Can anyone please tell me how to adjust (lower) the idle speed on my 92 Bronco 351W EFI motor?

Thanks, Dale

92 Bronco 351W EFI
71 F100 360

 

What specific prolems are you having? I think its all compter controlled. You might have one of the many sensors or servo bad, but probably have to run a diagnostic to find out for sure. I have an `89 model that decides to vary the idle at its discression. I would also be interested for some insight. I have been reading up a lot recently on what all of the components do on the EFI motors, most of which makes sense. But there can be more than one sensor causing similar problems. When my check engine light comes on, the idle will stay dropped down to 650-700 range. When the light is off it can hold around 800 or move to 1200 and then rise up to 2200-2800, where it wants to stay. In gear it loads down to 1300, which cant be that great on a automatic tranny sitting at stop lights alll afternoon.

 

fordtrucksforever is correct the idle is computer controlled.

If you check engine light is on, check the codes.
If not a possible culprit is a vacumm leak. I had a similar problem, it idled at about 1800 rpms when warm. Turned out to be a vacumm leak on the lower intake manifold.
Inspect your vacum hoses first. Then to check for vacumm leaks around the intake manifolds, take some carb cleaner and spray in around the gasketed joints with the engine running. What happens is the carb cleaner will temporalily fill the hole in a bad gasket, and cause the idle to drop to normal.

hope that helps.

89 351 bronc
65 mustang

 

My code list

 

I too was having problems with adjusting my idle. I knew all my codes were clear and yet the idle was difficult to control. I even took off my throttle body and cleaned the backside (which was quite dirty). Eventually I went to a hardware store and added an assist spring to draw the throttle blades closed. My Bronco is a 90 and I gather that the coil wound spring had lost some of it's potential. I struggled with the idle problem for quite a while before I did this. I also ensured that all codes were clear and the the vehicle was running properly prior to installingb the "assist" spring.

 

I dont think, and I can very well be wrong, the throttle valve itself is how the idle is set. The valves are supposed to be closed up against the body. There is a small bypass hole before the valves and directed to the other side of valves, and the opening is controlled by a small servo. The bypass adjusts airflow through a spool connected to the servo that opens and closes the bypass to effictively get the desired rpm from computer. Many times this assembly just needs to be removed and carefully cleaned up. My engine originally started up idled down below 450 then died. After cleaning, it overcame that obsticle. Now its off to the next hurdle.

 

My '92 does have an adjustable screw in the throttle body that does move the butterfly's if you turn it. However, since altering the depth of this screw could (and most likely would) be more detrimental than locating an electrical or vaccuum-related problem I marked the screw and use a dab of RTV to ensure it doesn't move under extreme vibrations. With the close tolerances that the computer controlled engines run under, you run the risk of throwing too many variables out of range and creating more headaches than solving the initial problem.

 

You should NEVER adjust the throttle stop screw on an EFI, unless you're SURE someone else did and you're trying to put it back where it goes.

Start the engine normally and, with screwdriver in hand, unplug the IAC. The idle should drop sharply and the engine should ALMOST be able to run, but it should die within a couple of seconds. Adjust the screw to get this effect, then plug the IAC back in and clear the codes.