Help, I have a problem with my 94 Explorer. When idling it will surge up and down from 500rpm to normal idle speed at random. The change is not gradual and I can actually feel the surging when I drive the truck. It does have an intermittent CEL also. I tried the shotgun approach and replaced the IAC (new) , TPS (new) and had the MAFS cleaned. No luck so I took it to the Ford dealer and when they pulled the codes it said 124 and 122 which is the TPS voltage is too high and the TPS voltage too low (explain that to me). They said that they needed to replace the PCM for $500 for starters and if that wasn't it, they would they would need to look elsewhere.YIKES!! Sounds to me like I need to look elsewhere for help. Any advise?

 

1st question I ask: Are those codes KOEO or continuous memory (CM) codes or both? What does the TPS output? Is the TPS seeing a consistent 5V Vref? Did the tech give you details as to how he came to that diagnosis? The usual procedure for determining the PCM is bad is by process of elimination: if all the possible causes outside the the computer check ok, then the PCM is the problem. Did your tech eliminate the wiring and the TPS as the possible causes?

 

I would have to agree with you mrshorty . i had a surging problem driving at low speeds untill the truck warmed up so what i did was put in fuel injector cleaner in the tank and since the it was ok. i for got to say i did have a slight surge at idle too and so far its fine.but checking the codes would do wonders as well

 

The person that I talked to was the service writer. I never spoke to the tech. I really don't know if it was KOEO or CM. I truly wish I had known to ask the right questions.
To tell you the truth after he said $500 all I heard was blah blah blah.

 

Wow, another bad PCM. I lived with a surging idle on my 94 for years, but it was only first thing in the morning and went away when the truck warmed up. Then one day the idle was constantly irratic and got to the point where it was hesitating on acceleration, and idling very rough. Changed the IAC and cleaned the MAF. That helped a tiny bit but not good enough. The tech narrowed it down to the PCM. Changed it out and no problems since. I think $500 is a little steep as I got mine for around $350 Canadian new. I think they gave me a deal though as I had just gotten the intake manifold gasket done a few months prior and I know the SM quite well.

If I were you, I'd first try a salvage yard as you'll probably pay less than $100 for a used PCM. Just make sure it's from the same year and try and match up numbers. You could probably swap it out right at the salvage yard as it's pretty easy to do. That way you could ask them if you can "try before you buy". Some places will let you do that.

 

I'm not going to 2nd guess your tech, since we don't know what he did. He could be right.

If you feel up to it, you could doulbe check his work yourself. It's easy to pull codes yourself on these older EEC-IV vehicles (see Ken00's tech info post at the top of the BII forum for an article telling how to. Only requires a paper clip to do it yourself.), then we would know the exact codes and what context (KOEO, CM) they appeared. Testing the TPS and its circuit shouldn't be too hard. The TPS is a basic potentiometer. With a voltmeter and basic understanding of DC electricity, we should be able to check the stuff outside the computer.

 

Thats a neat little trick! I never knew you could pull codes with the check engine light. The codes it gave were 124 and 157. (TPS voltage high and MAFS voltage too low). The truck seems to be idleing slow almost all the time now. It seldom speeds up normal idle now. NAPA has a reman. PCM for $129. if I order it online. I'll call the local store and see if I can get it at the same price. What do you think my chances of that happening are?MrShorty Thank you for all your help. Backin72 described my problem to a tee and I'm leaning the way of a new PCM also.
I'll let you know how it turns out and thanks again.

 

So you're seeing both a TPS and an MAF code (where the Ford tech only saw TPS codes). This is an interesting development.

EEC-IV computer's aren't always perfect at pinpointing problems. In a case like this I could see the computer is trying to say, "The TPS and MAF signals are inconsistent with each other, but I don't know which is wrong." So the computer may pick one (as it did for the Ford tech), or it may give both (as it did for you).

Again, I can't comment on what the Ford tech did. If you trust that he did a througough job of diagnosing it, then go ahead and replace the PCM. They do go bad on occasion, but it's not very common. One thing to note, the computer is telling you something a little different than what it told him. Where he only had TPS codes, he may have only looked at the TPS circuit. Now it's telling you that the problem may lie in the MAF circuits, which the other guy may not have checked.

If you want to investigate further, the first thing I will do is just what I did before: which type of code is each: KOEO (1st set of codes during KOEO test), CM (2nd set of codes during KOEO test), or KOER (if you ran the KOER test)? Becaue KOEO codes are set with the engine off, they almost have to be electrical in nature, and are almost always the best place to start.

 

Can I run the KOER the same way I tested the other codes with a paper clip?

 

Yes, instructions are in Ken00's tech article.

 

Great news! The local NAPA got me a reman. PCM for the internet price and it seems to have cured my problem. In all the problem cost me $110. from a local shop to scan and clean the MAFS, $88. from the local Ford dealership for a scan and advice (they recomended the PCM replacement but estimated @ $500.) and $136. plus tax to the NAPA store. But the advice I got here was pricless. Sounds like a Mastercard comercial. Thanks to all and especially to MRshorty. I think I may have learned something new.

 

Right on!

Glad everything worked out for you.

 

What's with the PCM's lately? Like Mrshorty said - ususally the last thing to go wrong.

www.Fordfueulinjection.com has a good listing of all the codes. For the future, hope you don't need it, you had a couple codes say component's below its minimal voltage. There are mins/maxs for TPS, MAF at least. These codes are supposed to ID a bad component - one that is not operating within its ranges.

Then you had some codes that said higher than expected. These codes that say higher or lower than expected are looking at the feedback values and seeing that they don't align to feedback from other sensors. This is what Mrshorty was getting at. So if the MAF is 3.5V and TPS is .5V, then the TPS is much lower than expected based upon the actual airflow through the MAF. Likewise, I don't know why the PCM wouldn't otherwise say MAF too high for TPS position at nearly closed. 6 of one, 1/2 dozen of another I guess.

Seems to be that when you stat seeing a bunch of just goofy unrelated codes and none of the typical cleaning, replace O2's...have no affect - start suspecting that the PCM is going bad. We might just now be getting to where the Gen 1 Explorers are getting board level component failures - resistors, diodes, capacitors... due to age.