Hi all. As long as I'm asking for help in another thread, I might as well report a 'field expedient' repair that seems to be working out well so far. This should be useful to owners of similar Astro or Safari vans.
I was getting a fast idle, frequent check engine light, and an IAC circuit fault code on my '94 GMC Safari with the 6 cyl. engine and multipoint fuel injection. Searches in this newsgroup turned up the fact that it's a pretty common complaint and usually caused by a plenum gasket failure on the front of the engine towards the passenger side which let's enough air through to completely swamp the effect of the Idle Air Control valve and cause the engine to race.
I got under the hood and took the air cleaner and intake ducts off so I could get at the area, and I confirmed that 1) the engine continued to idle *very* fast even with the intake hose completely plugged! 2) Squirting a little propane near the front of the engine caused the RPM to rise slightly; 3) When I probed the area with a thin piece of music wire, there was clearly about a 1" wide chunk of gasket that is just gone -- the wire slipped right through the gap!
So, I guess that's pretty clear, huh?!
I sprayed and scrubbed the area with throttle body and intake cleaner. Then I applied a glob of silicone to the length of the gap in the gasket and had an assistant engage the starter for about a half second. I could see it get drawn into the crack just enough to fill it without sucking a hole through the sealant. Then I let it cure overnight.
I used silicone without any problems, but there have been reports on this newsgroup that the acetic acid in normal RTV type products can trash your O2 sensor. So, it might be wise to use one of the "sensor safe" gasket compounds that are available instead.
The next day I tried starting it, and it immediately came into closed loop control of the idle, for the first time in months! No resetting of the ecm or recalibration procedure for the IAC valve was needed.
I can now idle along at 5 miles per hour, I have to actually apply throttle rather than brakes to go 25mph, and it takes much less brake pedal pressure to stop the vehicle.
Whew!! What a nice result. Now all that remains to see is how long it will last. It's been good for over a month now, but we haven't gotten into the hottest part of the year yet.
Can anybody give me a ballpark figure what it costs to have the gasket replaced properly by a good garage or dealer (US dollars)? Would such a repair hold up any better than the original assembly?
Is there even a slim chance that this repair will hold up? I figured it's worth a try anyway...
I hope this will be helpful to a few people.
Be well,
Mike