Saturn '98 SC2 idling problem after highway driving
December 13, 2006 9:07 AM   Subscribe

Why does my '98 Saturn SC1 have odd idling problems after more than an hour of highway driving (i.e. it sometimes revs high, sometimes it drops to nothing and stalls)?

According to the mechanic, the car's onboard computer shows no signs of trouble. Outside of this, it does not exhibit any other kind of issues. The stalling I could deal with, but the revving is problematic. At toll booths, I really have to hold down the brake to prevent it from lurching (or throw it into neutral, of course).
It has been tough to have it diagnosed because it only happens after an hour or more of driving at 65mph.
posted by Java_Man to Grab Bag (8 answers total)
I'm guessing a speed sensor of some sort is intermittently malfunctioning. The reading it is putting out is still in range, apparently, so it is not flagging a fault code the mechanic can see.

Might be best to take it to a dealer (assuming your mechanic is not at a dealership), and/or try to find an online forum for Saturns (try googling Saturn SC1 forum).
posted by Doohickie at 9:20 AM on December 13, 2006

Best answer: Sounds like the IAC (Idle Air Control) Motor is stuck to me. Your mechanic should be able to exercise it with a scanner and see if your idle speed responds to changes in commands. It could be a heat sensitive intermittant connection.
posted by rfs at 10:12 AM on December 13, 2006

I had a faulty Oxygen sensor that did the same thing. It was an older Oldsmobile. However, it did show up on the computer.
posted by advicepig at 11:58 AM on December 13, 2006

Another resounding vote for the IAC valve. I took my car on 3 separate trips to the mechanic for the same, exact thing you described. However, the problem was intermittant and would never "do it" when it was in the shop, but finally... finally it registered on a computer test that it was the IAC valve. The part was only about 35 dollars. My car no longer stalls out while stationary, idles high, guzzles gas, lurches, et cetera...
posted by goml at 12:30 PM on December 13, 2006

Typically the idle speed diagnostic codes are calibrated to take a fairly long time to set, so they don't get faked out.

IAC motor and IAC valve are names for the same thing - an idle air bypass valve that is controlled by a stepper motor.
posted by rfs at 9:45 PM on December 13, 2006

My apologies for answering this so late. I had a '97 Saturn SL1 that did the exact same thing for several months, and then died on the expressway - completely. It turned out to be the Crank Shaft Position Sensor (which was missed by three mechanics in two different states - didn't show up on the computer). Best of luck.
posted by betty botter at 10:58 AM on December 14, 2006

Also, to diagnose the problem in my Saturn, we ended up taking each part of the circuit out of the car, heating it, and testing it with a resistance meter, as the problem only occurred when the car was thoroughly warmed up.
posted by betty botter at 11:01 AM on December 14, 2006

Response by poster: I had a new IAC valve installed. Since then I have not had the problem. Fingers are still crossed...
posted by Java_Man at 12:35 PM on December 28, 2006

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