1994 GMC C2500 Sierra SL 7.4L - Idles Issues What Next?
 

Been looking at a lot of forums and posts regarding various idle problems on these TBI systems. Haven't got to the root of my problem yet so I thought I'd post here and see what y'all think I should try next.

I can't get the truck to idle below 875 or so. It's quite dangerous to drive in slippery conditions because the rear wheels will break loose from it idling so high.

No vacuum leaks. ECM, IAC replaced. MAP replaced. ISCA solenoid replaced. Coolant temp sensor replaced. O2 sensor readings seem normal. memcal prom data looks good. Timing @ 4 BTDC (w/ESC cable disconnected).

I replaced the entire TBI unit with a re-manufactured one and then it wouldn't stay running at all. Would rev up and just die...I could feather the gas but it ran really rough. I could unplug map sensor and it would stay running unless I put it in gear. Noticed when I plugged the MAP sensor back in at idle that the injectors would cut back the fuel so I think it has something to do with fuel delivery. But then again, I re-installed the old TBI unit and I'm back to square one with the high idle.

What are your IAC counts at idle?

I'll have to review TunerPro log this evening to make sure, but I want to say around 70. If I remember correctly the IAC counts didn't change much between old and new TBI unit but I have that log data as well so I can double check.
What should target IAC counts be at idle cold & warm?

By the way...I did return the re-manufactured TBI unit since I ended up with the completely opposite problem (from high idle to no idle) after installing it.

If your IAC counts are at 70 while idling that is where the ECM wants it too be.
Warm the truck so it's in closed loop, check and record IAC counts and then create a small vacuum leak. If the IAC counts drop then the system is working.

TBI 454s are programed to idle higher than I like. Why I do not know. Had new 2wd suburban with 7.4L engine that would spin its tires on snowy roads and confuse the ABS when stopping because of the programmed idle speed.

Looks like IAC counts at 85 when warm at idle of 825 RPM (see attachment).
I will watch IAC counts after creating vacuum leak and see what happens.
Is the idle speed setting in the ECM or the PROM? Must be ECM cause I don't recall seeing a setting for the idle speed in the PROM.

I would have to think it's in the PROM since the ECM can be used in different applications.

Old thread but still useful for someone...

Looking at the 1994 service manual for 454 TBI under fuel systems (Section 4-4A) the 'Min idle speed' setting is... (Factory set screw)

For the 7,4L auto trans in Drive is 625 (+-25rpm) Closed loop

Controlled Idle Speed is as follows:

454 7.4L in Drive 675 (+- 25rpm) IAC count 5-30 (Closed loop) at operating temp 196-222F (add 2 counts for every 1000ft above sea level

The 'controlled idle speed' is obtained by the control module positioning the IAC valve. A high/low coolant temp or AC clutch engagement may signal the control module to change the IAC counts.

IAC counts will be above 60 if idle is too low and zero counts if idle is too high. Things to check if so...

1. Vacuum leak/s

2. System too rich

3. TB obstruction

4. PCV issue

5. AC issue

For manuals here: https://www.gmt400.com/threads/88-98...manuals.43575/