Car shaking at idle?
Asked by 04SubaruForesterXS Apr 02, 2020 at 03:04 PM about the 2004 Subaru Forester XS
Question type: Maintenance & Repair
My car will idle at 750rpm while parked, rev up only the
slightest bit (not going above 1,000rpm) and then rev down
back to 750rpm but then shake only the slightest bit. Is this
normal? Or could it be a foreshadow to my
engine/transmission mounts needing to be replaced?
(Image is what the car revs up to before it goes back down)
5 Answers
TheSubaruGuruBoston answered 4 years ago
Pretty normal. You CAN bring the idle up slightly by loosening the idle control module on the right (pass) side of the throttle body and VERY slightly rotating the assy. Be satisfied with a bump of only 50-100rpm, as if you overdo it the ECM will get annoyed and slam it back down to 700 again, possibly throwing a CEL you have to erase. Generally these old SOHC 2.5 like to run around 850, but that's too fuelish for their fuel eco ratings, so Subaru squashed them down to a shaky 700rpm. Individual motors will vary, as micro-balancing the pistons and rods is something only Ferrari and Porsche do for their horizontally-opposed boxers.
TheSubaruGuruBoston answered 4 years ago
To max fuel eco ratings Subaru slams idle speed below the "happy" zone of 800-900rpm. You can usually successfully bump up into that zone by loosening the idle control module on the right (pass) side of the throttle body, and VERY slightly rotating it. Don't get greedy. If you raise it too much the ECM will complain, slam it back down to 700rpm, and possibly throw a CEL soon. I was usually successful in getting these old SOHC 2.5i up to 750-800, just above the shaky 650-700rpm factory setting. Don't forget to retighten the mounting bolts, and have fun. Ern TSG/B
TheSubaruGuruBoston answered 4 years ago
Two responses didn't post. Why?
04SubaruForesterXS answered 4 years ago
Am wondering the same thing. I have them in email, they're just not up. But anyways - thank you very much!!! Your answers were very helpful and insightful
