Ok here is the problem.My 94 Cherokee 4litre ltd has a very high idle,approx 2000rpm on start up (hot or cold) which then drops down to 1200rpm.The garage that services lpg said it does not have an inlet air leak and apart from this problem engine pulls well and has no other issues. I have changed: throttle body,lamda sensor and dizzy. Does the temp sensor in the head(at the back on the right) have anything to do with the cold start ? The only reason I ask is that the ceramic is broke and it can be pulled apart,the temp gauge reads normal. 
From my experiences with a face lift cherokee it sounds like your choke is working on start up which is why the idle is excessive, when the choke (cold start mechanism) then switches off thats when your idle returns to normal, the temp sensor at the back of the head (fitted at the front of the facelift, I think from memory in the water pump or top of rad?) will help to control the fuelling of the jeep by sending a resistance signal to the ecu which then adjusts the fuelling dependant on this reading, you can check whether or not the temp sensor is working by checking the resistance with an electrical multi meter, the resistance readings are given in the haynes manual or you can probably find them on the net somewhere. It is surprising how many sensors there are which control the fuelling on the cherokees (I speak from frustrating experience on this!) I'm no expert on this these were merely some of my findings when the electrics on mine played silly buggers! Mike Pavelin is far more knowledgeable than me on these matters though! if you need the resistance readings I still have the haynes manual for mine!
