I'm so frustrated about my idle issue I could, and have screamed. I've got a 1989 Bronco with the 351 which was rebuilt four years ago and now has 50,000 + miles. My idle on the Bronco is alomst always a pure 1300 to 1400 rpms. On a rare occasion the idle will drop down to 900 rpms. I've replaced the idle air control valve, throttle position sensor, MAP sensor and the computer with no change in idle. No engine light or codes are present and I've even changed out my throttle body with another one at a junk yard. I've been plaged with this issue for over two years and burned up a tranny and going on my second. I've taken the truck in to two Ford Dealerships and three local garages. Everyone has taken my money and said they fixed it but as you have guessed I'm pretty upset. Especially since I've spent well over fifteen hundred dollars on a simple idle problem. Anyone have any ideas I'm all ears. P.S. I usually do my own mechanical work and usually don't trust any garages.

 

Sounds like a vacuum leak. Check all of the vacucum hoses and intake gaskets.

 

Yup, it certainly does sound like a vacuum leak.

 

I'll give it a try and my thought is to replace all of the vacuum lines at one time. I feel the plastic lines are cheap. Does anyone know of a company that sells these vacuum lines? If you cannot buy them what do you guys do to replace them? Thanks for the quick responses.

 

Well tonight I went and capped off my vacuum lines one at a time at the intake tonight with no change in idle. I capped off the MAP sensor and the engine rpms dropped from 900 to 700. The truck ran rough and didn't seem to perform right. I'm figuring the MAP sensor would cause this problem if not hooked up. I then sprayed starting fluid all around and over the air intake system while the egine was running. Did not notice the idle change at all.

O.K., anyone else have any ideas. I did replace the sensors with O'Reilly brands and have not changed them out with Ford parts. I'm clueless.

 

How about your brake booster. That caused my 86 to run with an erratic idle. It had a vaccum leak after my engine rebuild.

 

Tricketts,

I pulled the vacuum hose that supples the brake booster and the cruise control at the air intake. I then capped both systems at the air intake with no change in idle. I was praying the cruise control system had a vacuum leak since it does not work. Thanks for the reply.

 

Sprout,

Out of curiosity, have you ever checked the throttle plate stop screw position? The IAC may work just fine and within its normal parameters (hence no codes generated) but with the stop screw run in too far, the idle will be forced up higher and the computer may be trying to compensate for the discrepancy thereby causing the erratic idle. The idle RPMs you mentioned seemed a bit high to me which is what leads me to this question.

With the IAC disconnected (electronically at connector), the truck should just barely idle and within a minute fall on its face and die. If it doesn't you either have a vacuum leak which from the sound of things, you have exhausted all chances of that being the case, or you have the throttle stop screw run in too far causing the airflow to be greater than normal. Its essentially the same situation as having a vacuum leak. The stop screw is on the top of the throttle body and its only function is to keep the throttle plates from jamming in the throttle bores when they slap shut.

 

Greystreak92,

Thanks for the informative post. I've already backed the screw out where it's not touching. I'll disconnect the idle air control valve tonight and let you know what happened. I just realized the two vacuum lines that supply the throttle body system was not capped last night and could also be a problem. I appreciate all of the great posts and will get back with you all later tonight.

 

So, how do know if the stop screw is not in far enough?

 

The throttle plates will stick in the bores if you let them slap shut. May not be mcuh but the potential is there. The screw only needs to touch the tab on the throttle shaft with the throttle closed. Running it any further in will begin to open the plates and increase air flow at idle.

 

Forgive me everyone for I have sinned. I decided to take my Bronco to the dealership to figure out the idle issue. If they happen to figure it out (Unlike the many shops before them) I will not only consume a beer in tribute, but will post the findings here on the forum.

 

could be a bad pcm

 

Please, please, please post what the dealer says! I've been fighting this idle thing for about 4 years now and have replaced everything anyone suggested and it's still happening! It drives me nuts that this is such a common problem with so many Ford vehicles and I can't find the answer! Well, I haven't tried a dealer yet either.

 

I'm wondering if it doesn't just happen...? My 351 does something very similar. Normally it idles ~800, but on cold mornings it will slip to 600, almost die, then rev up >1000, lather rinse repeat...

HOWEVER, I've noticed that it doesn't seem to do it when it's in Neutral... only Reverse and Drive... Could the cold fluid in the transmission produce a back-pressure on the engine enough to stall it?

I come up with some crazy theories, but I haven't seen anyone else actually solve the problem, so why not?