'89 Mustang check engine light

See:

I'm going to assume that this test was performed when the motor was cold..... giving the IAT and ECT (codes 21 and 24) readings are out of self test range. To eliminate these as concerns, we would run the motor to operating temp and rerun the KOEO test....

To address the memory code, we would need to perform the KOER test to see if we are dealing with a hard fault or an intermittent.... At any rate, the most likely cause for code 31 would be the EVP sensor on top of the EGR valve. Pay attention to "most likely"....

These "ancient" electronic control systems offered relatively little in the way of self diagnostic capabilities. Occasionally, parts replacement can be considered a viable diagnostic step... especially if testing the sensor would cost more than replacing it.

At the same time, it is important to pay attention to the base engine.... particularly the fuel filter which seems to be often forgotten.

While I am trapped to using the 1992 PC/ED (the oldest available on CD), the link supplied by the other respondant appears to contain some contradictions.

Hi Jim

You are correct. The codes 21 and 24 went away when warm. But the 31 remains. The car now is idling at a loping gate when cold, but ok warm. I mean ok/stumbling at about a 5 second rate each. Sounds like something leaking air in, like the PCV or EGR. I checked the PCV and it seems ok. A little leakage in the closed direction. Probably time for a new one of these. It has never been replaced. Also, It looks like I will have to take the EGR out and clean it or replace it.

Fred

I have been getting a code 31 lately. Idles bad cold and stalling. So today I changed out the PCV valve, the EVP position sensor on the EGR, and the gasket on the EGR valve. All three seemed ok before changing but I changed them anyway. The problem remains. EGR looks ok (sealing well with no vacuum). I cleared out the saved code 31. Now I will see what saved code turns up next. Anyone have any other ideas? Fred '89 LX 3.0

98k miles

Run KOEO (key on engine off self-test) and see if any on-demand codes show up. Check that the position sensor receives VREF (+5V). If you have VREF, the EVP signal (the wire that's neither VREF nor GND) may be open somewhere between the sensor and the processor (computer). Take a good look at the harness connector for the sensor. It's easy to damage contacts by pushing a probe into them. If at some point the processor (computer) has been removed and the harness probed, the same problem might exist at the other end. Make sure that both EVP and VREF contacts look good. These are frequently intermittent. My LTD used to stall in hot weather. By the time I got to test things, everything looked right. Finally I caught the entire VREF line at 0V when the engine stalled - a poor contact at the processor end. No output from every single sensor that uses this reference. Don't know what type of connector you have at the processor end. On the

3-row variety, the relevant pins are typically 26 and 27. Use an EVTM manual (or perhaps Alldata at the library) to find the connector pinout for your model year. Finally, there is always a chance of a defective processor, unable to read the EVP signal. Least likely, but not impossible.

I checked the VREF and I have the 5 volts. I then ran the car a long time in the yard. No code 31. Idle ok after warm-up.

I have a hunch its the fuel filter in front of the gas tank. Its never been replaced, but the fuel pump in the gas tank has. Maytbe its clogged and affecting the idle cold.

Fred

That's good news.

I doubt that poor idling would be caused by restricted fuel delivery -- the demand is very low. One reason could be a defective idle control (most likely a throttle bypass valve, but not sure what type your engine uses). Another could be a shorted ECT (coolant temperature sensor), erroneously indicating that the engine is warm. These older EECIV processors have a somewhat limited self-diagnostic capability, but before acting on any guess and throwing parts at the problem, I would run KOEO (with the engine cold), then warm it up and run KOER. Very simple to do and you may learn something useful from the on-demand codes.

Hi Group

Well, the problem is still the same. Ran all the codes and they all come up 11 now (ok). Still a galloping idle when cold. Looked at everything I can see but everything looks ok. If I put in in gear right away when cold with the brake on and give it some gas it likes that after. I wonder if its something major like the timing chain?

Fred

Hi group

Found the problem. I took off the hose from the air cleaner to the throttle body and found the throttle plate very dirty. After a thorough cleaning the idle went way up. This was probably from me fooling with the base idle screw over the years. Then I cleaned the idle air bypass valve (also very dirty but not stuck). Finally I reset the base idle and TPS voltage. Now the motor purrs like a kitten!.

Fred

never adjust the idle screw!!! this messes with the tps voltage at idle. When a (older) ford idle starts to act up clean the throttle plates!!!!!!!!!!