General Motors Tech Chevy tech talk within.

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

  #1  
Old 01-29-2013, 12:47 PM
Beginner
Thread Starter
Join Date: Jan 2013
Posts: 4
Default 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.
 
  #2  
Old 01-29-2013, 10:24 PM
Diesel Dan's Avatar
Senior Member
Join Date: Mar 2012
Location: Warren, Ohio
Posts: 190
Default

What are your IAC counts at idle?
 
  #3  
Old 01-30-2013, 07:20 AM
Beginner
Thread Starter
Join Date: Jan 2013
Posts: 4
Default

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?
 
  #4  
Old 01-30-2013, 07:23 AM
Beginner
Thread Starter
Join Date: Jan 2013
Posts: 4
Default

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.
 
  #5  
Old 01-30-2013, 11:58 AM
Diesel Dan's Avatar
Senior Member
Join Date: Mar 2012
Location: Warren, Ohio
Posts: 190
Default

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.
 
  #6  
Old 01-30-2013, 07:43 PM
Beginner
Thread Starter
Join Date: Jan 2013
Posts: 4
Default

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.
 
Attached Thumbnails 1994 GMC C2500 Sierra SL 7.4L - Idles Issues What Next?-truck_dash.jpg  
  #7  
Old 01-30-2013, 10:24 PM
Diesel Dan's Avatar
Senior Member
Join Date: Mar 2012
Location: Warren, Ohio
Posts: 190
Default

Originally Posted by dpc1969anoka View Post
Is the idle speed setting in the ECM or the PROM?
I would have to think it's in the PROM since the ECM can be used in different applications.
 
Related Topics
Thread
Thread Starter
Forum
Replies
Last Post
treasures4u1
General Motors Tech
0
04-29-2014 01:54 PM
BRiley3
Chevy Silverado/ GMC Sierra Forum
3
02-14-2011 07:51 AM
bradyjames
Chevy Silverado/ GMC Sierra Forum
6
06-11-2010 10:38 PM
karaz
Chevy Silverado/ GMC Sierra Forum
1
12-08-2007 12:20 AM
Jswerve
Chevy Silverado/ GMC Sierra Forum
16
04-28-2007 06:13 PM


Thread Tools
Search this Thread
Quick Reply: 1994 GMC C2500 Sierra SL 7.4L - Idles Issues What Next?


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service

© 2019 MH Sub I, LLC dba Internet Brands

We are a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for us to earn fees by linking to Amazon.com and affiliated sites.