The idle on my mustang gt is going up and down when i go to stop without touching the throttle. it doesn't do it all the time but has been doing it often lately

Hello Larry,

Without the car running... You should be able to open
the throttle.. and literally DROP it back to idle...

Observe the action of the THROTTLE RETURN
DASH-POTT should visibly SLOW the RETURN
of the throttle linkage like a door closer...

If you do not SEE the slowess... the dashpot
should be replaced (they are getting SCARCE).

SIX CYLINDER (EXAMPLE (only))
http://www.ebay.com/itm/NORS-65-67-MUSTANG-64-67-MERCURY-COMET-FORD-FAIRLANE-FALCON-DASH-POT-C2DZ9B549A-/142309387430?hash=item21224ceca6:g:kE8AAOSwo4pYmKnd&vxp=mtr


You will need to have the THROTTLE RETURN DASH-POT
on your (unspecified) carburetor
tested and adjusted... and the IDLE SPEED correctly set.

Idle Speed and a slow rate of RPM reduction is
critical to the proper function...

Some LESSOR "mechanics" will attempt to raise the
IDLE SPEED to compensate for a malfunctioning
DASHPOT... This will PROFOUNDLY exacerbate
the problem.

Throttle return dashpot (various pictures)
Go to Bing homepage

example DASHPOT (six cylinder)
NORS 65 67 MUSTANG 64 67 MERCURY COMET FORD FAIRLANE FALCON DASH POT...

example 1968 302/5.0L: Falcon, Mustang 2BBL carb
Carter 202 737 Carburetor Dashpot Ford 2 BBL Carburetors Replaces D7ZZ...

Hope this helps

Carnac the Magnificent

This issue can be related to a variety of problems. If the check engine light is on I would have the codes scanned for problamatic sensors etc. If no check engine light on, have a look at the throttle plate and bore by removing the air hose from the filter box to the throttle plate and inspecting for gumming up. ( Black/dirty) If so, use a carb cleaner spray and old tooth brush to remove any deposits that may cause base idle to be affected. Another problem could be the IAC ( idle air control ) motor not working properly

probably cause by the a/c cutting in and out or running at idle . Fit an idle compensator to the system throttle or raise the idle speed 300rpm.

Your problem is it is a ford,LOL:

Do an upper intake clean that will clean the throttle body and the Idle Air Control valve. Also clean the mass air flow sensor

Sounds pretty likely to be the iac. there should be a test for that - find it in the chiltons or haynes repair manual for your year, make model car - books found at yoru local auto parts store or local library - call ahead to be sure it is in stock
Best regards
Ck

It could be your idle air bypass valve. They get sticky with age and don't let enough air around the throttle body to keep the car running when it's at idle.

check the throttle body butterfly for excessive dirt build up. Mine had a rolling idle and would want stall in the mornings. The dealer said it was due to my aftermarket tuner and cold air intake..............just a dirty throttle body that i cleaned.

have you been tuning it? look for the vacuum hose you knocked off.

Make sure the inside of your throttle body doesn't have a gunk build up. If so, clean in out with some carb. cleaner really well. If the crack is plugged up in the throttle body, air cannot get into the engine at idle in which the car will die. Are you running an aftermarket cam? If you are, drill a 5/32 hole in the center of the lower half of the throttle plate in the throttle body. This helps a great deal as well.