Hi guys, this is something my car has been doing for a while but it seems like it’s gotten gradually worse, or at least I’ve been noticing it more. The car’s a 1996 Mercury Mystique 2.0L 4-cyl, 74000 miles. The issue arises when I am idling in drive (like at a red light). There is a distinct rumbling that you can feel through the seat and the steering wheel, and see if you look at the wheel or dashboard. It’s not visible from outside the car, though.
The vibration is worst when the car is idling low – typically around 500 rpm. When I’m driving, the vibration is not noticeable. Also, when idling, the rpms “pulse” upwards to about 700 rpm, and the vibration goes away. It will be rough for about 5 seconds, then bump up to 700 rpm for a second or so and smooth out. If the car is in neutral or park, the vibration goes away.
So I suppose there’s two issues: 1) the low idle that “pulses” upwards, and 2) the vibration itself. Is the vibration just an effect of idling too low, and raising the idle might fix the problem? What would cause a low idle? Or, if the idle does not seem too low, what would cause vibration that is dependent on RPMs?
The scheduled maintenance is up to date, but the car is due for 75K maintenance next weekend. The PCV system was cleaned and valve replaced, air filter replaced, and spark plugs and wires replaced 12000 miles (~1 yr) ago. The idle air control valve was replaced at around 55000 miles (~2 yrs ago), I believe (the previous owner, a friend of mine, had had trouble with the car stalling at idle, and replacing the IACV seemed to fix it).
Thanks in advance for your advice. You can tell how good your advice has been in the past that I haven’t posted on the message board in six months!