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Fuel Pump O-Ring Pulling Air?


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My son and I purchased a 1987 MJ for restoration. 97,000 original miles, 4WD, 5-speed short bed.

 

The engine is hard to crank. Turns and turns and turns before catching. The previous owner told us this problem started after he installed a new fuel pump at the gas tank. He said the gasket or o-ring did not seal correctly, and it is pulling air. He said that it is an easy fix - there is a bracket that spins around to release the fuel pump, and then the o-ring is simply replaced.

 

Does anyone have experience with this? I searched on the forum, but could not find anything specific.

 

Thanks in advance for any information.

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The fuel pump to tank Oring should have nothing to do with the pumping of fuel to the engine. I think you have other issues to contend with. Try replaceing the filter, do a pressure test to see if it is even getting fuel to the rail. I assume you have a 4.0, you should add your engine to your info. If youre not getting proper pressure, then pull the pump from the tank and check that all the connections there are good, and if needed replace the pump motor only. don't try to get a new assy form the parts stores as they will sell you one for a cherokee and tell you that it will work in a comanche.. it won't. No one makes new fuel pump assemblies for our trucks, but you can just replace the pump motor if needed. But check everything else first. Also check grounds... if you have a bad ground it may not let the pump get full power. Good luck!

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It's also possible the sock on the pump is clogged up. Especially if he replaced the pump without replacing the sock. Only way to tell is to pull the pump. The sock should be white. If it's brown or grey, replace it. If it's full of crud, replace it. Actually, you're supposed to replace it any time the pump is out of the tank for any reason.

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........if needed replace the pump motor only. don't try to get a new assy form the parts stores as they will sell you one for a cherokee and tell you that it will work in a comanche.. it won't. No one makes new fuel pump assemblies for our trucks, but you can just replace the pump motor if needed. .......

 

 

:hijack:

 

This is a good time to straighten me out on this subject. Can a new pump only be purchased at say NAPA, Orielly's, Autozone and a new sock so that an assembly be fixed(rebuilt) or are we talking about something(place) entirely different? Don't have this problem, yet. Just trying to understand so that it's not an emergency question later when everyone is off wrenching, wheeling, sleeping, or whatever.

 

Scott

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My '88 MJ 4.0L will crank and crank and crank before it fires up. I lose fuel pressure to the rail at a steady rate once the engine is shut down. I originally had leaking injectors, but once I fixed that, the problem persisted. I'm almost positive that there is a check valve in or around the fuel pump that has gone bad and I'm going to install an inline check valve one of these days to see if it cures the problem.

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also if you believe the problem to be fuel related, try priming the pump first...

 

basically what you do is...

when you're ready to start the truck, turn the key to the run position (just before start) and leave it there for 3-5 seconds,

turn it off.

turn the key back to run again leave it for another 3-5 seconds... now turn to the start position

 

if the truck fires up more quickly after following these steps then your problem is fuel related and can possibly be cured with injectors/check valve, whichever component is losing pressure after shut-down.

 

if the truck acts exactly the same the problem is ignition related and may be due to several things like CPS (crank position sensor) or normal renix starting. (usually takes a few seconds to start, NOT like a chevy where you bump the key and its running already)

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Thanks for all of the feedback.

 

We've had the MJ a week now, and we've had a little time to spend with it. What we have found is that it is almost impossible to crank - runs the battery down and we have to jump it before it will finally fire up.

 

Once it is running, you can restart it with no problem. I've had it sit as long as two hours and it still starts up fairly well. However, if it is left overnight, it is again almost impossible to crank.

 

In addition, most if not all of the fuel injectors are leaking a little.

 

Could the fuel injectors be the problem? Could they be allowing the fuel rail to lose fuel pressure, and everything has to prime after sitting for a long time?

 

I've checked the spark plugs, and they are clean and gapped correctly. I'm thinking if it were the CPS, it would continue to be difficult to crank after we get it started the first time.

 

As always, thanks for any ideas.

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Could the fuel injectors be the problem? Could they be allowing the fuel rail to lose fuel pressure, and everything has to prime after sitting for a long time?

 

Absolutely. But more importantly, the injectors leaking after the engine has been shut down will cause fuel to enter the cylinders, trickle past the cylinder rings and wind up in your crankcase oil. Drain the oil and give it a good whiff - does it smell like fuel? This is something you don't want to happen as the fuel will wash the oil off of the cylinder walls after shutdown. This will cause a direct metal on metal contact upon start up until new oil reaches the walls. My '88 MJ did EXACTLY this and it went this way for a couple years. It was being used strictly as a farm vehicle and saw very little on-road use so we didn't care. End result - the engine is trashed. The bearings are gone, the cylinder walls are likely worn so bad that the piston ring endgap is huge causing the massive amount of blowby I'm currently getting. The oil pressure is always low (due to the extremely worn bearings) and it feels like it has lost a lot of power. Kind of sad considering the engine only has about 105K miles on it.

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