Help! Fast Idle Issue Is Driving Me Nuts...

[Unthrottled]

I'm having trouble diagnosing an intermittent fast idle problem on my stock 1994 Bronco XLT, 5.8L. I'd sure appreciate any ideas you might share.

The problem occurs sporadically both in Park and in gear while I'm driving. It's worse when I first start the car up. After a hour or two of driving, it usually disappears. The more the car sits, the more frequent the problem. The more I drive the car, the less often I get a fast idle.

I can sometimes get it to go away if I put the vehicle in neutral and rev the engine. But this doesn't always work.

I can also sometimes get it to go away if I turn off the car and restart it. But this also doesn't always work either.

I replaced the Throttle Position Sensor, but this did not fix the issue. Also replaced the Idle Control Valve, but no improvement.

My mechanic friend wants me to keep throwing parts at it. I want to diagnose the problem first.

Oh, two last things. The problem first started after the car had sat unused for a few months while I was moving my family across the country.

Also, I occasionally get a rough idle at start-up.

I don't know if that might help someone with more mechanical knowledge than me narrow down a diagnosis.

I'd be very grateful for any ideas or suggestions. What could be causing my fast idle?

Thanks!

 

[Tiha]

There is always checking for vacuum leaks and that is a great place to start, but since you said it happens more often when it sits for a while.

I am wondering if you have a dirty, sticky throttle body. Or even throttle body linkage or accelerator cable.

Fire the truck up, open the hood, manually the throttle linkage. See if it will stick, see if you can manually close it farther when it is idling high.

 

[Unthrottled]

Great idea, thank you for offering to help. I appreciate the suggestion and I'll give it a try.

Again, thanks.

 

[johnnyreb]

Great idea, thank you for offering to help. I appreciate the suggestion and I'll give it a try.

Again, thanks.

Try oiling the gas linkage. Even behind the gas pedal

 

[L\Bronco]

Hey unthrottled, I think Tiha is on the right path, could be a dirty throttle body or an incorrect base idle setting AKA hard stop setting.
Try unplugging the idle speed control solenoid in neutral warmed up. The idle should be about 500 to 550rpm.
Let us know what you find

 

[johnnyreb]

Since it will go away--WHEN YOU REV IT UP. It is WARMING THE MOTOR UP MORE----which COULD CAUSE THE METAL AND GASKET TO SWELL UP----WHICH COULD SEAL OFF A VACUUM LEAK MORE. ALSO--think about the temperature outside---different OUTSIDE WEATHER CONDITION can make alot of difference. Go back and check for a vacuum leak. A little hose off ---off or bad hose is very easy to over look. Look also is hard to area locations---like behind the motor.

 

[Unthrottled]

Try oiling the gas linkage. Even behind the gas pedal

Thanks, @johnnyreb, will do. You've come up with two great ideas, and I'm grateful. I will try them both...

 

[Unthrottled]

Hey unthrottled, I think Tiha is on the right path, could be a dirty throttle body or an incorrect base idle setting AKA hard stop setting.
Try unplugging the idle speed control solenoid in neutral warmed up. The idle should be about 500 to 550rpm.
Let us know what you find

Yes, @Tiha and @L\Bronco , it's looking more and more like it may be the throttle body. Really appreciate the input. You guys are great!

 

[ShadetreeSeattle]

likely a vac leak. but* ensure that perhaps the original idle set point was not messed with on the throttle body. sometimes people will do this to get it to run. There's a window of control for the computer. think of it as like, the computer has no eyes, sort of speak. if it does not know any better, it will behave as it does, which includes not good lol. does the IAC behave differently when disconnected, how does it behave when you introduce extra air, or fuel, EG starting fluid... is it running lean or rich, that's a good place to start. those computers, (EEC-IV) are dumber than a bag of hammers. they will just try to do what they are told. researching how the EEC-IV derives it's fuel strategy is a great start.

 

[ShadetreeSeattle]

Also! failed throttle body gasket, i did a write up a few years ago about that one. If it's OG it may have become compromised and cracked, the original ones were made of some sort of a plastic that cracks, and little chunks break away and it becomes a vac leak. check that as well... Be careful with flammables, but do the check around things that may be an intake leak with a can of starting fluid or non-chlorinated brake fluid. *** always have a fire extinguisher at the ready when doing these kind of diagnostics. understand where you are aiming the flammable stuff. don't be dumb. it's an old school trick, but beware.

if it has a vac leak, the spicy spray will richen the mixture and make it likely pick up on idle, misfire less, etc.

bear in mind, this is a MAP sensor type of fuel system more than likely. Intake leaks before the throttle body should not be an issue. the EEC-iv derives most of it's fuel strategy from the manifold air pressure (map) sensor, at least for most pre 1996 big truck stuffs. Not to be confused with the MAF, (mass airflow sensor) that was typically used for more of the explorer/mazda b-series, or bronco 2's. those do use a MAF sensor. Beware also the intake air temp sensor, The coolant temp sensor, those will really do a number on fuel mapping. if it only has issues after warm, check the oxy sensors. the computer only reads the oxy sensors when it's at full warm, in closed loop operation. there's some good literature about how the ford EEC-IV derives it's fuel strategy out there, i'll try and keep an eye out for a reply. i think i have some good docs on that. Bear in mind that these computers are dumber than a bag of hammers. they only know what's told to them, they seldom ever throw a code for anything reasonable. most the pre-1996 codes they throw are seldom useful, but always worth doccumenting.
hope this helps.

 

[ShadetreeSeattle]

Computers can go bad btw, the capacitors are subject to age, and i have seen people fixing their rigs by swapping out the EEC-IV. I have never had such luck IMO, even when i opened the computer up and recapped it. it ended up being a much more trivial thing, like a vac leak. but that is also a thing. failed (PCM) can cause these issues as well, though ford's PCM's, in my opinion are pretty robust compaired to dodge/chrystler/chevy/gm stuff. I have recapped or outright replaced computers only to discover that it was something much less. Dodge/chrystler takes the lead in failed computers though. I don't want to talk ill of them, but yeah. they are not easy to repair either, given they *** the whole thing in rubber. basically impossible to repair.

 

[ShadetreeSeattle]

you can't even say P*o*t*t* on this site... as in potting a circuit board....

 

[goffprof]

I'm having trouble diagnosing an intermittent fast idle problem on my stock 1994 Bronco XLT, 5.8L. I'd sure appreciate any ideas you might share.

The problem occurs sporadically both in Park and in gear while I'm driving. It's worse when I first start the car up. After a hour or two of driving, it usually disappears. The more the car sits, the more frequent the problem. The more I drive the car, the less often I get a fast idle.

I can sometimes get it to go away if I put the vehicle in neutral and rev the engine. But this doesn't always work.

I can also sometimes get it to go away if I turn off the car and restart it. But this also doesn't always work either.

I replaced the Throttle Position Sensor, but this did not fix the issue. Also replaced the Idle Control Valve, but no improvement.

My mechanic friend wants me to keep throwing parts at it. I want to diagnose the problem first.

Oh, two last things. The problem first started after the car had sat unused for a few months while I was moving my family across the country.

Also, I occasionally get a rough idle at start-up.

I don't know if that might help someone with more mechanical knowledge than me narrow down a diagnosis.

I'd be very grateful for any ideas or suggestions. What could be causing my fast idle?

Thanks!

I had a similar problem on my ‘96 5.0 EB Bronco. I cleaned the throttle body, replaced the throttle position sensor, and then replaced several capacitors in the ECU (computer) from an inexpensive rebuild kit I bought on eBay. I think all 3 were causing idle problems.

Al