What year is the Grand? And what engine is in it? More info the better for problems like this. Oh, and welcome to the forum.
do you find that it only does it on a very short jorneys its just that i have a jeep gc lpg and mine does the same only on a very short jorney under a mile
That sounds like a problem with the LPG system.
Crank sensors can be affected by the heat as suggested. It's mounted at the front of the bell housing on the left side of the engine. Another thing that can also be affected by heat is the coil which is bolted to the engine.The crank sensor is a hall effect sensor similar in nature to ABS tone ring sensors. The coil primary should be 0.95-1.2 Ω The secondary resistance should be 11,300-15,300 Ω.If you turn your key on-off-on-off-on and leave it on the third time you will get codes displayed in the odometer display. A crank (or cam) sensor may have stored a code (11). 55 means end of codes. Try it, and post any codes displayed.
Link is up to 97xj...OBD1 codes..should be same for the Grand?http://go.jeep-xj.info/HowtoECUcodes.htm
Hate LPG.......cheap mileage but more to go wrong....
sounds like a trip to the stealership to me lpg can fool oxegen sensors so i would imagine it is where the fault lies but there is a possability that its the cam sensor as its a sneaky litle bastard when it plays up so dont rule it out