JustJim

I just picked up a high mileage '87 F150 with a 5L EFI V-8. If I 'rap' the gas pedal, the engine will rev up but then stay revved up for about 2 or 3 seconds after I release the throttle, then drop back to idle. I can watch the throttle plates return instantly to the idle position, so I know they are not actually sticking. I assume something is amiss in the idle bypass circuit. I removed the throttle body and cleaned the idle air bypass solenoid hardware and the TPS. I verified that the solenoid activates when energized. TPS appears to be a 3.5K ohm potentiometer with a tap range of 300 ohms to 3400 ohms. The problem persists. Any idea what causes this?
Any ideas or help would be appreciated.
Thanks
Jim

NGM

Your TPS is measured in voltage. The signal voltage to the tps is 5 volts. The TPS runs in a range of .5 - 5 volts. .5ish being closed throttle. Fords of those years had alot of problems with the idle air control motor. and the TPS so you are in the right area. Good luck. BTW Clean the throttle body with throttle body cleaner. DO NOT use regular brake clean or carb cleaner.

JustJim

I discovered that if I disconnect the electrical connector to the Idle Air Controller solenoid, the engine would idle back down as soon as I lifted the throttle. I replaced the entire Idle Air Controller valve/solenoid assembly, but with the electrical cable connected, the engine continues to rev on after the throttle is lifted, just as before.
I then replaced the TPS, since that seemed to be the only thing left to replace. Still no good.
So the bottom line is, with the Idle Air Controller and the TPS both replaced, the engine still continues to rev on for several seconds after the throttle is hit then released. If I disconnect the wires from the IAC solenoid, run-on discontinues, but steady state idle is not quite right.
It seems to me that the EEC is not properly controlling the fuel system.
Vacuum seems good, around 18 mm Hg.
Idle is high, around 1500 RPM but about once a minute, the idle nosedives for a second then catches itself and revs back up. Any ideas?

NGM

Try adjusting the TPS. Some TPS have slots in them. IF yours dosen't cut some and see if that helps. I have done that on a few Mustangs.

JustJim

Please note I have replaced the TPS. The new one did not change the way the engine performed. If the new TPS and the old TPS both result in the same operation, I would tend to eliminate the TPS from my list of possible bad parts.
Also note that disconnecting the wire from the Idle Air Controller seems to solve the run-on problem but causes idle problems. The Idle Air Controller has been replaced but problem persists.
It seems to me that the only suspect components left are the EEC and the wiring. I am only a shade tree mechanic, with no experience on computer controlled engines, so I welcome any technical insight as to what could cause this problem in light of what I have done so far. Thanks.

doughard78

where did u buy the parts??? I have had some new parts be bad right out of the box, won't say any names(autozone ,oriley's,pep boys opps lol)

JustJim

I decided to admit defeat and take it in to the Ford dealer. I'll tell you what they find out. After I recover from the sticker shock, that is.

NGM

If there is some play in the throttle blade it will throw off the tps. If thats the case you could throw 1090234892438594594594656 of them at it and would never get a good reading. I have run into that. That is why I suggested slotting the mount on the TPS and try adjusting it. I have had to do this on my own vehicles. I used to drive some old cars and trucks and be broke as hell and did what I had to do the keep them running. That is how I found all these tricks.

jakewash

I will take a guess at either an intake leak or a bad MAP sensor. I have seen both of these problems cause your situation.

JustJim

Well, I got the truck back from the dealer today. Here is their report:

DG ignition and fuel systems test fuel pump & regulator test. EEC-4 Code N 67-10-41-33 KOEO pinpoint test monitor sensor values found baseline idle not properly set, EGR valve sticking, EVP sensor out of range, injectors restricted, fan clutch locked up. Reset base idle speed, perform keep-alive memory reset, reset emissions module

Truck seems to run OK now, but my wallet is $1K lighter!