1993 Jeep GCL 4.0L Problems with engine not idling and trans not down shifting. Then it would crank, but not start (no spark). I replaced the ECM and it started right up, but still had the low idle and was not down shifting. The next day it was back to cranking, but not starting again! Any ideas?

SOURCE: 1991 jeep wrangler cranks but wont start

just a suggestion hows your leads?

SOURCE: HIGH IDLE

check for deposits in the airhorn at the breather end

SOURCE: 93 Jeep Grand Cherokee Won't start

Are you getting spark from coil wire when cranking? If so, check the cap & rotor. If not, the most common failure is the crank sensor. If you have a 4,0, you can find it on the drivers side of the bellhousing, in line with the flywheel, just where it's annoying to work on. There is a test you can make, but I don't have the specific test values or the correct terminals to check. (you can find that info in any half decent manual.
Good luck

SOURCE: 89 Jeep Comanche Fuel Pump. Won't start after running great.

Replace the crankshaft position sensor

SOURCE: 89 wrangler 2.5 5 speed.replaced crank sensor last

It sounds like your new crank sensor is defective. It should be under warranty.