So I bought an 89 Cherokee limited 4.0L as a build project for my daughter. I was able to get it running pretty quick. ( had a bad wire to the ecm ) Now it will idle fine but as soon as you give it throttle it starts backfiring. Here's what I've done so far. It has a new crank sensor, new distributor, new coil, new plugs and wires. Cleaned out the fuel tank and put new gas in it, new filter in the tank. I have gone through all of the sensor tests and all are good. I did notice it has a used ecm in it that had a tag for a 90 manual trans not sure if there is a difference. I did pull the plugs and they are carbon fouling. Still need to change the fuel filter and test rail pressure. any help would be greatly appreciated.

The last time I had a problem like that it was my fuel pump. Had just enough pressure to idle ok, but not enough pressure to move the car around.

While there certainly are many possibilities, I too would start with testing fuel pressure. Simple stuff first, always.

For your 89 (A Renix) you need approximately 31 psi. Must be tested with a fuel pressure gauge, call around to parts stores to rent one if you don't have access to one. Then disconnect the vacuum line leading to the fuel pressure regulator and the fuel pressure should jump to approximately 39 psi. If that vacuum line has raw fuel in it, replace the fuel pressure regulator as the diaphragm is compromised.

Also, check out this site. Cruiser54, a friend of mine through the years has a website with great information for Renix (87-90) owners like yours. Invaluable information; be sure to bookmark it.

http://cruiser54.com/

Good luck and be sure to keep us updated!

Are you 100% sure the timing has been set correctly with the new distributor?

Worth checking/indexing. http://cruiser54.com/?p=68

Ive been told that if it backfires through the tail pipe, its a fuel issue but if it backfires through the carb/throttle body, its a timing issue. Helps to determine a good jumping off point for troubleshooting in my experience. Hope it helps...

backfiring through the intake is generally a lean condition.

Thanks for all the tips. Today I tested the fuel pressure. Sits at 33 psi jumps to 39 psi with the vacuum line pulled from the regulator. Replaced the fuel filter and spark plugs. I set the timing per the tips given from the link supplied. Great info by the way. Very little change in the way it runs. Not sure where to go next. Maybe timing chain as it does have 200000 miles.

How are the cap, rotor and wires.

More info. Just found per the wiring diagram the #2 & #3 injector wiring and the #5 & #6 were swapped. Changed them to the schematic and still runs the exact same. Figured it would make a difference.

Cap, rotor, wires and plugs all new.