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Jeep backfiring I think I have checked all recommendations I have found

Precious88xj

NAXJA Forum User
Location
Los angeles
Working with 1988 jeep cherokee 4.0 auto trans 4x4 with a 96 jeep cherokee engine

Background that led me to current situation:

My original engine gave out I had about 70 psi of compression couldnt really drive i decided to swap in the a engine from a 96 cherokee which was in a wreck but had apparently been rebuilt. I did the swap and when I attempted to turn on the vehile it didnt run correctly. It stumbles and idles poorly. At about 2000 rpm it will backfire i saw fire out of the exhaust. Have not been able to actually drive it .

Diagnoses I have done
Fuel pressure 31 vacuum connected/39 disconnected
fuel pressure regulator not leaking
the fuel pressure holds after i turn off the jeep no leaks
compression test dry 1-159,2-159,3-170,4-163,5-163,6-170
multimeter tests map sensor,coolant temp sensor, tps(adjusted),intake manifold sensor they all tested to spec
spark on every cable
cleaned up the grounding connection

i also have nickintimes computer reader the codes that did show was loop fault rich and loop fault lean


New Parts:
Fuel Injectors,fuel pump,regulator,filter
coil,distributor,ditributor cap,plugs,cables, rotor
TPS,CPS,O2 sensor,IAC,


Im at a loss I applied all applicable tips by cruiser and I dont know what to do.

If necessary I would be willing to record all test procedures to make sure I havent been doing something incorrect.
 
First off, your fuel pressure is way too low. This could be caused by low vacuum, bad pressure regulator, clogged filter, bad fuel pump. Low fuel pressure can cause combustion temps to soar and cause any residual carbon in the cylinders to glow red hot, causing pre-detonation (not good).

I would put a vacuum gauge on it and post the readings at idle and at 2000 rpms no load. Is it steady or does it rapidly fluxuate.

I would also pull the plugs and do a reading/post up pix.

I would also put a timing light on it and see what it reads at an idle and at 2000 rpms.
 
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