Idle speed for F250

[Freightman]

My old 1989 F250 351 W C6 tranie has developed a fast idle 25mph. How do I set it back?

[Mk42gunner]

First thing I would look for is vacuum leaks. Second is the Idle Air Controller, it is the little fitting that bypasses the throttle body butterfly.

Robert

[Freightman]

Thanks will see if I can do it.

[djohns28]

you get it figured out?

[brass410]

ck them vacuum hoses rigourously, especialy the ones on the pass side rear there's lots of chafe points there at the rear. sometimes to check for leaks I shut down all sources of noise, radio, close shop door, total silience is a must, then 1 by one remove hose from vacuum applied side, then very carefully blow air from mouth or air comp (low press is key) and search for leak either with soapy water in a mister bottle or listen for a hissing noise, works for me 99% time, I also have a high quality traceing microphone from stellman but I like the air press better.

[Alan in Vermont]

GMs from that era would develop a leak in the gasket@ throttle body to intake manifold. The computer translated it to a running lean and would compensate by increasing fuel flow the symptom was an increase in idle speed that got worse over time

[Petrol & Powder]

I will second the advice to look for a vacuum leak.

[Freightman]

Haven't had a chance yet redoing a bath and I am old and slow. The vacuum leak is likely it as I have never replaced any lines and I am the original owner.

[abunaitoo]

Easiest way is to get a can of spray carb cleaner.
Brakleen will also work.
Engine running.
Spray it around the hoses, base of the throttle body, plenum to manifold, and where the manifold meets the head.
If there is a leak, you will know it.
Check engine light on?????

[brass410]

Easiest way is to get a can of spray carb cleaner.
Brakleen will also work.
Engine running.
Spray it around the hoses, base of the throttle body, plenum to manifold, and where the manifold meets the head.
If there is a leak, you will know it.
Check engine light on?????
I've tried this tech as well, but find it hard to specificly locate leak as surrounding air becomes fuel rich and engine speeds up anyway. not saying doesn't work because I know it does and I've used it, but I always find it a bit more difficult to notice rpm change unless leak is truly significant.

[brass410]

for some reason it sticks in my mind that there is a hard plastic vacuum line (form fit) that runs from the R/C on pass side around the back of engine and it rubs sometimes on the A/C plenum/housing and gets a hole in it. don't take this as gospel truth, its been awhile since I worked on a super d, my neighbor has one I'll try and get a look at it tonite and refresh the file 404 I'm getting right now.

[HATCH]

I would check the vacuum line that runs near the AC blower motor. Passenger side of the engine.
Don't use brake cleaner as it will damage the coating on the manifold.
Carb cleaner will spot the leak in under 15 mins.

[megasupermagnum]

If it is the IAC, often you can tap it lightly, and if the idle comes down, that is the problem. They can be cleaned. Also make sure your throttle is actually closed, simply close it with your hand. Sometimes the backside of the butterfly can build up carbon to the point they start sticking. This can also be cleaned. More than likely it is a vacuum leak as others have said. Any of the lines can leak and cause it. The very first place to check is the vacuum tree itself. Often the caps are what crack and break. The lines Ford used, while brittle, seem to hold up decent. Some say not to use carb spray for danger reason. I use carb spray myself. Don't go nuts near the exhaust, just little bursts on the vacuum lines and fittings, and you will be fine.

[Lloyd Smale]

older gms you could pull the idle air controller off and clean it. usually cured problems. don't know about a ford though.

[dagger dog]

Check the hose to the pcv where it attaches to the rear of the upper plenum at the firewall, it's a rubber elbow.

[popper]

And the carbon canister!

[fixit]

throttle position sensor is a potential for that, they can carbon track and stick on different rpm settings. had a chevy that would 'idle' at 2500 until tps was replaced!