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Old 05-13-2009, 02:01 PM   #1
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Default 1990 Cherokee 4.0 backfire misfire help.

Hello all, first time posting here and need a little help. Sorry for the long post but Iím about to scrap this if I canít fix it.

I have a 1990 Cherokee Limited with a 4.0, automatic with 130k miles. And I am having problem with it backfiring when I let off the gas when driving. If I rev it up in neutral it also hesitates and misfires/shakes badly. The fuel economy is also bad, I was getting about 16mpg now Iím getting 12mpg.

This is a list of what I have replaced recently.
Exhaust, from the manifold back including new converter.
O2 sensor
Intake/Exhaust manifold gasket.
Fuel filter
New distributor, distributor cap, plugs, wires
Water pump
Radiator
Harmonic balancer pulley.
Battery with new ends

I replaced the distributor because the old one was oscillating badly when running. The remanufactured one I put in still oscillates but not nearly as bad. I just got another remanufactured distributor to replace the other one thinking thatís the problem but havenít installed it yet.

Any and all help is much appreciated, Brad
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Old 05-14-2009, 09:38 AM   #2
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Check your fuel pressure WITHA GAUGE.... Harbor Frieght has a Fuel Injection pressure tester for less than $20. My 90 wasdoing similar but it got worse and worse. Couldn't drive it. Replaced lots of parts (and now I have a great set of spares for the trail), and the problem was now fuel pressure. MY resolution to the problem, was the little piece of hose on the pump INSIDE the tank, was rotted. Others have had similar issues. Still others have had bad pumps. Get a new fuel strainer/sock that fits the bottom of the pump if you pull it.

On your 90 the pump comes out without dropping the tank. Just make sure you use aluminum or bronze tools when takingthe pump retaining ring off (nonsparking tools), and that the tank is less than 1/2 full...
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Old 05-14-2009, 08:30 PM   #3
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That might be the problem. What should the pressure be? When I disconnected the fuel rail I tried to relieve the pressure how the book said and no fuel came out of the petcock.
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Old 05-14-2009, 10:47 PM   #4
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You should have about 31 psi.
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Old 05-15-2009, 12:33 AM   #5
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i had the same prob you do a long time ago. my 89xj had a bad fuel pressure regulator. DROVE ME CRAZZZZY!!!!! damn 50$ part right there on top
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Old 05-15-2009, 01:49 PM   #6
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I finally got it running right. I put another remanufactured distributor in and it runs perfect. What a PITA all the stuff I replaced and it was a bad new distributor.

The new distributor still oscillates when itís running. What would cause this?
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