I have a 1996 Geo Prism Lsi, 1.8 liter automatic, which I bought new.  It has a vibration problem when it is in gear and it is idling. The vibrations get worse when the A/C kicks on.  It runs great, idles fine, it just has a lot of vibration when stopped, which shakes the whole car.

Back in May of 2013 I had all four motor mounts replaced. The mileage on the car was 122,317. After the new mounts, the car ran great, no vibrations.

Timing belt was changed in 2015, mileage 130,847

Drive belts replaced in 2017, mileage 129,987

Starting in 2018 I noticed the car was vibrating when it was in gear and idling.

In October of 2018 I had a garage tell me I needed new motor mounts but they only replaced two the passenger side (belt side) and the front (next to the radiator).  The mileage on the car was 162,889

The car was still vibrating In November of 2018 so I replaced the plugs, distributor, rotor, plug wires, cleaned throttle body, replace PVC valve, I have a note that says I cleaned the MAF sensor (but I don’t think I have one to clean?).  The mileage on the car was 163,245.  The car was still vibrating.

In December of 2018 I took the car to a garage and they told me I needed an Idle Air Control Valve, the mileage was 163,727, after the new Air Control Valve the car was still vibrating.

Sometime in 2019 I ordered a set of four motor mounts. I installed one new mount on the transmission (driver side) sometime in 2020 (not sure of exact time frame), but wasn’t able to change the one in back (by the firewall) because I didn’t have a safe way to jack up the car.  So the remaining three mounts stayed in my trunk.  The mileage was 168,990.  The car was still vibrating.

A few days ago I took my car to a garage and was told I needed all new motor mounts. I had the mechanic replace the mount in the front by the radiator, and the one in the back by the firewall.  I told him I would do the one on the passenger side and I told him the mount on the transmission was replace about a year ago, the mileage on the car is 170,943 and the car is still vibrating.  The vibrations have not gotten worse but the have stayed about the same.

Today I looked at the mount on the passenger side that was installed in 2018 and it looks like a good quality mount, not like the cheep ones I bought, so I don’t know if I should change it or keep it.

I have so far, since 2018 spent around $1,300 trying to stop the vibrations.  One of my best friends is a Master Mechanic of around 45 years and his advice was to stop worrying about it because it’s a 26 year old car.  But I would like to stop the vibrations and continue to use the car for another 10 or 15 years.  Any ideas on what I can do next to stop the vibration would be greatly appreciated?  Should I change the remaining passenger side mount, or is there something else that could be causing the vibrations?