slimslow

I ran into a strange issue with a 1992 F150 4X4 I just bought.

It has a 302 with a 5-speed manual tranny. The idle is much too high most of the time, (but not always), around 1200-1500 rpm. I can goose the throttle a couple times and it usually settles down to 750 or so. But then when I begin to drive, as soon as I press the clutch in to shift, the RPM's soar to 2500-3000!! As soon as I find the next gear, it feels like the truck is on cruise control... press the clutch again, and the RPM's soar again. There is no check engine light on.

So far I have unplugged the IAC when it's running, and the truck begins to stall... removed it, cleaned it, measured it within specs and reinstalled it, so I've ruled out the IAC.

It looks as though the previous owner recently replace the TPS as well. Although when I tried to measure with a DVM, I barely have any voltage, .001 or .002 and it doesn't increase as I move the throttle plate. Now if I move my (-) cable on the DVM to the (-) of the battery, I get a voltage reading, but they are nowhere near specs.

During the original test, I had my (+) lead from the DVM to the Green "TPS" Wire of the TPS, and the (-) lead from the DVM to the Black "SIG RTN" Wire of the TPS.
I had the TPS unplugged from the harness as well (as indicated by the Chiltons manual).
Did I perform the test incorrectly??

Any ideas or suggestions are greatly appreciated!!!

BigGreyBox

sounds like a vacuum leak to me. Have you checked all your vacuum hoses?

slimslow

I checked all the vacuum hoses, and I only found one that was suspicious, so I split it and installed a vacuum line connector between the two.

Didn't change the performance at all.

ymeski56

When I'm reading low voltage or resistance, I've found it helpful to throw in some fresh batteries in the tester, especially if it looks like it can't decide on a reading or out of expected read range.

slimslow

Good Idea! Meter has been sitting awhile anyways.
Sometimes it's the simplest things...... Let's hope. I'll let you know what I find out.

slimslow

OK, new batteries in the meter and the TPS tested out of range. Replaced it, cleaned the throttle body and plate while it was apart, and installed a new gasket. Ran perfectly for about the first 15 miles, now it's back to same surging problem when the clutch is depressed during actual driving.

I did notice, however, that if I pull over and shut it off and restart it a couple times the problem goes away. If I restart the vehicle and the RPM's don't immediately drop below 1000, then the truck will act up during driving. If I start the truck and it immediately drops to 675 or 700, then it drives perfectly... Any ideas, anyone???????

Battery was disconnected during the TPS install, so it should have cleared any Keep Alive Memory that was remaining.

ymeski56

Lube the throttle cable and check the throttle's position when the your RPM's are wacked (determine if it's physically hanging up). Lube from both ends, look for binding.

hackersmovie

What about the "idle voltage", As I understand it, the computer is real picky about the TPS being set at something like .98 volts or thereabouts. I forget the actual voltage but, I know I've read somewhere that it is specific.

slimslow

I had the same thought about the cable, so I ordered one from Ford and replaced it. Same issue still. But even after I replaced it, I noticed that the very first time I start it in the morning, the gas pedal is like it has a very slight catch, and then it's fine the rest of the day...

I measured .90 on my voltage after the TPS was swapped, and I thought I was home free. The truck started easier, and idled as you would expect it to. The problem came back after I drove it a few miles, after the TPS replacement.

What's everybody's opinion on vacuum leak testing?? How do you guys do it???
The problem has to be there... I can't think of anything else.

Oh, and no codes now....

ymeski56

Do you have an adjustable , or non adjustable TPS? Also have you ever tampered w/ the throttle adjustment?