Bronco Forum - Full Size Ford Bronco Forum banner

1 - 5 of 5 Posts

2 Posts
Discussion Starter #1
I just bought a 1991 Bronco Silver Anniversary with the 5.8 engine. When first started it idles fine but if I drive it at all and then put it in Park or Neutral the RPMs jump to 1500. Occasionally it won’t give me this problem. It seems to idle fine in gear when stopped. I’ve checked the IAC and it’s fine. I don’t expect a vacuum leak or a worn throttle body because it’s not constant. I’m not sure where to go from here. Any help is appreciated. Thank you.

Super Moderator
27,220 Posts
Yo shoveloperator,

High idle with no applicable codes is usually caused by a vacuum leak,

Other perpetrators are:
Intake manifold leak.

Speed Control chain is binding, if equipped
Air intake tube is damaged
Throttle body return spring is loose
Throttle body is loose
Idle Air Control Valve (IAC); try these tests if you have any doubts the IAC is responsible;
"Start the Bronco and bring the engine up to operating temperature. Allow the engine to idle without any driver input to the throttle or pedal. Go under the hood, and disconnect the electrical connector to the Idle Air Control Valve (IAC). If the engine begins to stubble or stalls the IAC is functional and does not need to be repaired. If the engine idle does not change you should remove the IAC for inspection.

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. Remove the IAC and clean it. There are 2 halves to the IAC, and you can not buy just one half, but you can take it apart to clean it. But if the internal solenoid is faulty the IAC needs to be replaced." by Ryan M

How to Clean and Test your IAC by Seattle FSB @ How to Clean and Test your IAC

TPS Overview & Testing by Seattle FSB @ TPS Overview & Testing by Seattle FSB @ Bronco Forum - Full Size Ford Bronco Forum
Mikey350 wrote, "Does your throttle body have a yellow warning sticker that says not to clean the throttle bores?
Try this: 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 screwed 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
See my Vacuum leak test in post #11 @ Help with dtc codes and idle

"The vacuum gauge should read between 15 and 22 in-Hg depending upon the engine condition and the altitude at which the test is performed. SUBTRACT ONE INCH FROM THE SPECIFIED READING FOR EVERY 1,000 FEET OF ELEVATION ABOVE SEA LEVEL.
The reading should be quite steady. .
When engine is rapidly accelerated (dotted needle), needle will drop to a low (not to zero) reading. When throttle is suddenly released, the needle will snap back up to a higher than normal figure.

When vacuum leaks are indicated, search out and correct the condition. Excess air leaking into the system will upset the fuel mixture and cause conditions such as rough idle, missing on acceleration, or burned valves. If the leak exists in an accessory unit, such as the power brake, the unit will not function correctly. Or Air Conditioning when in MAX mode may switch to Defrost."

Here is the 1990 F Series/Bronco Pre-Delivery Shop & Electrical & Vacuum Troubleshooting Manual (EVTM), Partial by member Kingfish999 in Google Drive (similar to your 91) @ 1990 Ford Truck service manuals - Google Drive
Slow scrolling for my slow Comcast Highest Cost Blast Internet..
Suggest you download the docs for faster scrolling and to use the page index.

As you've done, for any questions, or to chat about it's planned build, it's better to post each seperately in Noobie section. This will get more attention and you can build up your post count to get into other sections such as Bronco and Ford Parts/Accessories that requires 75 non-padded posts to participate due to scammers who preyed on our membersn ).

Register with VIN to see most Ford dealer maintenance/repairs done @ any dealership nation-wide @ Welcome to Ford Owner | Official Ford Owner Site

To save you time and for better responses, please fill out your Bronco info with location, year, engine size, transmission type, transfer case type (manual or electric shift), locking hub type (automatic or manual) info & major mods such as a Lift, etc. .
Bronco info is now able to be put under your user name.
Click your profile button in the top right and go to account settings.

On that first page, named Account Details, scroll down to "Vehicle Info" and type in up to 100 characters.

Now you can simply enter your information in the text editor and click save.

Our Forum FAQs includes for example, How To Upload Images To Posts, How to Use Search and more tips!

Look @ Baba Looey's Favorite FSB Links (lots and lots of tech links)

Please find time to participate and VOTE in our Full-Size of the Quarter & Full-Size of the Year Contests @
You will get ideas by those competing. Also see the prizes⚠ They are awesome as compared to other sites' "contests"!


2 Posts
Discussion Starter #3

Thank you for your reply and help. It is much appreciated.

If there is a Vacuum leak wouldn’t it idle high all the time? I’m new to all this and just trying to understand.

Premium Member
US, 89, 5.8,C6, 1356 BW (manual), 5 Bolt Hub (automatic) No modes like to keep it or
279 Posts
If there is a Vacuum leak wouldn’t it idle high all the time?
Depends on what is causing the leak, with my 89 5.8L some portions of the emission system does not activate until certain temps, vacuum required etc..
I am also having and idle issue as well here is my post Idle Qs. To check hoses and items that require a vaccum like an EGR, EVR a vacuum gauge works good. For seals/gaskets Carb cleaner spray a little wait to listen if the engine RPMs change. If you spray on the exhaust manifold do it before it gets hot or you will be in for a show.

The post by Miesk5 with Mikey350's trouble shooting steps is what helped me find that my butterfly valve was not closing all the way. Still have an upper intake leak I am working through.

What do you consider good idle in Drive/Rev?
Do you have any codes, may have some even if Check Eng light does not come on?
  • Like
Reactions: miesk5

1986 Eddie Bauer 5.0EFI AOD Full length headers Y pipe into single 3" Magnaflow 3" factory exit
621 Posts
I just got my 86 to idle correctly after I replaced everything EFI related because it was all horribly old. Even after replacing it all I still had a surging idle on my "new to me" 86 302EFI.

Conclusion was, it was a vacuum leak in the cruise control module itself, not the vacuum lines going to it, the actual module was rusted thru causing the vacuum leak.

After capping that line to the cruise control from the intake manifold, I enjoyed a few weeks of a nice smooth 650 rpm idle. Then just yesterday i noticed my exhaust hanging down on one side, at the same time it started idleing at 950-1000 rpm when warm?

I went under the truck to look & noticed a broken exhaust rubber mount causing this hanging exhaust.... anyway picked up a new $3.00 rubber mount & when installing it, I noticed that the exhaust hanging down caused the bolts that connect the full length headers to exhaust to loosen up a bit. This must have allowed fresh air get to the o2 sensor that is in the exhaust, right after the header flange, which caused the high idle I'm guessing, because as soon as I tightened those header to exhaust bolts back up, the idle came right back down to 650rpm when warm & is back to idling perfectly fine now.

Just a thought from what I just had happen to me tonight, the good part was, I got to enjoy a 20 mile night time bronco cruise afterwards!
1 - 5 of 5 Posts