My '92 F250 5.8L has not been running right- its has a rough idle (rev real high then low like it is going to cut out then back high again), and while driving it does the same thing. I never had the check engine light come on until today (it started shifting hard when the light came on) and from browsing the forums I was able to check the codes.

I looked up the codes and from the engine-off test it looks like a TPS issue (code 123), an EGR issue (codes 327, 328, and 334), and a Transmissions issue (code 634). From the engine-on test it looks like there is an issue with the ISC (codes:411 and 412) which I'm guessing is the IACV?

So my question is- what do I do?!

I was hoping to make this truck last a while and because I don't know much about any of this I'm starting to get worried that it may be close to death. Can anyone help me figure out what to do to fix these issues?

EDIT: The catalytic converter was removed a few weeks ago, I dont remember it doing this before the cat was removed but I'm not sure (removed the cat a few days after purchase cause it was clogged). Is it possible this is all stemming from the sensors not working right after removing the cat? All I did was cut it out and weld a through pipe on it, I didnt change out any sensors or anything. If this is a possibility, could any one point me in the right direction of how to make the proper adjustments?

 

The transmission code is going to be a white/yellow stripe wire and a white/red stripe wire coming up from the tranny right under the master cylinder. The red/white stripe should have power when the truck is running and thats the cause of any hard shifting. This puts the tranny in default mode and all pressures are then run at their highest.

334 is the pintel sensor on the back of the EGR valve not getting a reading or resding too high a voltage. There are two replacements for this one is grey and the other is black and trust me when I say this the grey is the one you probably want.

The other codes I'd just have to look up and see but BEFORE you change any parts check all your fuses thats alot of codes all at once. As far as the engine on test it is mostly irrelevant until you get all clear codes on the engine off test.

 

Thanks for the response! So first step is to check the fuses, how do I know if a fuse is bad? Will there be a visual indication? And if things need to be replaced are you saying that I just need to replace a few wires and not actually replace any parts?

 

Fuses will have a visual indication or just check them with a continuity tester out of the vehicle. Then check wiring ad if there is a bad wire repair it to the affected sensors. Basically look for the low hangng fruit before ou throw cash at it. Also the 334 and 634 I believe ae "stored codes" so erase them. If it will not let you erase them they are stillcurrent.

 

Looks like you may have a bad PCM Computer.
Pull it out and open it up and see if you have leaking or bad caps in it.
It looks like the 5 volt voltage power supply has the bad caps.

 

I didn't get a chance to look at it today but I will tomorrow morning before work. Once I find where the PCM is, am I just looking for corrosion/damage? Is there any way to test this other than just looking at it?

Just some more information on the issues I'm having: As stated in my first post it has a rough idle and sometimes while driving the engine revs up and the truck seems like it has a mind of its own, like it wants to drive itself. I've read it could be a vacuum leak but I would think if that was the case then it would happen all the time, is that faulty thinking? Until a few days ago my check engine light would not come on. Yesterday it was having these issues but also started shifting hard, then my check engine light would come on and off while driving. There is a leak in the exhaust manifold ( makes a tick tick tick noise) that I haven't been able to fix yet and I had the catalytic converter removed last week because it was clogged.

I plan on looking at the fuses, wires, and the PCM but does anyone have any other ideas of what it could be, or how serious it sounds? Thanks for the help!

 

The PCM Computer is located in the drivers kick panel.
But it has to be removed through a gap you make between the inter and outer drivers fender. It has to be unplugged first from under the hood with a 10mm socket.



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Okay, I just got around to looking into these suggestions. I checked all of the fuses and they are all good. The power relay on the bottom right of the power distribution box (PCM Power Relay?)- there is no visual way of telling if it is good or not. I'm guessing that could be causing my issue if it was bad, is there any way to test that?

I took my PCM out (could someone verify that I actually took the PCM out and not something different?) and looked at the connections, they don't look good IMO but I posted pictures of them at the end of this reply, do they look okay?

I saw two other things that didn't look right to me. My air filter box is cracked, if the box is not sealed properly could that cause a vacuum leak? Also, there is a hose that runs from my air filter box to (what looks like) the engine (right above where the oil fill-up cap is). It was not completely attached at the air filter box. I reattached it, could that have been causing issues as well?

If all of these things check out okay, would my next step be to replace the MLPS, EGR, TPS, and clean the MAF?

I'm sorry this is so long, I really appreciate all of the help!

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The photos are of the PCM Computer and its plug.
Just make sure there are no pins push back in the plug that have a wire on them.
A crack in the air box will not cause a vacuum leak but may let un-filtered air into the engine.

 

UPDATE: put a new tps in and it fixed all the problems!