I have a 1991 Silverado 4x4 and it has been running weird lately. When I stop at a stoplight and have the brakes applied it will rev up briefly, SES light will appear and then it will continue idling. Kind of makes me nervous because it wants to jump out and go when I'm at a stop. I just recently ran some fuel system cleaner through it, but not sure what could be the problem.

Thanks in advance!

 

There are several things that can cause what you describe, but before you throw parts at it you may want to have a shop troubleshoot the problem.

It could be something as simple as a vacuum leak or an improperly routed vacuum hose, it could also be a faulty EGR valve or IAC motor or TPS or even an improperly set base ignition timing, there are many other possibilities but those are some of the most common.

 

I was thinking I may have to have a shop do a good diagnosis on it. Dang it! Oh well, I guess I can most likely do the repair once I find out what the deal is. This engine has ran great up until just a couple weeks ago. I like working on this truck. Parts are relatively inexpensive and it's pretty easy to work on!

Thanks for the information!

 

I'll second ZX's diagnosis on the IAC motor as you said that it "ran great up until...." In my experiance, an EGR slowly craps out, AND it shouldn't have any vacuum at idle. The IAC's job is to set the idle via vacuum through the throttlebody, and if the motor is going bad, hunting for an internal resistance reading on the solenoid, it will definately trip the SES light.

 

i'd go so far as to say Dave & 993500CS are correct (always a good idea to verify with a reading of the codes though). had the exact same issue with my '91 'burb- it would idle along at 20 KPH (a little less than 15 MPH). code came up for the IAC, cleared the code & changed the IAC, problem solved. have u checked your codes yet?

 

Man thanks for all the replies!

I haven't checked my codes yet, but they can't check the codes for my truck at Autozone can they? I didn't think they had the readers for these older trucks?

Edit-What is an IAC motor??? You mean IAC Valve? If it's the valve, I know it connects into the throttle body, but can anyone tell me where?

 

IAC is idle air control. If you are standing in front of the truck, it is on the left rear of the throttle body. it is basically a plunger that controls air flow to the throttle body. it will cause the engine to rev high at idle if it is letting too much air through

 

Okay thanks, I appreciate the help. I may gamble and do the famous "throw a part at it" on this one. I don't want to pay someone $100 to tell me it's the IAC.

 

One went out on mine once...I think it is around $70. Just unscrew it and install the new one

 

Thank heavens for this site. I think I will spending a lot of time here in the future.

I think I might be having a similar problem with my '98 Silverado 5.7L with 107,000 miles.

As I drive into the garage and lightly press on the brake, sometimes the engine will surge and I have to clamp down on the brake. Needless to say, this is a little scary - I'm afraid I am going to fly through the back of the garage. It seems to be more of a problem when I just ease on the brake. It also happens while at a stop light or stop sign, etc. Does this sound like a IAC issue as well? I am tempted to change mine out as well.

Unfortunately, I can't find my Chilton's, though I do have a Haynes manual.

Any help is appreciated.