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Shuddering Subaru Forester has stumped 6 mechanics

I have a 2005 Subaru Forester, 40,000 miles, with a 7 month problem that has stumped 6 different mechanics.

I turned on the car one day to find it vibrating at idle and when I put it into drive the vibrating got worse, as if the car were sitting through a sizeable earthquake. I didn’t drive it and instead let it sit for a few days. The next time I got in the car (to take it to the mechanic), the same thing happened and the check engine light came on. I drove it to the local mechanic, but on the way, the vibrating stopped, the check engine light turned off and the dealer could not replicate the problem. The scanner showed no engine trouble so I took it home the next day.

The car behaved normally for about two months until the same thing happened. As the first mechanic was no help, I took it to a different mechanic, who claimed the battery needed replacing. This solved the problem for about a month and the car drove beautifully…until I got into the car one day a month later and the same thing happened (shuddering, check engine light on- which stops after driving the car around for 10-15 mins).

This has happened to 6 different times now. Every time, the shuddering starts as soon as I turn the car on, the check engine light comes on a few seconds later, and usually the vibrating stops and the check engine light goes off after about 15 minutes (making it impossible for the car dealer to replicate the problem and see what’s wrong). The most recent time, I told the car dealer to keep the car until he could replicate the problem, and after a week he told me he said he had seen the problem,knew exactly what was wrong with it, “fixed it” and, again, it worked for all of three weeks before the problem recurred. For better or worse, the problem only happens intermittently, making it unpredictable and difficult to replicate for a dealer. No scanner has ever shown codes for engine trouble either.

The mechanics have so far:

replaced the battery

replaced the drive belts

replaced the brake pads and rotors

drained/cleaned/replaced the oil and other liquids

performed fuel induction service

Overall it has not been too expensive, so I do not feel like I am being gipped, but nobody has actually solved the problem yet and I am hesitant to take it in again without better advice. Any thoughts?

Is the problem related to rainy days?

Unfortunately not, nor is it related to the amount of time it’s been since I’ve driven it last. I’ve begun keeping a log of when it occurs, and sometimes it occurs after rainy days/weeks, other times it starts up again after dry/beautiful days/weeks. Also, sometimes it occurs when I haven’t driven the car in a week, other times it occurs when I drove the car all day the day before without a problem.

Another symptom I remembered just now is that the first time this happened the cruise control light came on (though the cruise control worked just fine). This only occurred the first time though, the cruise control light does not come on any more even though the problem remains.

There’s not enough info known to make much of a guess. At this point I would suggest getting an auto parts house to scan the car for codes and post the results back here.
They will do this for you free and it only takes a few minutes.

Before tossing out some theories (a.k.a. half-baked guesses) let me ask this.
Does it only do this on a cold engine start or will it also do it upon a warm engine restart?

I wonder if maybe the power to the engine’s computer is being interrupted, which wipes out the engine performance parameters. This happened in my truck, but the engine smooths out after a few miles as the computer “relearns” the necessary parameters.

Try to reproduce the problem by disconnecting the battery for a few minutes, then reconnecting it to see if it acts the same way. If so, then look for an intermittent short to the engine computer.

If the latest repair lasted only three weeks, then that repair should still be under warranty. I would take the car back, or wait for the next instance, and then have that last mechanic look at it again. The downside of shopping multiple mechanics for a recurring problem is that there is little to no accountability on the part of the work done previously. Decide on one mechanic and stick with him until he is proven to be incompetent.

The check engine light code should be set even if it goes off 15 mins later, so there ought to be some codes stored to be read. Get them and post them here.

It will do this with either a cold or warm engine start (though… only sometimes).