My 1992 F150 4x4, 5.0 EFI, 5spd manual is revving up when I push in the clutch. Not sure where to start, clutch, sensors or throttle. Any help would be appreciated. Thanks. -Al Nobles

 

Inspect for vacuum leaks. Clean your IAC valve. Perform an idle reset.

Steve

 

Does it rev more when you push the clutch than it does in N with the clutch pedal released?

 

Yeah. It revs high, and then if I just coast and hold the clutch in for about 50 feet or so, it goes back down to normal idle. I don't know what the hell it is. I'm planning on replacing all of the vaccum lines this weekend.

 

So, just so I'm understanding the symptoms:

When you're driving, and you let off the throttle & slip the trans into N without touching the clutch, the engine does NOT rev up.

When you're stopped with the engine idling & the trans in N, and you press the clutch, it revs up.

Is that right?

 

Thanks for your help. Not quite right, but sort of. When I'm just driving normally, and press in the clutch to change gears, it revs high. Then, I can either hold down the clutch for about 25-50 yards and it goes back down, or I can tap the accelerator and it goes back down, just like if the choke were stuck or something. I went ahead and cleaned the IAC valve, and that didn't fix it. So, I just replaced it. I figured, what the hell. It's $70 bucks, it's going to go out eventually anyway. I thougt it fixed the problem, but then today, it started doing it again. This is an old truck that I'm planning on fixing up and keeping 'till I die, so I don't mind sinking some $ into it to get it right. Besides, it annoys the hell out of me when it's not running perfect and I refuse to take it in until I've tried everything short of replacing the engine. Thanks again for any advice you might have. -Al Nobles, Fort Mill, SC

 

OK So what happens in the 2 situations I described?


At this point, it sounds like you need to just pull your floormat back. Seriously.

 

Good point. Try unplugging the IAC and retesting. This will determine if you have an electrical (eec related) or mechanical (e.g., floor mat) problem.

 

What do you mean about the floor mat? Do you think something's in the way of the clutch pedal or the clutch safety mechanism or something? I looked under there first, and all I could find was a tiny broken piece of plastic that attatches the clutch pedal arm to the plunger. I fixed it with a small clamp and it seems to be fine. I've been suspicious of that ever since, since it does seem to be clutch related vs. throttle. However, what else could be wrong with it? It just clips on to the plunger...I think. -Al

 

Floor mats are notorious for catching the gas pedal if they slide too far forward.

The clutch pedal has nothing to do with the throttle, so it can't be the CAUSE of this symptom. What happens if you try the 2 things I described above?

 

Only in a round-about way and it would only be the trigger to the event.

Then the clutch pedal is depressed, the CPP switch tells the PCM that the event has happened. The PCM responds by making adjustments to the IAC valve's duty cycle to compensate for the change in engine load.

When the IAC valve fails, idle speed control is usually lost resulting in an overspeed engine condition.

You're still looking at an IAC fault like I mentioned earlier.

Clean it or replace it, then perform an idle speed reset.

Steve

 

But immediately before a shift, when the throttle is well above idle (closer to WOT), the IAC duty cycle is nearly 0%, so if it was sticking, the idle would fall to stall. If the IAC was stuck open so badly that it was still open after he went thru a gear, it would be stuck open all the time - it wouldn't drop a second after pressing the clutch.

That's why I think it's a mechanical throttle issue - not IAC or EEC.

 

Hey Steve. Thanks again for the help. I replaced the IAC valve. I'm not sure if it needed it or not, but I did it anyway because even if it was okay, it will probably go bad eventually. How do I do an idle speed reset? I haven't tried the other thing you told me to do yet, and I don't have floormats in the truck. I'll try it tonight on my way home. Thanks again.
-Al Nobles, Fort Mill, SC

 

Steve83: Consider the requirements for dashpot control function....

Al: Disconnect the battery negative terminal. Wait 10 minutes and reconnect (there's a fuse I usually recommend pulling instead, don't have my notes handy...). Turn all loads off. Start engine and let idle for half a minute. then, one at a time, turn on headlights, step and hold the brake, turn on AC and give each 15 seconds or so before proceeding to the next. Go for a test drive and drive normally. The computer will relearn how to control everything. Within a few drive cycles, it will have made its greatest corrections.

Steve

 

Thanks again for the help. I'll do it today. -Al