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Torque454 12-04-2012 06:31 PM

87 Ford 5.0 revving up and down and occasionally stalling
I am having a problem with my 87 ford f-150. The engine revs up and down randomly at idle and sometimes stalls, especially when I pull into my driveway, right before I stop. Half the time its ok the other half of the time it revs up and down. I suspect a vacuum leak but I cant find one anywhere and i would think a vacuum leak would be a constant problem not an intermittent one. Is there anything else it could be?

LATECH 12-04-2012 06:37 PM

It may have a bad IAC (Idle Air Control)
There are a few other things also.
Is The Check Engine light On.?

mrhp 12-04-2012 07:33 PM

Either I.A.C. valve or T.P.S. Both commonly fail and would give you that drivability concern. Remove and clean I.A.C. and see if problem gets better/worse. Usually an I.A.C. valve will act up upon first start of the morning, but not always. On the T.P.S. backprobe the 3 wires going to it. One is ground( black I think) the 5 volt reference is ( orange I think) and the return sensing wire is green, I think. With the key on engine off, backprobe the green wire with the throttle at idle. Should be around 1 volt. Slowly have someone step on the throttle and you will see the voltage climb to appx. 5 volt at W.O.T. If the voltage increases cleanly with no drop-out or spiking, it is O.K. My money is on the I.A.C.

Torque454 12-04-2012 09:33 PM

Thanks guys. There is no check engine light on. I should also mention the hesitation it has that is cured by giving it a little more throttle. Does this sound like a possibly related issue?

Torque454 12-04-2012 10:38 PM

OK i went out to test it and couldn't figure out how to get the probe on the sensor wires without poking a hole in the wires insulation. So I just disconnected it. It wouldn't idle but it seemed to possibly run better. It certainly didn't run any worse, except of course for not idling. It did however seem to still rev up and down for the second or so that it did idle before stalling so I suspect there is another problem, but I do think the TPS is bad as well.

mrhp 12-05-2012 03:45 PM

Take a pin and push it alongside the wire where it goes into the connector. That will go in and touch the wire crimp and not hurt the integrity of the weather pack connector.

Torque454 12-05-2012 07:33 PM

I cleaned the IAC tonight with carb cleaner and it did not change anything. It was dirty, but its still doing the same thing. It may not be as bad (wont know until i drive it more) but its still doing it.

I think it might possibly be something to do with the throttle position sensor but i dont know. I disconnected the throttle position sensor and drove the truck without it and it acted the same as it did when it was plugged in, with the exception of the fact that it wouldn't idle.

Other symptoms that I think are leading to the TPS are: hesitation at light throttle - cured by giving it more throttle. Jerky at low speeds.

LATECH 12-06-2012 03:25 PM

You can clean an Idle motor and the throttle body, but not the TPS. They are sealed. It wont happen. If the tps fails a sweep test replace it.
My advice is for you to get a Key on Engine Running diagnostic performed with an OBD 1 Scanner to see what codes it generates. It will generate a code, and you can use that to determine which flow chart ( Greek for diagnostic procedure) to use to further the diagnosis.

After doing the basics...Like cleaning the IAC and the throttle body you are pretty much done with theeasy stuff.
Injectors on those are very troublesome and get dirty fast. That can affect the idle if an injector is spraying wildly.
Try running Seafoam in the Gas tank for the next couple of fill ups, it cant hurt.
I suspect there is more to the problem than that, like I said it could be many things.

Torque454 12-15-2012 05:30 PM

The problem seems to have gone away after changing the oxygen sensor. Truck drives alot better now, no more hesitation on acceleration either.

Irelands child 12-15-2012 07:17 PM

An '86 Mustang that I owned used to fail TPS's often - I ended up carrying a spare in the glove compartment for a 5 minute change out whenever....

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