I have a 1990 Jeep Charokee with a 4 literthat back firers through the exhaust and intake when fuel is applied

Question

it has new spark plugs, cap, rotor button, MAP sensor. When it starts up it will run for 30 Seconds and then cut off like some one has cut the key off

Answer 1

could be that exhaust valves are not closing 100%, due to carbon build up try a drop of red x in with the fuel.
If you cant get red x where you are, there are other upper cylinder lubricant degreasers/cokers on the market .
Not sure if you have the old points type ignition on that engine.
If you do and you have replaced the rotor arm, the chances are your ignition timing could be too retarded.
The fuel air mixture explodes slightly to late after full compression has been achieved.
that being the case the fuel can escape passed the exhaust valves and some of it will explode in the exhaust system.
If you do have this system you need to reset ignition timing.
Its a five minute job and does not cost much.
Again if you have this system and you have replaced the rotor arm, i reckon that's your problem
One last thought. Some of those old 4 litre jeeps have a mega size engine control fuse could be an intermittent fault with the fuse as far as i remember they are on the left hand side of the inner wing looking towards the back of the car( on right hand drive models)

Answer 2

I worked in a jeep dealer and Cherokee have lots of problems with the ECM (engine control module). purtty sure that's your problem, or maybe your crank sensor. that's the second most common problem. My suggestion is to take it to a very very good electrician.

Answer 3

could have jumped time. check number 1 for top dead center,and ck rotor position at distributor. you should have pretty high mileage on it by now.

Answer 4

Check out this web site it may have the answer you are looking for. www.jeepforum.com I hope this helps... Keep on Mudding!