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Gremlins at long lights

Dear cartalkville,
Car: 2010 Forester
Situation: Occasionally (3x a week or so) the car when recently started will, at a long light, start to stall and then catch itself. The last time rpms got so low the dashboard lights lit up for a fraction of a second before the engine caught itself and revved up to a normal 650-750 rpm. I have taken to the dealer 3 times; one time they kept it for 3 days, they can’t reproduce it. Took it home and the next day the gremlin was back. Ideas? Help?

I would suggest that you focus on the Idle Air Control Valve (IAC) and/or a possible vacuum leak. Since the car is relatively new, I think that the chance of a disconnected vacuum hose is more likely than a gummed-up IAC.

Is there more than one Subaru dealer near you? If it is something like a disconnected vacuum line, another dealer might find it. It seems to me that if the line was disconnected at the original dealer, they may overlook it several times.

Automatic transmission? If so, I assume you leave it in gear and hold the car using the brakes. I also assume that the dealer just let it idle in neutral and that may be why they can’t duplicate the problem. I would start by listening for the sounds of a vacuum leak in the power brake booster.

Thanks, everyone! Yes there is another dealer (though 1/2 hour away and no mass transit), but I think it’s time to go to them. Yes, it’s an auto, and yes this happens in gear with foot on brake, so, Keith, good idea.

I concur w/@VDCdriver and all, this seems like an air leak somewhere. If unmetered air gets into the engine, it causes a lean mixture at idle. The effect of this lean mixture will be most noticeable when the car is stoped at a stoplight, the brakes applied, with the transmisison in gear.

PS. The coil went. They replaced it. But I swear the leak/problem is still there. Would these conditions be incidental. I can’t reproduce the idle-down/near stall systematically.

I can’t see how a failing ignition coil would cause the problem you’re having. If your car has coil-on-plug technology, the engine would miss and turn on the check engine light. Ditto if your car has a single coil or coil pack, except the engine would probably just stall with no warning and not just slow down.

You might see if the dealer has a firmware update for your car’s PCM (computer)