I have a 1999 GSR with a terrible vibration right at 1500 RPM. If you sweep from 1000-2000 or back down, the dash shakes like crazy right at 1500 RPM. If I hold the throttle constant right in the vibration range, I can tell the engine is pulsing. According to my OBD bluetooth adapter and Torque, it pulses + or - about 50 RPM. Throttle position and manifold pressure remain constant (according to Torque). I unbolted the exhaust at the catalytic converter to see if it was related, but the vibration was still there.

I'm all out of ideas. I've checked timing (mechanical and ignition), checked valves, and replaced the following parts:
All motor mounts (even did tightening sequence twice to ensure that wasn't the issue)
Alternator
Distributor

I'm baffled because it vibrates as RPM's go up, or as they drop down, meaning it isn't ignition related. I also ran the engine with the PS and AC belts removed and it had no effect. For a '99, the car seems to have a strong exhaust smell. Not rotten eggs or anything, more like gas fumes, reminds me of driving my old carbureted farm truck in high school.

Can anyone help?

 

Update
I just did the exhaust leak test shown here and found a hairline crack in my exhaust header on cylinder #4. The previous owner put a Comptech header on the car and when I got it, there was no lower support near the catalytic converter, i.e. the whole exhaust was cantilevered off of the head, which would put a lot of stress on these welds.

Looks like I'm in the market for an OEM GSR exhaust manifold. Could this be the source of my 1500 RPM vibrations?

 

First thing I would suggest for fluctuating RPM for Honda/Acura (all models) are cleaning following 2 valves

https://honda-tech.com/forums/acura-...valve-1575913/
https://honda-tech.com/forums/acura-...valve-1564019/

Both are common preventive maintenance for cars older than 10 years, especially they never been clean, or you bought used.

Exhaust header leak should not cause fluctuating RPM. So if above 2 cleaning jobs doesn't resolve it, then look for vacuum leak, or replaces all vacuum hose with new 8mm hoses. This is another cheap (< $15) preventive maintenance for old car

 

Thanks for the advice. I have cleaned the IACV but my car doesn't have a FITV. I think replacing all the vacuum hoses like you mentioned would be the next step to address the fluctuating RPM.

As for the vibration, I have a few updates. I took off the timing belt tensioner and it seems to spin a little rough. Not bad, but I think I'll replace it anyway. Also, I noticed that the lower bolt of the three that hold the driver's side motor mount to the block was loose. Not bad, but definitely way below spec. Once I get everything back together I'm hoping that the vibration is gone. If not, I'm out of ideas!