The idle goes back and forth from 1700 to 2500 when slowing down for a stop light

when your engine speeds up or slows down the voltage will increase and decrease,, probably has nothing to do with your problem.. Its a 2002 car so first check all the hoses and pipes on the engine that provide vacuum to the different parts of your engine.. check the hose that goes to the brake booster. plug the hose and try running the engine.. If you DRIVE THE CAR YOU WILL HAVE MUCH REDUCED BRAKING EFFIGNCY.. BE CARFULL. to see if you have an internal vac. leak in the booster. Read out the codes to see if there are any set. Your problen sounds like a vac leak to me.. Post here and let us know what you find..

Possible idle air control motor failing/sticking.

You could have a clogged fuel filter, if you haven't changed it in awhile. You could have a leak in the intake system somwhere also. When you slow down, you get less power to the engine. It may need spark plugs and wires as well, if in need for a tune up.

EGR valve and PDFE sensor control idle speed. The idle speed valve you replace very rarely goes bad.

Do you have a check engine light? Which engine? My 1996 Trooper with a 3.2 D.O.H.C. is awful for plugging up the E.G.R. I have cleaned it several times! Good Luck!

One suggestion:
Speed sensor failure.

Info:
Some of the classic symptoms, stalling at dead stops. failure to auto lock doors. poor fuel economy. Failure to go to overdrive.


Recommendation:
Go to your local parts store and run the failure codes.

Shop notes:
Your ECU thinks that the engine is running with out moving, During slow down the engine steps down the idle to match ground speed and as a result the engine will not stall. At present it goes directly to idle and stalls due to drag on motor.
Hope this tip helps.

R/
David

low oil,bad oil,or to light of oil

check the motor ground wire it is a braided copper wire usually off the back of the motor to the frame of the car

THis has happened to me in the past. My fix was to clean the throttle body. The problem began after my battery died. The computer was defaulted back to its original settings after replacing the battery. Once I cleaned the throttle body, everything was fine. This happened at about 85,000 miles.

it moves back and forth even if its at a half a tank when i take off it goes to full and when i slow down it goes to e