View Full Version : Setting Base Idle

09-14-2015, 01:21 AM
Have 91 GT 5sfe and 93 GT 5sfe as daily drivers.

91 bought new with 300K, redid the motor last year (rering lower and rebuilt head). All vac lines replaced, all sensors tested and tuned. Car ran like new for a while but ended up lately running rich.

93 bought used with 180K. Some prior owner must have damaged something as I have found evidence that the motor was pulled. Car lately has lost power i in the mid-upper rpms. Figured it was time to get the car back to spec before investigating true problem. Compression checks out, spending a lot of time undoing all the stoopid things done to it by prior owners. While undeleting the IAC water and air hose bypasses, I have had the chance to dig into idle and fuel controls (ECU, IAC etc) and general AFM's.

This has led me back to tearing down and cleaning the throttles and then rebuild and readjust everything. In doing so, I found a predicament that has no answer in the BGB's or is discussed in this or other forums.

Assuming everything is perfect (i.e no leaks) The idle speed is controlled by 4 items (3 in rev 2)

- Throttle butterfly stop screw. This is mounted on the throttle and controls how far the butterfly can close - or opens it enough to allow some air by.
- Idle Air Control Valve (IAC). Mounted under the throttle and allows air to bypass the butterfly. This is the primary mechanism that the ECU uses to control idle speed. If rpm's are too low, the ECU will open the IAC (and visa versa). This has bimetal spring in it to allow for increased air flow during a cold start. The coolant water passages allow the IAC to warm up and close up the air passage. (I Think? I've seen other discussions suggesting the coolant sensor signals would be used by the ECU to control Idle - could be on a different motor/yr though). There is some adjustment of the IAC allowed via two screws and twisting of the bimetal spring thus further closing or opening of the standard setting. In theory, this is set by the factory and never needs adjustment. There are plenty of discussions abound that suggest that the older IAC's will need adjustment to adapt (to a gate that is worn down?)
- The throttle opener is a vacuum operated device that holds the throttle open when the vehicle is off but under vacuum, holds the plunger back allowing the ECU to control idle with the IAC. Since this device is not used for base idle, it can be ignored for the following question:

If I am setting base idle - do I start with the throttle stop screw or the IAC adjustment? I've adjusted the stop screw in the past but after cleaning the IAC, it's clear that the stop screw needs to be adjusted back. But when I back them all the way out, I find the IAC is still too open to get the rpms down - leading me to adjust the IAC spring.

Also, under a normal tuneup procedure, I would jumper the TE1/E1 to stop the timing advance control by the ECU. This jumper also has the ECU open the IAC for a bit and then close it down to some level to allow for setting of the idle speed. I have found on the 91 that the tuneup jumper does not close the IAC completely - is this normal?

Soo, assuming I have to fix/adjust both the stop screw and the IAC - what is the proper procedure?

09-14-2015, 04:26 AM
Oh, forgot the 4th idle adjustment. On the rev1's, there is an idle adjustment screw on the body of the throttle that also allows air to bypass the butterfly. I assume that this was spec'd to allow for adjustments to be made as the IAC became dirtier and restricted flow. The rev2's have a different ECU/TPS configuration that provides for finer adjsutment of the IAC air flow - thus compensating for the variable dirtiness. Yes?

So, to add to the question, Where should the Idle Air Screw be set for a base idle - all the way closed? 2 turns open?