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2002 Acura RSX. Sudden high idle speed on startup, slight rumble, goes away after a few miles

need some help, guys. I’m the loving owner of an amazing '02 RSX. She is just about to hit 100K, and has been completely trouble free. This past sunday, I started it up in the morning and immediately noticed a semi-harsh vibration, with high idle speed (1500ish, normal is 500). If I give it gas, the vibration and rumble noise goes away, but if it put it in reverse, it seems to get worse. After a few miles of driving, the idle speed returns to normal and seems to feel/sound fine. Also of note, if i hit a dip in the road, i get a vibration and engine sound that resembles that of a flat tire. I know this sounds random, but it’s the best I can do at describing it. There are no check engine lights, temp is fine, and otherwise seems to be driving great. I initially thought bad spark plug or air filter, but have since thought of vacuum pressure or O2 sensor, maybe even an engine bolt break (from the sound it makes driving into a dip). Could this be as simple as getting a bad batch of gas? I’m a grad student in Baltimore (i.e. poor) and hope this is an easy fix/solution. All the garages around here want at least $90 just to plug in the diagnostic tool. I appreciate your help, and if you solve my problem on the cheap, I’ll even offer to throw in some world famous crab-cakes from charm city itself! Thanks!

“engine bolt”=mount screw

Hondas and Acuras have a history of bad motor mounts, particularly as the miles add up, so I would suggest that you have your motor mounts (and transmission mounts) checked a.s.a.p.

As to the fast idle that goes back to normal after a few miles, this could simply be a function of lower temperatures in the A.M., as the seasons change. Fast idle in colder temperatures can easily go to around 1,500 RPM for a few minutes.

Thanks VDC. I plan on looking at the mounts this weekend, that would account for the shaking. As for the temps, I don’t usually leave in the mornings until around 10, so it’s at least up to about 60 here still. Also, when I’ve had it parked all day at my lab, I’ll come out at 5 and it’ll do the same thing, but fine when I get home.