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  #1  
Old March 23rd, 2016, 10:39
SBpunk SBpunk is offline
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4.0 Issues on idle/power

1992 w/ 4.0 AX15 231

So having a rough idle and a few other things I'll list below. Worried the issue might be more than I can deal with in the garage. Hoping its just a fuel injector or spark plugs so going to be changing them out.

Rough Idle
Oil w/ Gas smell
Sometimes hard start (have to hit the gas pedal)
Backfire through intake (this one scares me a bit)
Lack of power (gearing and tires suck but still lacking)

Things I'm hoping would solve it is replacing the tail pipe after the muffler since theres some holes from rust. Testing out the fuel injectors to see if one is leaking/not working at all. Changing spark plugs because I'm pretty sure the PO said he put the 4 point ones in. Other than messing with the rotor, cap, wires and other general tune up stuff I'm concerned the problem might be in the pistons or valves.
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Old March 23rd, 2016, 11:10
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Overland Overland is offline
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Re: 4.0 Issues on idle/power

Showing any codes?
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  #3  
Old March 23rd, 2016, 11:48
SBpunk SBpunk is offline
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Re: 4.0 Issues on idle/power

Nothing just flashing 5 5 if I remember right or whatever the end of codes is
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  #4  
Old March 23rd, 2016, 13:19
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Ecomike Ecomike is offline
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Re: 4.0 Issues on idle/power

It is running rich or has a clogged cat converter, or both!!!

Rich running can be a bad O2 sensor (that can tested with a volt-ohm meter), leaking fuel injector(s) (test with a 9 volt battery and blow through to see if they open and seal), gas in fuel can also be fouled plugs and or worn gap on plugs, bad plug wires, cap and rotor, weak spark, criss-crossed plug or injector wires for starters.

Look for a bad spark, or injector or cylinder issue by disconnecting one injector wire at a time (or plug wire) to see if the engine runs the same or worse.

Change the oil real soon if it has gas in it!!!!
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34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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Old March 23rd, 2016, 13:34
SBpunk SBpunk is offline
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Re: 4.0 Issues on idle/power

I've changed the oil a few times and haven't been driving it as much. I've read you can test the injectors by pulling them off the intake manifold, putting water bottles on each and hitting the key to see if any leak. Then try to crank to see if any aren't flowing. Sounds pretty simple.

Thanks for giving me a few more things to look at. She doesn't have a cat but the down pipe back is pretty rusted out so I pulled everything. Going to clean the intake really well specially where the injectors go in to make sure I get that seal.

Picked up new exhaust manifold, down pipe, muffler, tail pipe and the accel tune up kit (wires, plugs, cap, rotor) and some used/tested sensors (MAP/IAC)
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Old March 23rd, 2016, 14:30
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4.0 Issues on idle/power

Is the timing correct? A warn distributor maybe
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  #7  
Old March 23rd, 2016, 17:59
SBpunk SBpunk is offline
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Re: 4.0 Issues on idle/power

Quote:
Originally Posted by Green Mesa XJ View Post
Is the timing correct? A warn distributor maybe
I thought the timing was controlled by the computer in the new XJ's. If everything else doesn't work I'll give it a shot. Rather do that than get the motor rebuilt. Really hoping its not the valves or pistons
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Old March 23rd, 2016, 18:20
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Green Mesa XJ Green Mesa XJ is offline
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4.0 Issues on idle/power

A worn out distributor can get sloppy and retard or advance timing. The rough running and backfiring made me think of it.

Edit: also sloppy timing chain. mine was in good shape at 200k I heard of others who had too much slack with fewer miles.

But excessive gas smell from the oil is a concern, if it really high mileage it may be on its way out but these 4.0 engines are tough to kill. If they remake die hard it will be about a 4.0 Cherokee.
Btw I have the same year with 300k everything major is original (engine, trans, transfer, axels) but the compression is getting weak. hunting for a motor for the eventual replacement.

Last edited by Green Mesa XJ; March 23rd, 2016 at 18:32.
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Old March 24th, 2016, 08:48
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Re: 4.0 Issues on idle/power

I'm hoping the gas smell in the oil is just a fuel injector staying open when the vehicle is turned off. I purchased some 4 point injectors from a guy here and didn't test them out before installing them.
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Old March 24th, 2016, 12:39
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Re: 4.0 Issues on idle/power

Quote:
Originally Posted by SBpunk View Post
I'm hoping the gas smell in the oil is just a fuel injector staying open when the vehicle is turned off. I purchased some 4 point injectors from a guy here and didn't test them out before installing them.
A bad O2 sensor also makes the engine run rich enough make the oil smell like gas. It can also burn up the Cat converter when not fixed right away
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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Old March 24th, 2016, 12:54
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Re: 4.0 Issues on idle/power

Quote:
Originally Posted by Ecomike View Post
A bad O2 sensor also makes the engine run rich enough make the oil smell like gas. It can also burn up the Cat converter when not fixed right away
Sweet hopefully that'll be squared away. I have a new sensor and downpipe I'm put in this weekend.
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  #12  
Old March 24th, 2016, 17:07
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Re: 4.0 Issues on idle/power

I fear that my O2 sensor is toast also. I think I got brake fluid on it, and if it has silicone in the fluid it is bad for the O2 sensor.
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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  #13  
Old March 25th, 2016, 13:07
SBpunk SBpunk is offline
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Re: 4.0 Issues on idle/power

Quote:
Originally Posted by Ecomike View Post
I fear that my O2 sensor is toast also. I think I got brake fluid on it, and if it has silicone in the fluid it is bad for the O2 sensor.
That might actually explain a bit too. I did a booster swap to the WJ model and pretty sure I got brake fluid all over the place.
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  #14  
Old March 25th, 2016, 15:23
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Re: 4.0 Issues on idle/power

Looks like DOT 3 and 4 are OK, but DOT 5, the new purple ?? brake fluid is 70% silicon based.

The old style green antifreeze does have silicates that will screw up an O2 sensor and cat converters when a head gasket leaks.
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Quote=8Mud: "Go ahead and put up the best fence you can build, I'll bet on some Mexican with a few dollars of nails and a pile of scrap lumber."
34 MPG , '85 2WD Cherokee Pioneer with custom installed, 64 hp, 2.2 L Nissan SD22 Diesel 5 spd Manual; & 4 Renix XJs, '87 Wagoneer 4.0, 4WD, 89-Cherokee, 4WD, '87 Cherokee 2WD, & '89 Cherokee Pioneer 2WD, all 4dr. #2091
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