Fox Owner 88

I am new to this Forum and I have owned my 88 Mustang 5.0 for about a year now and I love it! When I start the car up cold it goes into high idle fine and then settles down to 800rpms and runs a little rough, nothing crazy. Once I start to drive it the idle smooths out fine, but then as I'm driving the car, it will at times (not all the time) start to idle at 1200rpms and stay that way until I turn the car off. When I'm at a stop it's at 1200. I put new plugs, wires, K&N air filter, and cleaned the throttle body and IAC out and it didn't fix it. I am think about changing the TPS. Would this make sense to do? Any help would be great. Thanks

88 orangepeel notch

I'd check the TPS voltage first, idle and sweep. Maybe there's a dead spot. Otherwise "base idle reset".

Have you pulled codes? Seems weird that its a steady 1200rpm idle, or is it surging?

projectresto83

Unplug the IAC and drive it around...or if you can catch it at a stop or something hop out and disconnect it and see if the idle drops back down to normal..be aware your idle might drop low enough to kill the car though so do not be afraid of that....

The IAC and TPS are notorious problems with the cars because of their age

Fox Owner 88

Thanks guys, appreciate it. I will try everything recommended. It does surge sometimes like it's trying to figure out where to settle but mainly just stays at 1200. I have heard of checking the voltage, pulling codes, and re-setting the idle but have never done them. I know there are instructions on here. I will also remove the IAC while it is running and see what happens. I'm hoping I have time this weekend to do it. I will let you know. Thanks again

mongolchuck

I know yours is an 88 and therefore probably doesn't have a maf unless its a Cali car; but k&n filters and the like will eventually leave an oil film on the maf coils making them read incorrectly. This is especially true if the filter is over oiled like most of them are. If your car does have a mad make sure to clean it too with sensor safe non residue cleaner.

88 orangepeel notch

Great idea mongo, I've never thought of that

Fox Owner 88

Thanks, mine doesn't have a MAF. Could the filter oil still be a problem? Or is it just with the MAF?

88 orangepeel notch

Nope, you'll be fine if there's no maf. I thought 87 was the last year for speed density, shows you learn something everyday.

dawson1112

In 88 all California mustangs went maf, In 89 all others were maf.

dawson1112

Op check all your throttle linkage as well as the cruise cable if your car is equipped with cruise. Sometimes the throttle plate doesn't close all the way especially if there is crud in there. Ive seen the cruise cable just a tad to tight and cause the throttle plate to hang just enough to create a high idle once in awhile .