need help diagnosing an engine problem...

[exploded99]

I like the idea of checking the throttle body and making sure that the throttle plate is not sticking open. I would remove the throttle body and clean it - make sure the operation of the throttle plate is smooth from closed to wide open throttle. While you have the throttle body off, and your throttle cable is loose, move the cable with your hand and feel if it sticks / binds anywhere from idle position to full throttle position.

 

[jekbrown]

first of all, I just wanted to say a general "thanks!" to everyone. I am an old carb'd chevy guy and don't know crap about early 90s Ford FI'd engines... so the info you have gave me already has been very very useful. thanks!

getting on to the specifics...

The IAC isnt going to make the truck shoot up to 3000 rpms.

I've had it get as high as 3500-3600 in the last few days... its still intermitant, but seems to be getting a little worse. Hopefully I can get her fixed fast... need two cars to get to work and there is no way in hell I'm giving my wife the keys to thing thing while it tries to drive itself. lol!

Have you tried popping the hood, and having a buddy start the truck for you while you watched the position of the throttle?

no, but I'll do that. I planned on it already, since someone suggested it could just be a simple throttle cable problem. Based on how it acts, I really doubt its something simply being "gummed up" though... when the throttle is high, if you tap the gas a bit it gets even higher and settles back down to being high again (ie, 3000, to 3500... back to 3000). Also, when coasting on the freeway in stop and go traffic you can hear the engine "blip" up a few hundred (usually) or a few thousand (rarely, but its happened) without ever hitting the gas. If it was something sticking... it would stay stuck after gassing it... not just start winding up all on its own.

Try disconnecting the Cruise control, so we can elimnate that.

First thing I am going to try... what exactly should I disconnect? my K5 doesn't have CC...

This might be off the wall, but is just the RPM gauge going up to 3k or is the actual engine running at 3k?

I don't know if the guage is 100% accurate... but I can hear the engine winding up really fast... its not just a nutty guage, I'd ignore that on an old rig... they go belly up sometimes.

Any progress?

I have kidney surgery tommorow... and will be in the hospital for 3 days. Hopefully sometime next week I can tear back into it and see if I can't figure it out. Based on what everyone has said, I am suspicious of the cruise control. If I knew how to disconnect it, I'd try that first...

j

 

[Creager]

dude, your problem is kind of weird.

My thing is, i dont see how the engine can burp with out the throttle being moved, thats my whole theroy.

The other thing is, the computer doesnt do as much as we all tend to think, it merly tells functions of the engine yes and no. Its almost simpler then old carbed engines yesteryear, esspecially to kids now-a-days who have experince programming computers... pascal, c, basic, dotNet... whatver... it all makes sense.

good luck with the surgery, we're here to help how ever we can.

i cant wait to see what actually is causing your problem.

i ment to ask, are you seeing any other issues when this happen? black smoke? or something?

 

[410Fortune]

I have seen PCM's do odd things on these trucks, but not like you describe.

the cruise control cable is piggy backed on top of the throttle cable at the throttle body. Disconnecting it is easy, just be careful not to break the plastic mount.

Dont forget to check around your gas pedal, I have seen stranger things happen.
Cable could be hung up at the pedal, spring is broken or skipped over, etc.

 

[jekbrown]

The morning before I went under the knife, I went out and figured out which cable was the CCs and pulled it. Haven't had a single problem yet.. not even one throttle blip. Since we've driven the car for several days and a couple of hundred miles, its safe to say that we "nailed it" this time. Thanks guys! Only question I'd have now is, is there anything wrong with leaving the CC disconnected like it is? It won't send any wierd trouble codes to the computer or anything will it? If so, is there any way to further disable to CC so it won't be a problem any more? I'm not going to bother with fixing it, we never used it anyway and I really don't want to go through the time/effort/$ it might take to fix it. thanks again... surgery went ok... feel like poop, but I'm getting better.

j

 

[410Fortune]

The cruise control cable not being hooked up will not effect anything.

Let us know if the problem comes back.

glad you found it and are on the road to recovery!

 

[jekbrown]

uh-oh.... damn near a year now after I fixed the problem by disconnecting the cruise control, I have a very similar problem. The RPMs are not getting as high (2000-ish at idle or less) but its still revving independant of gas pedal-pushing and fighting the brakes. I double checked the cruise control cable thing and its still disconnected.

I was reading a post on ck5.com about TPS sensors and one guy was saying that aftermarket ones tend to suck. I got mine from Napa... any chance it went belly-up on me in a 9 month span? Maybe I'll try to swap the OEM one back in (since it wasn't the problem in the first place!) and see if it calms down. Any other ideas? The problem is very similar to the way I described it in the original post... its just less extreme... RPMs are quite a bit lower... but still too high for idle.

j