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Help!!! 1990 Jeep XJ starts & idles, but backfires through the intake & wont rev...

This is a discussion on Help!!! 1990 Jeep XJ starts & idles, but backfires through the intake & wont rev... within the Automotive Support forums, part of the Tech Support Forum category. I had to replace the original engine; i found one in a running and driving 1989 XJ, after i got


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Old 10-25-2010, 08:34 PM   #1
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I had to replace the original engine; i found one in a running and driving 1989 XJ, after i got the engine out i noticed the block casting was 10-12-96... not sure if that means anything. I put the engine in my 90 XJ, everything fit right so i re-used my distributor and fuel rail, it started & idled perfectly- when i gave it gas it started backfiring through the intake and wouldnt rev... I replaced the fuel pump and filters before the swap, and i have checked all the wiring & connections, & vacuum lines. I went and got the fuel rail, injectors, distributor, and computer, from the jeep that i took the motor from, still does the same thing... start and idle perfect, wont rev, backfires through the intake, it gets real hot real fast... dont know if that means anything... Please HELP!!!! any ideas or info would be greatly appreciated!!!

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Old 10-25-2010, 09:09 PM   #2
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Hows your ignition timing? If it is off it can produce the symptoms you described.

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Old 10-25-2010, 09:19 PM   #3
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Hi Scott Fleenor


Intake backfiring is caused by the timing being completely off or an intake valve that is stuck or broken. First thing you need to do is verify how much travel the timing chain has. To do this set cylinder no.1 to TDC and remove the distributor cap, note the position of the rotor and begin to turn the crank shaft by hand stop when you see a little movement. Then reverse direction and see how much a distance it takes for the rotor to move . If the gap from forward motion to reverse motion exceeds 1/4 inch then the chain is loose or the cam sprocket has stripped teeth.

If the timing chain is OK, then do a leak down test on all cylinders to verify that the intake valves are not bent or broken.



post back your findings.
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Old 10-25-2010, 09:55 PM   #4
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you are getting spark to a plug when the intake valve is open ... if its just a spit thru the intake you're getting compression when the valve is open

since you installed the distributor i would look there for the problem an check to make sure you dont have any plug wire's crossed

check timing chain like octaneman said
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