My 93s 4.3 Vortec starts and idles normally (around 7-800 RPMs), but after it warms up, the idle will stay around 1500 RPMs and NOT drop.

If I unplug the IAC Valve (when the engine is warm), it has NO Effect on the idle. (??)

I also started it up Cold and it still idles around 800RPMs or so. I unplugged the IAC after running the engine about a minute or two. It made absolutely NO difference to the idle speed.

The Check Engine light will come on after the engine is warmed up and it's in gear (like when stopped at a Red light). But after I start driving again, the light goes back out.

BTW, I have a IAC Valve coming to me off Ebay....

Besides a IAC Valve, what else would cause the idle speed to climb after the engine is warmed up ??

 

the 4.3 L vortech has a history of smoking the plenum gasket, its not hard to replace and its not expensive.

can you hear any whistling coming from the engine?

 

Hmm interesting..... No whistling or vacuum leak noises. How can I rule out the intake gasket ??

 

i have no idea, sorry, im not a mechanic, i just know 3 people that have had that gasket fail, and i dont know many people with 4.3L's

i really dont have much other than their experience that tells me that the gasket likes to blow.. but i figure if 3 of maybe 6 people that i know that have that motor have had it go.. it must be common

 

Yea, I'm starting to hear about this gasket issue from some other boards.

Just to confirm, are you referring to the Upper Intake gasket ??

 

yes sir, plenum, upper intake same thingy