i have a 90 cherokee 4.0 6cyl 4-w drive , hard to start and eng turns off when it's put in drive but it will go in reverse.we have replace the ignition coil and the rotor and cap .had severel wires checked and still don't know what to check next  

i dont know why the problem it has with starting but its prob stall into drive becuz the rpms drop with the shift and there isnt enough power to keep going, and reverse doesnt drop the rmps as much... but have you checked the fuel pressure at the rail, spark plugs, and used a spark tester (one of the wires made for checking spark with the light on it)... if so and its still a problem i would be leaning towards sensors... just use a multi-tester and check to the normal values...  

P.S. hows your cat? and the rest of your exsaust, a clogged exsaust can cause issues  

A CAM SENSOR WILL WILL CAUSE HARD STARTS AND WHEN IT DOES FINALLY START ITLL DIE...THE THROTTLE POSITION SENSOR WILL DO A SIMILAR SIMPTOM (DIE AT IDLE OR LOW SPEEDS IN GEAR)BUT ITLL BE EASIER TO START.  

a good point, did you drive through any large deep puddles or streems? cuz if your got your CPS wet that would happen... when it happened to me it took 5-6 days for it to fully dry out...  

eng ran fine it just died on me last week this prob happend all at one time,cam sensor sounds like a good start. just one of 20 sensors this jeep has .cac is fine just about a year old  

fuel pres seems fine dont have a psi tester for that. but i'am woundering if the fuel injecter  system my be going under or settings my be off