance and is properly wired to the ECU. In addition, the engine sounds like it runs a little rough. Pardon my lack of engine knowledge, but it almost sounds like a V8, which leads me to believe that I have an intermittent misfire on one of the cylinders. The car has never stalled on me, but the idle is not particularly smooth.

To be clear, the previous owner of this car seems to have screwed up some stuff. When I first bought the car, multiple sensors were outright unplugged, and several vacuum lines were disconnected. I believe that I have fixed all these issues, but I could be wrong. There is still one electrical connector sitting unplugged directly behind the intake manifold, but I have no idea what it could go to, as there is no sensor in that location. If anyone has a clue what that connector is supposed to go to, I would love to have some input. Finally, there is some weird cable with translucent red insulation that appears to connect directly to my battery positive terminal, run to the driver's side of the car, and then go into the cabin; it is currently disconnected, and I assume that it is for some kind of aftermarket stereo that a previous owner tried to set up. I will upload photos of these connectors when I get home later.

Sorry for the huge post; TLDR, the car starts and runs rich and rough on the incorrect 88 ECU, but refuses to start and/or idle properly on the proper 86 ECU.

Thank you in advance for the help!

DREAMYZ

I believe it has a MAF ( mass air flow ), My brothers 87 had the kinda same symptoms, ended up being the MAF. Welcome to Z world, in 5 years you will be a top notch mechanic. At least your car gives you codes, my 82 you trouble shoot in the dark.

I dont think your problem is serious, when you find it, the car really screams, my brothers car was a smooth runner, but I got tiered of working on it so we gave it away, One Z is about all a person can handle. Good Luck, hang in there you will find it.

Sebastian Bauco

Thanks for the encouragement, haha. I considered the MAF, but it won't even start on the 86 ECU with the MAF unplugged; shouldn't it at least start and then only rev to 2000 rpm? That's what it does when I unplug the MAF while running on the 88 ECU.

Some other notes: I measured resistance on one of my injectors and it was ~2.6 ohm. That means I'm running low impedance injectors, correct? Also, I forgot to mention that the few times I have managed to start on one of my 86 ECUs, the engine has made a sort of pinging noise when I apply throttle. I looked this up, and it appears that this indicates an ultra lean condition, correct? In that case, it looks like the 88 ECU is making me run super rich, and the 86 ECU is making me run super lean.

86tcoupe

so a 30 yr old in a junk yard ECU is bad ...what are the chances of that ...pretty high ..people used to sneak the old/bad one in to swap , also there is a organic residues that will get on to the motherboard and change the IC's (dog breath a big thing)

if you unplug the o2 sensor it should put it in fail safe , but I would do some other checks to see what you have , you said it ran ok for a while ?

I recommend checking fuel press. and condition of the tank , I'm fighting to replace mine now , it sounds a bit like a low fuel condition ( open the mil sec time of the injectors because of low pres. then pres comes back )

I would check the compression ,done cold
also the conn. pins are an issue after this amount of time you need a great connector for micro amps , time consuming operation for sure
a logic probe would be something to try to see if the MAF, others sensors seem to put out data , but that is tricky

DREAMYZ

Yes...Low Impedance

To bad you dont know someone else who has the same car, you could swap ECUs, When I first bought my 82, I had a friend in the Z Club that had a like new 82 turbo, never touched, never driven, approx 15,000 on the clock, I matched up the cars and found so many Vac hoses and such had been messed with over the 32 years of life, but I just copied his car and now I pay close attention where everything goes.
Bet you, as the last commenters said, I believe your looking at a ECU, I have 7 Ecu's, 4 are good, Ebay 80 % chance it is a good computer, Junk Yard 60% chance. I run a modified Jim Wolf Ecu, After I put it in the car, it was like I was running a totally different car. But if your car is like my brothers car, You have a nice car, and like I said, they are very quick, and purr like a kitten, tho the V6 is not easy to work on physically.
I like straight 6...solid lifters

Each bad computer will give you a different problem, your lucky when its a no start. And your lucky ....you have codes at least, helps somewhat, even tho they are Elementary at best .

86tcoupe

How is the exhaust ,no leaks ? to the cat conv. It causes a lean reading from the O2 ,that will richen up the mixture .
a leaking injector a possibility ,is one cly the problem?

Sebastian Bauco

Hey everyone,

My apologies for the huge gap in response, and thanks to everyone for their suggestions! I wanted to update this thread just so future visitors know what the problem was. It turns out that after the previous owner ran the low impedance injectors on the ECU designed for high impedance injectors, it just destroyed the injectors in some way. After spending an entire weekend replacing all the injectors, the car started right up on the correct ECU. It took a while to figure that out, so hopefully this helps someone else at some point!