I've noticed this on my sport before I sold it also, but the other day while sitting in a parking lot I noticed that at idle the vehicle will move slightly every few rotations. It's always done that but while sitting it started to get into my head. I opened the hood and you can see the the motor moving slightly, which yeah is kinda normal but I'd expect it to be smoother.

Just did a new oil change with M1 5-30, cleaned the TB at the same time (no low idle / dip problems or anything now). Engine has 94k on it.

It has a strong idle at the correct RPM, just seems like something's throwing off the balance. Oddly enough it vanished entirely when I turned off the A/C, but I feel like this is probably more due to the load on the engine than the A/C being able to move the vehicle/engine like that (I mean the compressor is tiny.. )

I've also noticed a slight knock/thunk type noise (not valvetrain or at the front/top of the engine) sounds like once per RPM, so who knows - again, at idle.

I just drove round trip Northern VA - Lexington KY about 1100 miles and it was great, on the first leg I actually went over 400 miles on a tank before filling up, and averaged 20.3mpg @ 65mph so I wouldn't think there's a miss or anything significantly wrong hopefully!

Also if you rev the engine up slowly from idle there's a span of RPM where it'll shake the car a little bit, then smooths out again. Maybe just a funky thing with this bore/stroke perhaps, as I remember the sport doing the exact same thing.

 

I'm no expert but that all sounds normal or similar to what I've observed from time-to-time.....

 

Do these engines have a fan clutch, or an electric fan?

An unbalanced mechanical fan blade will cause a vibration at a particular RPM range.

 

Yeah and yesterday she was really smooth so who knows.

I just read some horror stories and it makes me even more paranoid than I already am about random noises / NVH on engine and driveline.

This is supposed to be a decent motor though, well compared to the Disco II era. I suppose even Hondas have some horror stories out there.

I really would like to keep this thing to 250k so, just keeping an eye on everything. I really need to replace the thermostat/housing in the next month or so though.

Oh and FWIW I believe they use electro-hydraulic fans, I don't even think there is an aux fan..

 

Throttle body/idle control sounds like it is still gunked up. Did you clean it real well?

Check your engine mounts?