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Fuel/Performance Issue... immediate help requested

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Started today. Out of the blue.


Patient: 87 MJ, 4.0L 6 Cyl


Symptom: VERY throaty under load, first gear, second gear are both hard to accelerate, its bucking and coughing so bad, third and fourth better a little, but still very hard to speed up. Fifth is non existent because i can't GET THERE. Smells rich in the fuel. throttle cable moves and springs back with little trouble.


Signs: rough engine is not present, or very little when in neutral. Only appears in gear. Very light gas makes it drivable, but when given a little more, roughness begins.


Treatment done: Changed cap and rotor. No effect.


This is a daily driver MJ. Immediate help requested.

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Went to a boneyard today that has a hefty amount of XJs, and pulled a throttle sensor off another RENIX engine. (And saw a VERY nice MJ and took the tailgate...)


Replaced the sensor... it did NOT fix the problem.


Do we have a picture or guide to all the vacuum hoses so i can know which to trace?

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I don't know what "throaty" means to you ... to me a "throaty" exhaust would be something deep, mellow and powerful, like a Ford Mustang V8. But ... for your symptoms, it sounds like a clogged catalytic converter.

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I had a distributor that was grounding that caused the same problem , took me forever to find it because it only did it under

heavy throttle in gear . 1-2-3 were terrible and yeah no 5th . Thing is once I tested the Dist. , It worked fine .


Under heavy throttle older motor mounts flex and torque out and caused a clearance issue with the block and the dist.


It could be a number of things but I'd start with spark since you can still smell fuel ... :dunno:

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if it was a fuel pump issue it wouldn't be THAT you were getting fuel it would be HOW MUCH fuel you were getting.


i replaced the fuel pump in an astro van over the summer to fix a no-start, and it came back 6 months later with an intermittent loss of power, stumbling, just generally running like crap. warrantied the (ac-delco) pump, and its good again.


do you have a pressure gauge to use on the fuel rail? if so you could also test the fuel pressure regulator

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if you think your fuel has water in it you can pour about half a bottle of rubbing alcohol in your tank. Gasoline (especially octane) is hydrophobic and won't mix with water, and of course water won't burn on its own, but if you put soem rubbing alcohol in your tank it is hydrophylic and will mix with the water and cause it to burn. Its a pretty simple solution to a big problem

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What does the top of the spark plugs look like ?


Yesterday I had the same problem , so last night I drained the tank , added 87 with some Techron fuel system cleaner and

some Lucas oil fuel system cleaner and lubricant . Drove it around for a while and got some of that crap out of the injectors .

Then this morning I let it idle for about 30 min .


It already runs better , starts up better and took some of the valve noise out of the eng.


It will probably take a full tank or two to fully do its job .


I also cleaned all of the spark plug prongs and put a new fuel filter in , checked all connections on the plug wires and vac. lines .

Some of those vac. lines were lined in oil .


I would still check the fuel pressure , if you don't notice any difference . Get that throttle body/ injectors cleaned as best you can

and go from there .

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