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2 Posts
Discussion Starter #1
Hello! I have a 1993 Bronco 5.8L, and as of recent the RPM has started to fluctuate quite severely. Upon turning the ignition on the car seems to idle ok every once in a while the RPM will raise itself to around 2000RPM while in park at idle, however with a little gas the car seems to figure itself out. Now while I have the car in drive and come to a stop the RPMs drop quite severely almost causing the car to stall out. With that said the RPM's also raise while driving and at a stop I will be driving the car down the road maybe going 30MPH and I can feel the car accelerating more, and when I come to a stop the car is almost fighting me trying to accelerate when at a dead stop and the breaks are applied. Also when these issues are occurring and I put the car in park sometimes it will automatically rev the engine to around 2000RPM. Now these issues are intermittent and for a week the car drives fine but then the issues start back up. Do any of you know what it could be? My guess would be the TPS sensor?

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25,324 Posts
Yo R8t56,
See my Vacuum leak test in post #11 @ Help with dtc codes and idle
One way to do a quick check is to grab a vacuum gauge. Some parts stores will loan you a gauge with refundable deposit.
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.

"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)

ASAP, find out if speed control if equipped, recall worked out of was completed @ Recalls Look-up by VIN (Vehicle Identification Number); or @ Welcome to Ford Owner | Official Ford Owner Site; or ... have VIN ready. While there, see most Ford dealer maintenance/repairs done @ any dealership nation-wide.
See this guide by jowens1126 to confirm recall status @ 93 & 94-96 Cruise Control Recalls Repair
Note that the 93 recall is different than 94-96.

For any Bronco questions or to chat about desired modifications or build, it's better to post each seperately in Noobie Bronco Tech Questions. Flame free zone. 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 (75 posts required to participate due to scammers who preyed on our members ).

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. .
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See Baba Looey's Favorite FSB Links (lots and lots of tech links)

Here is the 1993 Bronco Dealer Brochure in Google Drive @ 93Bronco.pdf

1993 Bronco Dealer Training Video via silver70 @

1993 Bronco, Venezuela Commercial @

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4,826 Posts
The IAC (Idle Air Controller) is responsible for controlling idle speed. Check to see if it is clean. The TPS also has a role in idle speed, check the voltage when it is closed. The throttle cable and return springs also can have a role. Check that the throttle spring is returning the throttle plates all the way to the throttle stop.
And as @miesk5 related, check your vacuum lines.

2 Posts
Discussion Starter #4
Ok thanks for the responses I actually already replaced the IAC and there was no change the car doesn't have an issue with idle as much as it does while driving and if I hit the gas and accelerate at a fast speed the car works perfectly fine these issues usually occur when driving normally I will definitely attempt to check the vaccum lines
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