Catastrophic failure

[scyattica]

Greetings all. I have a weird problem. When the car is at operating temperature, it runs horribly. It's missing, surging, and spews black smoke (unburnt gas). Odd thing is, when I start it cold, it's fine, no surging, no missing, exhaust is normal.

I pulled the KOER codes(no codes until it's at operating temperature):
412 - Unable to control RPM During high RPM self test.

159 - Mass Air Flow Sensor out of self-test range.

121 - Throttle position censor out of self test range.

129 - Insufficient Manifold Absolute Pressure/Mass Air Flow change during Dynamic Response check.

167 - Insufficient throttle position sensor change during dynamic response check.

I had no problems with anything, till this all happened at once. A day or so before this, CEL turned on, and it started missing occasionally when idling. I think it's the computer. I doubt that all those sensors failed simultaneously.

 

[410Fortune]

You need to check your wiring harness, sounds like a plug either at the PCM, or at the engine, or at where the engine harness connects to the PCM harness may be loose or have internal issues.

You can try locating a computer with the same calibration code as yours and swap them out.
Junk yards get $35-80 it seems for used PCM's

Ford can do a diagnostics for you for around $100 to determine where the problem lies for certain (in PCM or in wiring)

 

[scyattica]

You need to check your wiring harness, sounds like a plug either at the PCM, or at the engine, or at where the engine harness connects to the PCM harness may be loose or have internal issues.

You can try locating a computer with the same calibration code as yours and swap them out.
Junk yards get $35-80 it seems for used PCM's

Ford can do a diagnostics for you for around $100 to determine where the problem lies for certain (in PCM or in wiring)

Why would it work cold, but not warm?

 

[KPT]

When its cold the PCM is operating in open loop. Its ignoring the O2 sensors (I believe those are the only sensors it ignores) and IIRC runs slightly rich to get the engine warmed up.

 

[scyattica]

When its cold the PCM is operating in open loop. Its ignoring the O2 sensors (I believe those are the only sensors it ignores) and IIRC runs slightly rich to get the engine warmed up.

Okay. so, it is agreed that the problem has SOMETHING to do with the PCM, be it wiring, power, the PCM itself?

 

[MrShorty]

Or something in in the engine management system. Especially when it comes to fuel control, there are a lot of interrelated sensors etc. that figure into determining the fuel mixture. EEC-IV isn't always smart enough to identify the circuit that is at fault, but it can see that something is wrong, so it makes it's best guess, sometimes outputting multiple codes, but they are all from a single fault. The trick is to figure out what the problem is.

Did you try the KOEO test? We're not too worried about CM codes at this point, but we need to know if it passed the KOEO test before we want to spend any time worrying about the KOER codes.

 

[scyattica]

Or something in in the engine management system. Especially when it comes to fuel control, there are a lot of interrelated sensors etc. that figure into determining the fuel mixture. EEC-IV isn't always smart enough to identify the circuit that is at fault, but it can see that something is wrong, so it makes it's best guess, sometimes outputting multiple codes, but they are all from a single fault. The trick is to figure out what the problem is.

Did you try the KOEO test? We're not too worried about CM codes at this point, but we need to know if it passed the KOEO test before we want to spend any time worrying about the KOER codes.

I ran the KOEO tests, and it basically gave me the same codes. except the ones that are KOER only, I forget which ones exactly, though.

Edit:Reran KEOE.Codes are as follows (in this order)

159 - MAF
159 - MAF again
177 - heated o2 sensor
452 - vehicle speed sensor
177 - o2 sensor again
452 - vehicle speed sensor

 

[MrShorty]

You don't indicate where the separator pulse is between the KOEO codes or CM codes, but, since my code list indicates that a 159 can't be a CM code and a 177 can't be a KOEO code, I will assume that you have a KOEO 159, CM 177, and CM 425.

When resolving EEC-IV codes, the best place to start is with KOEO codes. Because KOEO codes are set with the engine off, these faults almost have to be electrical. So I would start with the KOEO 159 code.

I don't have a pinpoint test in front of me. I expect the first part of testing this is to make sure the MAF has +12 V and a good ground on the excitation voltage circuit. Then make sure the signal wires to the PCM are good. Then check the output signal on the MAF to see if it's generating a reasonable base KOEO signal.

It seems to me that it wasn't too long ago (last month or so) that someone else was having MAF difficulties and there were MAF wiring diagrams posted and some other information. You might see if you can find that.

 

[scyattica]

Update: I pulled the PCM, and I still think it's the problem, seeing as it has what I think to be rust or some kind of corrosion on it. I dunno, I'm going to get a new one tomorrow, so I'll know then.

It was the PCM. Replaced it with a '92 PCM and it's running fine now.