possible to have a pcm with intermittent failure?

[1992mustangLX]

Had a issue with the pcm a couple weekes ago, i thought i had resolved the problem but the symptoms have returned. It seems to almost missfire at higher rpms. I had initially searched and found a tune on the car that was for a automatic and thought it had caused the issues because i took it off and the next day the car was running awesome! This issue seriously robs me of major power because the difference of driving it with this issue and with out was very significant! I have replaced everything involved with the ignition except the wiring itself and the pcm. I planned on replacing it anyway because i am practing doin the whole car over so it wasnt a shot in the dark waste of money to replace everything.
So my question is it possible to have a pcm that operates perfectly one moment and then to shut the car off and a couple hours later to start it have it not working correctly. It hasnt gone away since it came back. Is it likely to have a pcm that would do this? Thanks for any help

 

[greg251]

Did you replace the TFI module? They have been known to cause high rpm misfires before.

 

[1992mustangLX]

yes the tfi has been replaced. Along with the ignition coil, distributor cap and rotor, plugs, plug wires. Basically everything but the wirng and the pcm itself. The only thing else i can think of, which i highly doubt is the distributor and the ignition pick up. However i took the distributor out of my cousins 5.0 and it did the same thing so i am pretty sure it would be that. have pulled the codes before but only got two codes that didnt invovle anything to do with the functioning of the motor (67 and 81).

 

[HISSIN50]

It is very very unlikely to be a PCM issue. Stresses are greatest on mech parts at high RPM so any number of issues can cause a high RPM miss.

 

[1992mustangLX]

well the other reason i fing it to be the pcm other than the fact that i have checked or replaced everything involving the ignition except the pcm and wiring, is that it almost seems to be hitting a rev limitor in some ways. It seems like it is bouncing off this particular high rpm and causes it to back fire and from reving any higher. The things that worry me is the ignition pick up (distributor), which i switch with another 5.0 and did the same thing so i think i can rule that out. A fuel pump (?) i think is fine because the fuel pressure is reading properly at 42psi which was recommened by trick flow. But i am not sure if that pressure changes as the motor is under load. Im hoping that someone can spark maybe another idea for me that i am overlooking. Thanks for the reply

 

[Daggar]

I've read through your symptoms and none of them suggest to me that you have an PCM issue.

List all of your ignition components, whether they are stock or not, and their approximate age (right on down to the brand and age of your plugs and wires).

What RPM does your ignition hit the wall?

Are you running any power adders? Boost? Nirtous? High Compression Postons?

When the thing acts up, what are the weather conditions? Raining? Humid? Dry? Cold? Hot?

How old is your alternator? Have you ever had it checked? How about the battery?

What MAF are you running?


This should help in getting down to what ails you. At first glance, my suspicion is that some portion of your charging system is not up to snuff. Backfiring is a classic symptom. A PCM problem would not usually cause a backfire. The injector will either fire or it will not when the EEC is in control. Since you're not getting a complete burn I'd say that your EEC is doing it's part.

 

[1992mustangLX]

I've read through your symptoms and none of them suggest to me that you have an PCM issue.

List all of your ignition components, whether they are stock or not, and their approximate age (right on down to the brand and age of your plugs and wires).

What RPM does your ignition hit the wall?

Are you running any power adders? Boost? Nirtous? High Compression Postons?

When the thing acts up, what are the weather conditions? Raining? Humid? Dry? Cold? Hot?

How old is your alternator? Have you ever had it checked? How about the battery?

What MAF are you running?


This should help in getting down to what ails you. At first glance, my suspicion is that some portion of your charging system is not up to snuff. Backfiring is a classic symptom. A PCM problem would not usually cause a backfire. The injector will either fire or it will not when the EEC is in control. Since you're not getting a complete burn I'd say that your EEC is doing it's part.

Ok. The whole top end of the motor is new. New ignition coil (oem replacement from napa), new msd distributor cap and rotor, new sparg plugs (nkg), new plug wires (ford racing 9mm), new tfi module (replacement from napa), granitelli maf sensor (calibrated for 24#inj; which i have), new throttle body and tps sensor (set at .88v @ idle, 4.77v@ wot), new fuel filter, timing set @13*, new O2 sensors (bosch), i put a smog delete kit in (wanted to ask what i did with the vaccum lines that use to go to the smog line? Do i cap them? What would happen if i routed them to eachother?) The only thing else suspect is the the distributer (pip inside too) itself, which i already switched with another mustang and had the same issue. And the fuel pump but dont know if a fuel pump would cause this, the fuel pressure stays at 42# when i rev it till the symptoms occur. What about the egr valve on the egr plate? Could that get stuck open and cause this?

oh by the way im running nothing but motor, no nitrous, etc

 

[Daggar]

You fuel pressure should fluctuate as you accelerate. It should change with the amount of vacuum that you pull.

Make sure that you get a change in fuel pressure at idle with the vac line to the regulator on and off. Check all of your vac lines to ensure that you don't have something sucking air from somewhere.

The lines that you pulled for the smog need to be capped.

 

[HISSIN50]

Do you use plat plugs? If so, swap 'em out.

Have you tried closing the gaps a little to see if it helps?

 

[1992mustangLX]

well guys. I think i got it. I took it out after checking vaccum lines, fuel,etc. and when i got back i threw the code scanner on it again just caase i was running into dead ends. I mentioned this before but it was a guess but never checked. I got the original codes 67 and 81 and then to my suprise it beeped code 31.

code 31-EGR Valve position (evp) sensor or pressure feedback EGR sensor-signal voltage is below minimum. OR..EGR vaccum regulator solenoid circuit problems. OR... EGR valve is not in its normal closed position.

So im gonna take the egr off and check to make sure its not stuck open but if it isnot how do i go about tackling the other possibilities? Thanks for all the replies!

 

[1992mustangLX]

So what do you guys think? I didnt have time to do it today but i will tomorrow, but if you give me your thoughts on how to go about tackling this i am sure they will help. Thanks

 

[jrichker]

CODE: 31 (KOEO) - EVP circuit below minimum voltage. Vref (5 volt reference voltage supplied by the computer) missing or broken wire or bad connection in circuit. Use a DVM to check for 5 volts on the orange/white wire. If it is missing, look for +5 volts at the orange/white wire on the TPS or MAP sensor located on the firewall near the center of the car. Use the black/white wire for the ground for the DVM.
With the sensor removed from the EGR and still connected, press the plunger and watch the voltage change on the brown/lt green wire. Pull the passenger side kick panel and measure the voltage at the computer. You will need to remove the plastic cover over the wires and probe them from the backside. A safety pin may prove very useful for this task. Use pin 27, EVR input (brown/lt green wire) and pin 46, signal ground (black/white wire) to measure the voltage. The orange/white wire is Vref and should always be 5 volts -/+ .25 volt. Be sure to measure Vref at the EGR sensor to rule out any broken wires or bad connections.

Measuring the voltage at the computer helps you spot broken wiring and intermittent connections.

 

[1992mustangLX]

Awesome. Thanks for the reply and ill let you know how i make out! Hopefully it solves the issue. Thanks.

 

[1992mustangLX]

ok so my buddy gave me a brand new egr and sensor for free and threw it in there. Still having the issue however. I ran it for a little while and then put the code scanner on and now the code 31 is gone. However now i am getting code 34, which also has to do with the egr. I must have a wiring problem like you said already, can you (jrichker) give me more details on how to test the wiring i was a little confused before maybe you can help clear it up a little. Thanks.

 

[jrichker]

Code 34 Or 334 - EGR voltage above closed limit - Failed sensor, carbon between EGR pintle valve and seat holding the valve off its seat. Remove the EGR valve and clean it with carbon remover. Prior to re-installing see if you can blow air through the flange side of the EGR by mouth. If it leaks, there is carbon stuck on the pintle valve seat, replace the EGR valve ($85-$95).

If the blow by test passes, and you have replaced the sensor, then you have electrical ground problems. Check the resistance between the black/white wire on the MAP/BARO sensor and then the black/white wire on the EGR and the same wire on the TPS. It should be less than 1.5 ohm. Next check the resistance between the black/white wire and the negative battery post. It should be less than 1.5 ohm.

Note that all resistance tests must be done with power off. Measuring resistance with a circuit powered on will give false readings and possibly damage the meter.

Let’s put on our Inspector Gadget propeller head beanies and think about how this works:
The EGR sensor is a variable resistor with ground on one leg and Vref (5 volts) on the other. Its’ resistance ranges from 4000 to 5500 Ohms measured between Vref & ground, depending on the sensor. The center connection of the variable resistor is the slider that moves in response to the amount of vacuum applied. The slider has some minimum value of resistance greater than 100 ohms so that the computer always sees a voltage present at its’ input. If the value was 0 ohms, there would be no voltage output. Then the computer would not be able to distinguish between a properly functioning sensor and one that had a broken wire or bad connection. The EGR I have in hand reads 700 Ohms between the slider (EPV) and ground (SIG RTN) at rest with no vacuum applied. The EGR valve or sensor may cause the voltage to be above closed limits due to the manufacturing tolerances that cause the EGR sensor to rest at a higher position than it should.

The following sensors are connected to the white 10 pin connector (salt & pepper engine harness connectors)

This will affect idle quality by diluting the intake air charge