I have 1997 4wd 4.0 XJ that I have had now since Sept. Soon after I bought it I would feel a vibration around the mid rear of the vehicle only under hard acceleration between 20 and 35 mph. I suspected the rear u-joints so I replaced both u-joints in the rear and its still there. Any ideas at what it could be?

Also after I bought it I did a full tune up. Plugs, wire, and new distributor(it was squeaking)plus new fluids in both diffs, transmission, and engine. One day I was accelerating harder and heard a bang under the car, when I looked under the car the exhaust came off at the CAT probably because of the vibration and the exhaust clamps were rusted and not clamping anything. When it fell it also broke my O2 sensor. I then bought new clamps and a new O2 sensor. Since then my jeep idles low and almost feels like its going to die sometimes. It gets better once its warmed up and driven for a bit. I also changed my other O2 sensor and cleaned the IAC. Problem got a bit better after the new O2 sensor but not much. After this exhaust fall my MPG is all over the place. If I reset the ECU I can get about 16 mpg mixed which I would say is good for a stock 160,000 mile engine. Also when I reset the ECU the idles gets better. Then after about a full tank of gas the mileage goes down to about 12 to 13 mpg with the same driving and if I reset again it will go back up.

Overall the jeep drives great as long as I don't leave it to ideal for to long. Once it ideals for a long time(about 20 mins) it starts acting like its about to die and also when its acting like that if I accelerating it hesitates and then it sounds like its misfiring but once the RPM stay up it drives fine. Any ideas on the idle and MPG? Im stumped. The only things I can think of now is a new IAC, TPS, or Fuel Pressure. Im wondering if you guys can point me the right direction? Thanks a lot.

 

is pre o2 sensor replaced with correct part? or are the wires ok?do you have rich mixture smell at back when engine is warm? if nothing is wrong, TPS can cause also crank position sensor can caure weird idling too. May be crank position sensor.

plus, worst thing in electronics which i hate most is they may not be broken enough to trigger codes.

 

Thanks for the info, I will check and replace both the TPS and CPS if needed. Any ideas on the vibrating?

 

CPS's dont wear out. haha i just said this on another thread. if a CPS goes bad your car will die or refuse to turn on. theres no rough middle. its worth replacing but will show no difference in performance.

 

few years ago we own an opel sedan. its crank position sensor went bad. there was a check engine light on dash. It appeared and disappeared time to time. It was obvious that something was wrong on car and it was occasionally stalling and engine revs were changing when clutch engaged. when engine light was on, you cant even move car, it only was vibrating like you forgot to press clutch pedal because of lack of power. no power at all. that is why i stated so.