86 NA Z31 won't start on 86 ECU, but will start on 88 ECU

[spynoodle]

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!

[evildky]

Welcome to NICO!
Long but pretty thorough. I wonder if the entire engine had been swapped? Only california compliant cars got the EGT, I've never seen one but I'm a long way form CA. You'll want to identify which injectors are in the car. Should be pretty simple as the 88's used high impedance dual feed injectors, the 86 would have standard low impedance top feed injectors. If the car has the 88 injectors and an EGT then you should probably be running the 88 ecu. Swapping the O2 sensor is pretty simple.

The way the injection systems work on these cars is generally default rich. When an ecu doesn't see an O2 sensor it assumes the system is lean and it adds fuel, If the ecu doesn't see the fuel temp or chts, you guessed it it adds fuel. So anytime you unplug pretty much anything the car should run rich. When the car is running rich it's a safe place to start is to assume that something is not working or unplugged or has a bad connection. You didn't say if you ran codes using the 88 ecu? If the car ran better on the 88 it might be a better place to start. If you do have the low impedance injectors you can wire in resistors to correct the impedance.

[8631z]

what does your unplugged connector look like? the only one i could think of on the drivers side would be o2 sensor. not sure where the exhaust gas temp sensor would be so maybe its that?