Pressing down on brake pedal causes high idle

[shawnkelly1970]

I have a 91 Ford bronco with 150,000 miles on it that was rode hard before I got it. I have done all the work myself on the engine and it runs so much better now. The problem I am having right now is chasing a height and idle that comes and goes. I have the 5.0 with a five speed and it usually starts and idles just fine. I noticed it when driving around town pulling up to stoplights the idle jump up to 3000 RPM usually. I have to tap on the gas to get it to drop. The frustrating thing is it’s inconsistent. I have done an entire smog delete on the engine. I have replaced all the sensors related to an idle issue. The last being the engine temperature sensor. It ran fine after installing it. Finally finally finally I found the problem or so I thought. The next day the problem was back and my wife got an earful of all the foul language I learned when I worked in the oil field. I have eliminated every vacuum line except for the brake pedal and map sensor. I noticed that when parked in the driveway idling the RPMs will increase when I step on the brake pedal. Also when I first started chasing the problem I removed the throttlebody and cleaned it and Unfortunately I messed with the idle setscrew because of a worn throttle cable. I have read that that is a huge no-no because the computer adjusts the idle from the original position. How do I reset the computer to the new location of the setscrew? Also can the brake power booster have an intermittent vacuum leak causing the issues while driving?

 

[Tiha]

Brake booster was going to be my first guess.

If you can get it to act up in the driveway, plug the line to it and try it again. Intermitten can come from a lot of things, but really if the booster is leaking, what is happeneing is unmetered air is entering the intake and leaning out the fuel mixture. Which throws the fuel mapping off and the computer has to take those readings and play catch up.

 

[shawnkelly1970]

Brake booster was going to be my first guess.

If you can get it to act up in the driveway, plug the line to it and try it again. Intermitten can come from a lot of things, but really if the booster is leaking, what is happeneing is unmetered air is entering the intake and leaning out the fuel mixture. Which throws the fuel mapping off and the computer has to take those readings and play catch up.

Since my first post I went back out and stepped on the brakes again. It increased about 200 RPM then leveled back out when I pressed on the break again it remain steady so I’m guessing it’s just the initial priming of the brake booster and then it’s fine after that. I was able to set my idle ***** back very very close to the factory location thanks to the paint they put on the *****.
I went back through my checklist of things I have already done and tried them again. I just unplugged the IAC when the vehicle is idling smoothly at 800 RPM and the idle remained steady. If I recall it should’ve dropped. I cleaned the original IAC twice before replacing it with one from O’Reillys with no improvement at all. After doing this test I am wondering if the new part is bad also. I am so frustrated because it makes it unsafe on icy roads not knowing if or when the idle will jump up in between shifts or stopping at a red light.

 

[miesk5]

Yo Shawn,
Ford states "Excessive deposits, especially on high-mileage engines, can build up in the IAC passage and on the IAC pintle. Excessive deposits can also build up on the throttle plate, so check for a sticking throttle plate.
Use of unapproved solvents may damage the IAC valve."

The IAC can pass and still need repair, or it can fail and not need replacing. The plunger and internal spring can get clogged with dirt and oil. This will slow down the air flow and not allow the IAC to function properly.
If the internal solenoid is faulty, the IAC needs to be replaced.

 

[pltommyo]

I have a 91 Ford bronco with 150,000 miles on it that was rode hard before I got it. I have done all the work myself on the engine and it runs so much better now. The problem I am having right now is chasing a height and idle that comes and goes. I have the 5.0 with a five speed and it usually starts and idles just fine. I noticed it when driving around town pulling up to stoplights the idle jump up to 3000 RPM usually. I have to tap on the gas to get it to drop. The frustrating thing is it’s inconsistent. I have done an entire smog delete on the engine. I have replaced all the sensors related to an idle issue. The last being the engine temperature sensor. It ran fine after installing it. Finally finally finally I found the problem or so I thought. The next day the problem was back and my wife got an earful of all the foul language I learned when I worked in the oil field. I have eliminated every vacuum line except for the brake pedal and map sensor. I noticed that when parked in the driveway idling the RPMs will increase when I step on the brake pedal. Also when I first started chasing the problem I removed the throttlebody and cleaned it and Unfortunately I messed with the idle setscrew because of a worn throttle cable. I have read that that is a huge no-no because the computer adjusts the idle from the original position. How do I reset the computer to the new location of the setscrew? Also can the brake power booster have an intermittent vacuum leak causing the issues while driving?

Pull the line for the brakebooster from the intake, plug them both (cap on intake, big bolt in hose). Repeat your testing - but if you try to drive be aware the brakes will be HEAVY. If problem is gone then you know booster is leaking. If problem persists ... hmmmmm

 

[miesk5]

Yo Shawn,
FYI
EFI High Idle Tips by Mikey 350:
"Take off the air intake hoses so you can access the throttle bores. Start the engine and place a piece of stiff cardboard over the throttle bores. That should NOT change a correctly configured engine, as the idle air is only from the opening of the IAC, and nothing through the throttle bores.
If that make a difference, then the throttle plates are slightly open due to the removal of the Teflon coating, or the linkage/return spring/throttle stop is ******* up.
If the engine still runs too fast with the throttle opening blocked, try taking off the IAC and blocking those openings.
What you have done is (supposedly) removed all the "normal" air intakes, and if the engine still runs, you have a vacuum leak.
You could then try leaving the air intakes (throttle and IAC) blocked and cap off the vacuum lines coming off the vacuum tree. (except the MAP line) If one of those makes a difference, investigate the leak.
Check the vacuum line from the vapor canister (on the left side of the throttle, when facing the throttle assembly) (cap it, on the outside chance that the solenoid has failed open) (if equipped)
Pull the EGR connection to the intake and plug or cover it. (if you have wide duct tape, use that to cover the hole)
If it still is running with the throttle blocked and the IAC and the lines from the vacuum tree capped, then you have a manifold or PCV line leaking. Plug the PCV line to the intake, then the vacuum line to the brake booster (if it has it's own fitting on the manifold)
Something is causing that high idle, and 95% of the time it is a vacuum leak."

 

[RockNBronco96]

I have a 91 Ford bronco with 150,000 miles on it that was rode hard before I got it. I have done all the work myself on the engine and it runs so much better now. The problem I am having right now is chasing a height and idle that comes and goes. I have the 5.0 with a five speed and it usually starts and idles just fine. I noticed it when driving around town pulling up to stoplights the idle jump up to 3000 RPM usually. I have to tap on the gas to get it to drop. The frustrating thing is it’s inconsistent. I have done an entire smog delete on the engine. I have replaced all the sensors related to an idle issue. The last being the engine temperature sensor. It ran fine after installing it. Finally finally finally I found the problem or so I thought. The next day the problem was back and my wife got an earful of all the foul language I learned when I worked in the oil field. I have eliminated every vacuum line except for the brake pedal and map sensor. I noticed that when parked in the driveway idling the RPMs will increase when I step on the brake pedal. Also when I first started chasing the problem I removed the throttlebody and cleaned it and Unfortunately I messed with the idle setscrew because of a worn throttle cable. I have read that that is a huge no-no because the computer adjusts the idle from the original position. How do I reset the computer to the new location of the setscrew? Also can the brake power booster have an intermittent vacuum leak causing the issues while driving?

Shawn,
I've got a 96 with a 351w, and had the exact same problem and basically did everything you did and still had the problem as well. When the IAC was replaced and something that was turned way up was turned back down the problem was solved. The mechanic said the main issue with the IAC, sorry I can't say what was turned down as I don't recall and I haven't found the receipt as it was filed in whatever month the work was done, and I have no idea when that was... I do know it was a meter reading and dealt with something electrical not fuel. Good luck.

 

[shawnkelly1970]

Thank you everyone for your input. Sorry it’s been so long getting back to you. I am dealing with my second bout of Covid in four months. It’s been absolutely miserable. So my IAC looks like it is brand new. I only have three or 400 miles on it since replacement. Last night I drove it after dinner and it ran fine. This morning I got in it to take my son to school and the high idle was back. I’m beginning to wonder if maybe my throttlebody is wore out or there is an issue with my EEC. I have tried the carburetor cleaner around all the vacuum lines, brake booster and intake manifold to see if it increases the RPM and I get nothing.

 

[shawnkelly1970]

Thank you everyone for your input. Sorry it’s been so long getting back to you. I am dealing with my second bout of Covid in four months. It’s been absolutely miserable. So my IAC looks like it is brand new. I only have three or 400 miles on it since replacement. Last night I drove it after dinner and it ran fine. This morning I got in it to take my son to school and the high idle was back. I’m beginning to wonder if maybe my throttlebody is wore out or there is an issue with my EEC. I have tried the carburetor cleaner around all the vacuum lines, brake booster and intake manifold to see if it increases the RPM and I get nothing

Yo Shawn,
FYI
EFI High Idle Tips by Mikey 350:
"Take off the air intake hoses so you can access the throttle bores. Start the engine and place a piece of stiff cardboard over the throttle bores. That should NOT change a correctly configured engine, as the idle air is only from the opening of the IAC, and nothing through the throttle bores.
If that make a difference, then the throttle plates are slightly open due to the removal of the Teflon coating, or the linkage/return spring/throttle stop is ******* up.
If the engine still runs too fast with the throttle opening blocked, try taking off the IAC and blocking those openings.
What you have done is (supposedly) removed all the "normal" air intakes, and if the engine still runs, you have a vacuum leak.
You could then try leaving the air intakes (throttle and IAC) blocked and cap off the vacuum lines coming off the vacuum tree. (except the MAP line) If one of those makes a difference, investigate the leak.
Check the vacuum line from the vapor canister (on the left side of the throttle, when facing the throttle assembly) (cap it, on the outside chance that the solenoid has failed open) (if equipped)
Pull the EGR connection to the intake and plug or cover it. (if you have wide duct tape, use that to cover the hole)
If it still is running with the throttle blocked and the IAC and the lines from the vacuum tree capped, then you have a manifold or PCV line leaking. Plug the PCV line to the intake, then the vacuum line to the brake booster (if it has it's own fitting on the manifold)
Something is causing that high idle, and 95% of the time it is a vacuum leak."

This makes total sense to me to diagnose the issue. Going out to try it now.

 

[shawnkelly1970]

Yo Shawn,
Ford states "Excessive deposits, especially on high-mileage engines, can build up in the IAC passage and on the IAC pintle. Excessive deposits can also build up on the throttle plate, so check for a sticking throttle plate.
Use of unapproved solvents may damage the IAC valve."

The IAC can pass and still need repair, or it can fail and not need replacing. The plunger and internal spring can get clogged with dirt and oil. This will slow down the air flow and not allow the IAC to function properly.
If the internal solenoid is faulty, the IAC needs to be replaced.

I think I’ve finally sold my problem. Can you rebuild the throttlebody?

 

[miesk5]

I think I’ve finally sold my problem. Can you rebuild the throttlebody?

Yo Shawn,
Ford states for 92-96, "The throttle body (9E926) for this vehicle is a sludge-resistant design and should not be cleaned."

See

Proper throttle-body cleaning guide by Mobil @ https://www.mobil.com/en/lubricants...-yourself/proper-throttle-body-cleaning-guide​

&

https://www.google.com/search?q=thr...e&ie=UTF-8#kpvalbx=_-EP1YcrpA8qZptQP4a6q0Aw15

Overall, there's not much point in cleaning just the throttle body because that same gunk, etc. is all over the inside of the plenum & intake, so I'd either pull the plenum for good thorough wash, or have a pro use a hi-pressure system that works with the engine running.

 

[Tiha]

I think I’ve finally sold my problem. Can you rebuild the throttlebody?

Actually good point, I have seen the throttle shaft start leaking air/vacuum. If that is the case they are not rebuildable, need a new one.

Spray starting fluid around the shaft where it enters the housing. See if the RPMs change.

 

[johnnyreb]

I think I’ve finally sold my problem. Can you rebuild the throttlebody?

I hope you have a speedy recover and get your truck fixed soon.

 

[johnnyreb]

Since my first post I went back out and stepped on the brakes again. It increased about 200 RPM then leveled back out when I pressed on the break again it remain steady so I’m guessing it’s just the initial priming of the brake booster and then it’s fine after that. I was able to set my idle ***** back very very close to the factory location thanks to the paint they put on the *****.
I went back through my checklist of things I have already done and tried them again. I just unplugged the IAC when the vehicle is idling smoothly at 800 RPM and the idle remained steady. If I recall it should’ve dropped. I cleaned the original IAC twice before replacing it with one from O’Reillys with no improvement at all. After doing this test I am wondering if the new part is bad also. I am so frustrated because it makes it unsafe on icy roads not knowing if or when the idle will jump up in between shifts or stopping at a red light.

Are you feeling better Shawn. Hope so.

 

[captsb890]

I have a 91 Ford bronco with 150,000 miles on it that was rode hard before I got it. I have done all the work myself on the engine and it runs so much better now. The problem I am having right now is chasing a height and idle that comes and goes. I have the 5.0 with a five speed and it usually starts and idles just fine. I noticed it when driving around town pulling up to stoplights the idle jump up to 3000 RPM usually. I have to tap on the gas to get it to drop. The frustrating thing is it’s inconsistent. I have done an entire smog delete on the engine. I have replaced all the sensors related to an idle issue. The last being the engine temperature sensor. It ran fine after installing it. Finally finally finally I found the problem or so I thought. The next day the problem was back and my wife got an earful of all the foul language I learned when I worked in the oil field. I have eliminated every vacuum line except for the brake pedal and map sensor. I noticed that when parked in the driveway idling the RPMs will increase when I step on the brake pedal. Also when I first started chasing the problem I removed the throttlebody and cleaned it and Unfortunately I messed with the idle setscrew because of a worn throttle cable. I have read that that is a huge no-no because the computer adjusts the idle from the original position. How do I reset the computer to the new location of the setscrew? Also can the brake power booster have an intermittent vacuum leak causing the issues while driving?

Re-set your computer as it has RAM so disconnect the negative and positive lead---put a jumper on the negative to positive lead ( battery is out of the picture so no problem) Might turn on the light switch---helps with the discharge even though no lights will come on-----worth a shot-- leave it like that for a while ---connect the battery back up and then drive for about 20 miles so the computer will know your driving style!---also make sure you have a good vehicle battery---Optima is good-( expensive though)--also put a voltmeter in you cigarette light receptacle and keep an eye on the voltage as you drive---a failing EEC computer will drag the voltage down somewhat--------------