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Old 12-04-2012, 06:31 PM
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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?

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Old 12-04-2012, 06:37 PM
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plenty.
It may have a bad IAC (Idle Air Control)
There are a few other things also.
Is The Check Engine light On.?
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Old 12-04-2012, 07:33 PM
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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.
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Old 12-04-2012, 09:33 PM
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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?
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Old 12-04-2012, 10:38 PM
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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.
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Old 12-05-2012, 03:45 PM
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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.
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Old 12-05-2012, 06:32 PM
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This happens all the time. Just take the iac off the throttle body and clean the inside with carb cleaner get all inside the ports and around the piston. Then reinstall and drive.

Is problem fixed? Drive it for a few months and see if ti starts acting up if so clean the intake and check the valve seals and clean the iac again.

If it doesnt fix it replace the iac.

Simple its always the same thing dont hurt your self looking for any other cause. This happens to all 5.0 80s fords. Single or dual throttle body. Car and truck.

Hope this helps. I see your last photo posted is what looks like the iac so i am assuming you got a new one.
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Old 12-05-2012, 06:36 PM
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Quote:
Originally Posted by Torque454 View Post
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.
Dont bother its not electrical. If the iac was bad it would not move around and run the same speed all the time. Its moving cause it is clogged and rpm goes down faster than the computer can tune. So it keeps trying to adjust:pimp the iac to keep the engine running at its programmed rpm but that position is jammed up with carbon and oil goo so it just keeps hunting as its commonly called.
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Old 12-05-2012, 07:33 PM
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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.
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Old 12-05-2012, 07:58 PM
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Quote:
Originally Posted by Torque454 View Post
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.
Usally you can unplug the iac motor it should stop the madness. Try to clean the tps up with wd40 some of them can be cleaned. They are cheap and so just replacing it is not a big deal to be sure. Motorcraft ones seem to last the longest but cost the most.

When you cleaned the iac did you clean the throttle plate where it seals up around the body. Also the holes that lead around the throttle is where it gets clogged. Make sure you can see spray coming out the back side when the throttle plate is open and you spray into the holes.

Also it will take a few minutes for it to smooth out. unhooking the battery for an hour should reset the computer and force it to start over from scratch with factory settings. Once it gets confused it will start up in the wrong mode and try to postion it back to the old spot that wasnt working. Best to clear the computer and start it learning over.

When you reinstall the tps try to twist it out of the same postion it was in before. Computer will adjust and some times you can move it around a bad spot. You can test it if you have an ohm meter just connect to the wires and open the throttle and see if the ohm meeter goes flat or jumps aorund when you move on and off idle. Car does not need to be on of course. I think it should be around 5 ohms at idle. But its been a while.

Sorry for the book i have done this so many times just trying to help.
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Old 12-06-2012, 03:25 PM
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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.
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Old 12-15-2012, 05:30 PM
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The problem seems to have gone away after changing the oxygen sensor. Truck drives alot better now, no more hesitation on acceleration either.
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Old 12-15-2012, 07:17 PM
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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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