1990 jeep cherokee don't start

I don't think so. It has a returnless fuel system, so i don't see any way that coolant could go to the fuel tank.

Must of burned one and put the water in the gas tank.

http://jeep.off-road.com/jeep/voice/the-jeep-creep-questions-and-answers-15727.html

click this link and read the FQA

hope you fill find you problem solution

All I can say is do some research online find a REPUTABLE shop and let them drive it until it dies. Three things are needed for an engine to run: spark, fuel, compression. I doubt it is a compression problem being that it is a new engine. Something is causing either spark or fuel to drop out, a good mechanic should be able to quickly identify what is missing from the equation. Find a shop with a lab scope that can test sensors that send signals to the engine computer. It could be as simple as a fuel pump getting hot and quitting. Good Luck!

Does your check engine light illuminate on the dash, when you turn the ignition on, right before cranking? If it does not light up, at those times when it's hard to start, then the main ecu might be in need of repair.

you can have a bad coil you can have a bad crank sensor a bad cam sensor a bad throttle posistion sensor etc. if you can have someone do a diagnostic on the car to see if any codes are stored this will give you a good starting point.

either fuel pump relay,or ecm

Check the crank position sensor. I had the same problem in a 1989 cherokee 4.0, and the solution was simply to acomodate the sensor wiring. No replace was necesary

Try replacing the wire from the positive battery cable to the small terminal it is attched to. It is the smaller gauge wire that is attched to the battery terminal. Be sure to use the right gauge, don't over or undersize it. I had this problem and it did'nt stop until I changed this. Without this wire in good condition, the starter will crank too slow or not crank at all, mimicking other problems (like bad battery, bad starter , etc.). Post if it works.

Try replacing the engine relay switch. It is located under the hood with the other relays. If not check the wires for ground. Check the battery cables , especially the cable that goes to the positive of the starter.