terrible idle/93 GMC jimmy


Old 12-21-06, 10:10 PM
cheese's Avatar
Forum Topic Moderator
Thread Starter
Join Date: Jul 2001
Location: USA
Posts: 16,888
terrible idle/93 GMC jimmy


I am working on a friends 1993 GMC jimmy with a 4.3 liter engine (vortec). It runs awful at or near idle. After idle, it runs good. I started out with a full tune up. I replaced the plugs, wires, cap, and rotor. I also cleaned the throttle body, and removed the IAC motor and cleaned it and it's seat well. It still runs badly. Sometimes the idle will be very high, and gradually work its way down to a stumble and stall. Sometimes it will idle considerably well, but if I drive it to the end of the road and back, it stalls before I get it parked. It rund fine all the time it has a bit of throttle, but as soon as I let my foot off the gas, it stumbles and eventually stalls.

I noticed if I give it just a hair of throttle, it revs up and down to a very consistent pattern, high for a little less than a second, then low for the same amount of time, and cycling this way continually until I let off the gas. Then it's back to stumbling again when I let it idle. I am thinking the IAC motor is bad. What do you folks think?
Sponsored Links
Old 12-22-06, 03:47 AM
Join Date: Dec 2001
Posts: 2,538
would start by pulling off the egr and make sure its not got a piece of carbon holding it open they are fairly notorious for the egr to stick open and cause bad idle if it even idles at all, usually also has a check engine light on and a with a code 32, the egr is just below the throttlebody 2, 10 mm bolts hold it on you can clean it with some carb cleaner if its stuck open and work the pintle back and forth a few times to get any excess carbon build up out of it.
the only other thing that I can think of would be a fuel leak under the intake the cpi injector or fuel lines that run under the upper intake can start to leak they are made out of plastic, if they leak alot they will quickly foul one bank of spark plugs and will run bad even off idle due to the fouled plugs but a very slight leak will just cause it to run rich and load up a little most noticable at low rpm such as idle to check install a fuel pressure guage at the test port and cycle the key a few times to build fuel pressure and with the engine off see if you lose any pressure over a 5 minute period if you do you probably have a leak under the upper intake, but would check the egr first.
Old 12-22-06, 06:30 AM
Join Date: Feb 2005
Posts: 72
i dunno what sort of car this is but sounds american, if its anything like the australian commodore which it probably is similar to, if you have done all that stuff you have said you would have upset it, you need to reset the computer and let it relearn idle etc.. parameters then it should be fine again. just make sure you have the base idle set correctly. also make sure there are no vacuum leaks form any pipes youve left off or knocked off.
Old 12-22-06, 01:26 PM
michaeljp86's Avatar
Join Date: May 2005
Location: USA
Posts: 478
My dad had this problem in his buick with a 3.8L and it was the egr valve.
Old 12-22-06, 05:45 PM
Join Date: Apr 2006
Location: Floriduhhhhh Us of A
Posts: 611
Agree with Bejay 150%..........4.3 Vortecs are GREAT for getting carbon stuck in the pintle of the EGR valve.........Car will run like poop when at idle........as you come off idle you won't notice it as the engine is now gulping air and the EGR is SUPPOSED to be open
Old 12-22-06, 07:19 PM
Join Date: Nov 2006
Location: Jersey
Posts: 526
I was thinking along the same lines as everyone else.... vacuum leak. A stuck EGR valve is like having a vacumm leak.
Old 12-23-06, 10:03 PM
cheese's Avatar
Forum Topic Moderator
Thread Starter
Join Date: Jul 2001
Location: USA
Posts: 16,888
Well, I started it up the next morning, and it ran fine. The IAC motor worked like it should, and I drove it awhile. After 15 minutes or so, the idle went to crap again. Brought it back, pulled the EGR off. There wasn't a lot of carbon, but still some, so I cleaned it. When I pushed the valve back to clean the seat area, it stayed stuck back. I cleaned it and worked it in and out several times until it quit sticking, and reinstalled it...and it ran like a top the rest of the day. Problem solved, you all are right on the money! Thank you for the great advice, and have a merry and blessed Christmas!

PS...I am one of the mods in the small engine repair forum, so maybe you folks will give me the chance to help you out one day!
Old 12-31-06, 08:43 AM
Join Date: Dec 2006
Posts: 4
Exclamation Where is it again?

I've got a '93 GMC Jimmy S15, with the Vortec 4.3.

Where exactly IS the EGR valve? Anyone have a link to a picture of it? Or could give me good directions to track it down. I'm REALLY getting sick of the stalling and bad idle.

Also: I'm getting HORRID gas mileage, like a couple miles to the gallon... With gas prices as high as they are, I can barely afford to get anywhere. I also can't afford hefty mechanics fees or a new vehicle.

Any help would be much appreciated.

Old 01-01-07, 01:25 AM
cheese's Avatar
Forum Topic Moderator
Thread Starter
Join Date: Jul 2001
Location: USA
Posts: 16,888
Look at the front of the engine, where the air cleaner hose connects to the throttle body. Then look down under the throttle body. It's right there. Kinda looks like a little motor with a connector, held in place with 2 10 mm bolts.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Thread Tools
Search this Thread
Display Modes