karl's-92'-z28

iTrader: (5)

I recently bought a 91 Formula with 59000 original miles. It has a 5.7 TPI. my problem is the engine idles high after she warms up, around 1500 rpm's with the AC off, it goes down a little when i tirn on the AC. What cold be the problem? Thanks for your help.

Brighton

I have a 1990 5.0L TPI Formula. Check the throttle cable and mechanism on the throttle body. After removing my throttle body and cable for injector replacement I discovered that the throttle was not closing completely. I needed to loosen bolts on cable and reposition it to get it to close completely. Make sure the throttle body is able to return to the end stop on the outside of the throttle body where the end of the throttle and cruise cables connect to the throttle body.

Other thoughts would be IAC motor issues or maybe vacuum leak.

Also clean the two throttle blades with throttle cleaner. Dirt on the blades will cause a stalling problem when my manual transmission is not in a gear.

I bought an USB interface box from Moates.net and use Tuner Pro RT to monitor and log data even though I have access to a Tech II. This is very useful for trying to figure out what is going on.

Blazer79

The best way to find out what's wrong is to datalog. It could be a CTS gone bad and the ECM thinks its freezing out there so it pushes the idle.

karl's-92'-z28

iTrader: (5)

Datalog, meaning do a diagnostics check?

Brighton

You could flash out the codes (if there are any) by ground the correct pin on the ALDL connector. If the CTS (coolant temperature sensor) is bad you may be able to just check the resistance with an Ohm meter. When mine went, it set a CTS DTC (Diagnostic Trouble Code). I think it opened up (2 Megaohms) which mean cold. I could have that reversed or otherwise wrong. The symptoms were hard starts and way too rich--I could smell the rich condition from inside the car while idling with the windows open.

To datalog you need something like what I use. I use TurnerPro RT software (free but paid registration is encouraged) and the ALDU1, CABL1 XtremeALDL with NewCABL1 combo from moates.net. These will allow you to view and record to a file various parameters from the ECM such as disired idle speed, actual engine RPM, IAC motor position, TPS, MAF or MAP, etc.

CamKid1992

i just had this issue on mine and it was as simple as the lever on the tps that controls the lever on the throttle body were not in contact. bent the lever a little and runs drives and idles great.

uemf

could also be wear on your throttle body. with engine off, open throttle a little and see how much you can move the shaft up/down/side/side. or try squiring liquid around shaft end n see if idle changes. (been there)
Vaccum leak ?

ASE doc

The fact that the engine idle decreases when you turn on the AC tells me that the porblem is in the IAC or control circuit. Perform the minimum idle setting procedure, I believe it's outlined in a tech article here on TGO. Also, clean the throttle body bores.

Monitor data and watch desired idle and IAC steps to see if the IAC is responding to ECM commands. Try inducing a small air leak at the PCV valve hose and look for desired idle and IAC steps to come down. turn on all electrical loads and watch for desired idle and IAC counts to increase. If IAC doesn't follow desired idle, replace the IAC.