1998 Grand Cherokee
Six cylinder 4.0
The engine idles roughly.
The eninge wil rev but it shakes and bucks.
There is no backfire.
The check engine light is not on.

Things my mechanic tried -
replace the TPS with an OEM unit
disconnect/re-connect the O2 sensor
change the ECU – it’s not giving any codes
disconnect the clockspring by pulling the horn relay
engine grounds cleaned and checked
different fuel injectors
cleaned the throttle body
replaced the IAC
checked the condition of the battery – it’s less than three months old
checked the fuel pressure
checked the plugs, wires, distributor cap and rotor – they are about five months old.

The airbag light is on but it has been on for two years.

Any help is greatly appreciated.
Peter

 

It might be a mechanical issue. A compression test and cylinder leak down test should be performed to see if there's a dead or weak hole.