my 91 f150 idles at 1200 rpms i have tried everything to idle it down changing the iac and throtle positioning sensors but no luck. i had a guy tell me there was a idle screw for the throttle body butterflys but i cant find it. it does idle down with the air on but only to about 800 rpms. the truck is a five speed. could i have a vaccume leak or is it something else. also i did move the outer stop by the throttle linkage but to no avail. thanks.

 

i think it just might be the way these trucks are. when i first start up my truck my idle goes from 600rpm to almost 1600 and then back down to 600. it does this numerous times and i dont know why. also the screw to set your butterflies should be by the end of your throttle cable running from your pedal to your throttle body, right above your first butterfly on the outside on top

 

thanks, but my idle dosent go down after warmup im not quite sure what it is. i tried unscrewing the screw that you talked about but no luck. thanks for the post though.

 

You may have a vacuum leak keeping the idle up.

The adjusting bolt is under the factory black roundish cover on top of the throttle body "4X2Mudder shows" you may be able to "knock it down" with the adjusting bolt .........

But if the idle was normal before, then you are just covering up a problem, not fixing it, by turning this bolt out.

Plug off the vacuum line to the PCV valve with your thumb/Vacuum cap/ or a big bolt.

It should drop your idle WithOut adjusting the screw. And it should run smoother with the line plugged. This would tell you your getting an internal leak comming through the crank case via the PCV.

This is why it's better to buy the more expensive PCV's in my opinion.(if you use them)

But Seeing how the PCV dose not "help" the engines preformance at all, and is just an emmissions part.

I can tell you how to fix it and not ever worry with it again......

Pull the the hose off the back of the metal elbow on the back of the intake.
Unscrew it out of the intake and take it with you to Home Depot.
Go to the plumbing section where the brass fittings and connectors are.
You will need to match the threads to a short Double Male Fitting (@ 3/4 " of threads on both sides) then you will need a brass cap for it, and some high temp. teflon tape.

Tape the threads and bolt on the cap to the double male fitting, then add some more tape to the other threads and screw it in to the back of the intake. I did mine 5+ years ago, never worried with it again.

Now buy 3' or so, of vacuum hose to fit the PCV valve.
Put the hose on the PCV. Then replace the PVC back in the valve cover.
(givin room for slack/ slight curve will do ) run the hose over to the plastic inner fender well, down to the side of the frame , and hold the hose in place with zip-ties attached around the hose and through some of the existing small holes in the frame and plastic inner fender. Once they are fastened tightly enough. Trim the hose to the bottom of the frame leaving about 2-3" under hanging.

The PCV will still breath this way and you wont get any moisture in the covers from an open hole of no PCV there.

You may see a puff of light smoke occasionally at the stop light when the crankcase "burps" itself . Normally this would be sucked back in to the intake .

And you wont have any more future problems with a possible air leak comming from the crankcase via the PCV. I did mine 5+ years ago and have never had another problem there again.

Get this............Some people run the cover hose (PCV) up to holes in there hoods. The hoods will have a small fake scoop(s) and the light burp of the smoke makes the hood scoops look like they are "breathing" slightly.

The smoke is just the mynute amount of combustion gas that slips past the piston rings into the crank case. It's just a light thin single blow off ,puff that will bleed out , as your sitting at a stop.

It gets there from the prior engine rpms , it's hardly noticable most of the time.

This is just a neat way to fix a problem , and change it to some style at the same time with no harm to anything.

I would not adjust the idle by the bolt untill I went over all the other hoses and lines first ( in fact the little black and yellow factory sticker on the side of theT.V.Bores tell you not to adjust the bolt .......But....)

Just snip back a section of the vacuum hose ends and replug the connections.

I have done plenty of modifications to my 1991 - 5.0 (pictures in profile), And it is now 1 hassle free truck, spacious and easy to work on when needed,

I even got rid of the A/C , (we have vent windows and sliding back glass), but the a/c radiator condensor (in front of the radiator ) is soon to be a super effecent transmission fluid cooler.

And look closly at the right side of the throttle valve , theres 2 vacuum line plugged there (1 on top of the other) that tend to be well hidden places of those sneaky leaks that develop over time.

 

I have a 92 F-150 with the 5.0. I had the same problem, the engine just ran fast at all times, I wore out alot of brake pads just trying to keep it slow. It turned out to be the intake pendium gasket was blown. Once I got that fixed it runs great now.

 

you shouldn't play with the screw to adjust the idle if you have an AOD you could ruin the trans by dropping the line pressure too far down it's really easy to do

 

Isn't there a sticker on there that says to "not mess with this screw???"

 

Does the idle drop very low if you disconnect the air bypass valve? If so, you may have a misadjusted throttle body or TP sensor. The TP signal has to be in a specific voltage range at idle or the PCM will go into part-throttle mode and open the air bypass valve. You need a shop manual and follow the procedure to set the base idle.

If the idle speed doesn't change when you disconnect the air bypass valve, you probably have a vacuum leak. Vacuum leaks can come from the uppper or lower intake gaskets, broken vacuum lines, injector o-rings, or from a bad brake booster. Carefully use a can of brake or carb cleaner and spray around all of the intake gaskets. If the engine speeds up, check for bad gaskets. Also, disconnect the vacuum line to the brake booster and plug it. If the idle comes down to normal, the booster is bad. Also, verify you have the correct PCV valve, as they have different flow rates. For the two or three dollars they cost, replace it if you haven't already.

 

Hey tclymer83,
I had the same problem on my 90' f150. The engine just started to run at high idle all time. Checking for vaccum leaks etc.. I found out that the intake plenium gasket was leaking/blown. Once I got a new gasket installed the leak/high idle problem was fixed and it runs fine now. Good luck!

 

I'm getting in late on this thread but I have a somewhat similar problem. I just bought a 1988F250/5.8L/5spd. Upon startup the idle wavers from 800 to 1300. After warmup it stays at about 1200-1400 constant. The seller, a friend, said he had a general local shop look at it. The mechanic said the throttle body (dual throat) butterfly shaft bushing was worn between the two throats. He claimed this causes the butterflies to stay partially open, leak air, and the PCM keeps trying to adjust the mixture by raising and lowering the throttle. After it gets warm it idles high. That doesn't sound completely consistent to me but maybe it is. I tend to think it is the IAC or maybe the problem he described. Anybody else hear this claim about the throttle body leakage issue causing this? If I can't get an answer I may have to use justask.com again. It isn't too expensive at about $20 per answer from experienced Ford techs.