High Idle...

Sorry to ask what may be an obvious question, but have you cleaned the throttle body?

You may have a vacuum leak somewhere that lets in extra air. The computer see a lean mixture and compensates by adding more fuel and that raises the idle speed.


You guys with idle/stall problems could save a lot of time chasing your tails if you would go through the Surging Idle Checklist. Over 50 different people contributed information to it. The first two posts have all the fixes, and steps through the how to find and fix your idle problems without spending a lot of time and money. It includes how to dump the computer codes quickly and simply as one of the first steps. I continue to update it as more people post fixes or ask questions. You can post questions to that sticky and have your name and idle problem recognized. The guys with original problems and fixes get their posts added to the main fix.

It's free, I don't get anything for the use of it except knowing I helped a fellow Mustang enthusiast with his car. At last check, it had more than 200,000 hits, which indicates it does help fix idle problems quickly and inexpensively.

If it is idling so high that the only way you can get it to idle down is to back the idle screw so far that the throttle butterfly sticks, it is sucking air from somewhere. That means there is either a vacuum leak or the IAC isn't closing like it should.

take the inlet tube off the TB and then start the car. put a rag or your hand over the inlet opening, if the car continues to run, you have a vacuum leak or faulty IAC.

Did you try hooking up a smoke machine? It pumps vapor into the plenum with the car off, where the smoke comes out is the vacuum leak.
If you still have smog stuff the tad tab solenoids are prone to leaking, as are the connections for the ac doors.
Good luck.

Here's the full scale vacuum leak test path. There is a link to YouTube on how to build and use a homemade smoke machine.

Finding vacuum leaks

Revised 6-Feb-2016 - added how to build and use a smoke machine to find vacuum leaks YouTube link

There is no easy way to find vacuum leaks. It is a time consuming job that requires close inspection of each and every hose and connection.

Small vacuum leaks may not show much change using a vacuum gauge. The range of "good readings" varies so much from engine to engine that it may be difficult to detect small leaks. The engine in my first Mustang pulled about 16.5" of vacuum at 650-725 RPM, which I consider rather low. It was a mass market remanufactured rebuild, so no telling what kind of camshaft it had. Average readings seem to run 16"-18" inches at idle and 18"-21" at 1000 RPM. The only sure comparison is a reading taken when your car was performing at its best through all the RPM ranges and what it is doing now. Use one of the spare ports on the vacuum tree that is mounted on the firewall near the windshield wiper motor.

Use a squirt can of motor oil to squirt around the mating surfaces of the manifold & TB. The oil will be sucked into the leaking area and the engine will change speed. Avoid using flammable substitutes for the oil such as starting fluid, propane or throttle body cleaner. Fire is an excellent hair removal agent, and no eyebrows is not cool...

After you have done the simple visual checks and the check for vacuum leak on the underside of the intake manifold, consider doing a smoke test.
Some of the guys here have built smoke machines used to find automotive vacuum leaks. They seem to work quite well and are made mostly with parts you would have laying around in your garage. Check out smoke machine vacuum leak - YouTube and see if there is one that you could build.

The vacuum line plumbing is old and brittle on many of these cars, so replacing the lines with new hose is a good plan. The common 1/8” and ¼” vacuum hose works well and isn’t expensive.

The PCV grommet and the power brake booster check valve grommet are two places that often get overlooked when checking for vacuum leaks. The rubber grommets get hard and lose their ability to seal properly. The PVC grommet is difficult to see if it is correctly seated and fitting snugly.

Fuel injector O rings can get old and hard. When they do, they are prone to leaking once the engine warms up. This can be difficult to troubleshoot, since it is almost impossible to get to the injectors to squirt oil into the fuel injector mounting bosses. If the plastic caps on the fuel injectors (pintle caps) are missing, the O rings will slide off the injectors and fall into the intake manifold.

Fuel injector seal kits with 2 O rings and a pintle cap (Borg-Warner P/N 274081) are available at Pep Boys auto parts. Cost is about $3-$4 per kit. The following are listed at the Borg-Warner site ( http://www.borg-warner.com ) as being resellers of Borg-Warner parts:
http://www.partsplus.com/ or http://www.autovalue.com/ or http://www.pepboys.com/ or http://www.federatedautoparts.com/

Most of the links above have store locators for find a store in your area.

Use motor oil on the O rings when you re-assemble them & everything will slide into place. The gasoline will wash away any excess oil that gets in the wrong places and it will burn up in the combustion chamber. Heat the pintle caps in boiling water to soften them to make them easier to install.



Diagram courtesy of Tmoss & Stang&2birds



Vacuum leak due to slipped lower intake manifold gasket...

Ask Nicoleb3x3 about the intake gasket that slipped out of place and caused idle and vacuum leak problems that could not be seen or found by external examination. I don't care what you spray with, you won't find the leak when it is sucking air from the lifter valley. It simply isn't possible to spray anything in there with the lower manifold bolted in place.





Determining if you have a leak due to a slipped intake gasket as shown above. This test is only good if you can get the engine to run somewhere in the 1000-1700 RPM range
If your valve cover oil filler & PVC systems are still in the original configuration, try this:
Cap or plug the hose from the intake manifold to the PVC valve with a bolt.
Cap or plug the PVC valve with a piece of hose with a plug or bolt in it.
At that point the only vent for the crankcase is the tube from the oil filler neck to the throttle body.

Disconnect the tube that runs from the oil filler neck to the throttle body. Make sure the oil filler cap is on securely. Start the engine and put your thumb over the end of the tube that comes from the oil filler cap. If you feel suction, there is a leak. Another thing to do is to extend the tubing from the filler neck so that there is enough to stick the end in a jar or cup filled with motor oil. If it sucks up the oil, you definitely have a leak at the underside of intake manifold.

This isn't necessarily the definitive test, but it is the best thing I could come up with on short notice. If there is a lot of blowby, this obviously won't be of much help.

See the picture below to see the breather tube where in connects to the throttle body. It is close to the TPS and runs over the top of the IAC.

See the following website for some help from Tmoss (diagram designer) & Stang&2Birds (website host) for help on 88-95 wiring http://www.veryuseful.com/mustang/tech/engine/ Everyone should bookmark this site.

Ignition switch wiring
http://www.veryuseful.com/mustang/tech/engine/images/IgnitionSwitchWiring.gif

Fuel, alternator, A/C and ignition wiring
http://www.veryuseful.com/mustang/tech/engine/images/fuel-alt-links-ign-ac.gif

Complete computer, actuator & sensor wiring diagram for 88-91 Mass Air Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif

Vacuum diagram 89-93 Mustangs
http://www.veryuseful.com/mustang/tech/engine/images/mustangFoxFordVacuumDiagram.jpg

HVAC vacuum diagram
http://www.veryuseful.com/mustang/tech/engine/images/Mustang_AC_heat_vacuum_controls.gif

TFI module differences & pinout
http://www.veryuseful.com/mustang/tech/engine/images/TFI_5.0_comparison.gif

Fuse box layout
http://www.veryuseful.com/mustang/tech/engine/images/MustangFuseBox.gif

all of the above is absolutely useless, if your throttle body is bad then replace it

Problem solved

Decipha said:

all of the above is absolutely useless, if your throttle body is bad then replace it

Problem solved

Click to expand...

Do you typically throw money and parts at a problem without doing the diagnostic legwork?

Ford knew about the tendency for throttle bodies to stick when they built the car. The problem arises with production tolerances on mating parts are too close to the minimum value on one part and too close to the maximum value on the other part.

the OP did the diagnostics alreadyz he said in the OP that it was sticking when we dropped the idle, the problem isnt the idle its the sticking throttle body

Decipha said:

the OP did the diagnostics alreadyz he said in the OP that it was sticking when we dropped the idle, the problem isnt the idle its the sticking throttle body

Click to expand...

Spend some time with Google and you will find this, which is the OEM adjustment procedure...

The idle stop should be set first. Back out the idle stop screw, away from the bell crank arm, until about 1/2 turn past the point where it no longer makes contact (blade fully closed). Using a 0.010" feeler gauge, tighten until gauge just drags between screw and bell crank arm. Remove feeler gauge. Tighten screw exactly 1 1/2 turns. If the screw is very loose, put a drop of loc-tite or silicone on it, so it doesn't work out of adjustment

See the Chilton Ford Mustang 1989-1992 Repair manual, page 5-12 for the verification.

No need, I have no use for others thoughts or processes on how they recommend a specific process should be performed, I too write publications myself so chilton, decipha, and henry may not all agree. The advantage I have over chilton and henry is that I get to play with vehicles after the fact with modifications and in situations they never anticipated nor ever expected.

Their recommended process may have been valid in the 80s when these vehicles were factory new but believe me, the air requirements of a vehicle factory new and 25years later is going to be significantly different.

In any case, the ONLY way to CORRECTLY set the throttle body stop according to the ford engineers design spec is to log IPSIBR and adjust it until it returns a 0 correction.

Have you done the vacuum check as suggested by mikestang63?

mikestang63 said:

take the inlet tube off the TB and then start the car. put a rag or your hand over the inlet opening, if the car continues to run, you have a vacuum leak or faulty IAC.

Click to expand...

How about some vacuum gauge numbers to use as a guideline in determining if you have a vacuum leak?

damn wish I could get my car to idle at about 1200, or even 1k would be decent