I have an 89 Ford F150 4.9 that when idling the RMP's go between about 1500 down to about 500 and back up and back down. I cant seem to get a wrench on the idle adjustment screw but I think this wouldn't fix the problem anyway. Does anyone know what could cause this?

Truck Information:
1989 Ford F150 Custom 4.9L Inline 6-Cylinder 300 4x4 Manual Locking hubs 4 speed(1 = granny) 8-ft bed and standard cab. Fed. Emissions.

 

When my '85 started doing that, I checked the vacuum lines and had to replace a few lines and some caps, then she ran just fine. I'm not saying that's whats wrong, just someplace to start. Maybe check your spark-plugs and dizzy cap too. You could even run some Seafoam through it.

 

im goning to guess that your tps may be bad.. ist an easy ans cheap fix

 

Try researching here...also the screw is not for that

Ford Fuel Injection » Home

 

How would I test the TPS? would it hurt to just unplug it? I figure if i unplug it and nothing happens everything still works the same then the sensor is bad.

I took a loog at the vaccum hoses and i will take a deeper look.

Thank you guys, I'll get back to you.

 

Some good info in this thread: https://www.ford-trucks.com/forums/1...code-53-a.html

The Idle Air Control (IAC) is also well known to cause idle issues. Cleaning or replacing it has been a common fix as well. A hunting idle like yours could be a combination of sticky IAC and a vacuum leak(s).

 

I had that same problem with my 89 f150 4.9. Take the IAC valve off, take it apart and clean it with carburetor cleaner. If that does not work then you will probably have to buy a new one, which I had to. If that doesn't fix your problem the next thing is your TPS. I'm not sure how to test if it is bad. After I replaced my IAC my truck still idled like yours did some so I replaced my TPS and now my truck Idles like it is new.

 

And if you do buy new parts go with motorcraft. This is my third IAC and TPS including the original.

 

Wouldnt all of those cause a check engine light?

 

A sticking or stuck IAC will not typically trigger a Check Engine Light. A TPS that has a dirty internal wiper will not necessarily trigger a CEL either. As long as the closed throttle output signal falls within 0.6-1.0VDC the PCM will be happy. A TPS with a dirty/bad wiper will output a signal of varying amplitude every time you move the throttle. Because of the ratchet function built into the PCM programming it is always looking for the minimum output signal then re-adjusting based on that input. When the TPS returns to a somewhat normal state, the PCM thinks the throttle is open more than it really is so it increases the duty cycle to the IAC which in turn increases the idle RPM.

 

So, IAC or TPS first?

 

Measure/test the TPS, then replace if necessary. If the issue is still there, go for the IAC.

 

I have replaced the IAC. pretty easy. I measured the TPS, I think right but if so its good too. What else could it be?

 

What was the closed throttle value? Did you open the throttle slowly while watching the output signal? Were they are indications of a bad spot on the wiper?

How did you check for vacuum leaks before? the visual and carb. cleaner spray method are not always the best way to find leaks. I use a hand held vacuum pump/gauge to test each circuit.

 

Well I broke down and took it over to a mechanic that helps me out every now and then left it with him because he didnt know about 4 hours ago. He called me and said its a bad fuel pressure regulator.