Gas mixing with oil in crankcase

I've managed to stump numerous mechanics, XJ, and MJ owners alike. Here is the issue I am facing with my trucks: I own a 1986 MJ with the 2.8L V6 (bleh), a 1990 MJ with the 4.0L and I just received a 1988 MJ with the 4.0L. Every single truck is exhibiting the exact same problem - gasoline is mixing in with the crankcase oil. I have to change the oil on these trucks every 100 miles or so and when I do, I get an additional 4 or 5 quarts out of the crankcase. The oil comes out super thin and is one of the worst smelling substances I've come across. It is unmistakably gasoline mixing in with the oil. I can tell this by the scent and the color of the oil coming out of the engine.

These three trucks are (well, used to be) my work trucks and I would love to bring them back to life and completely restore them when I get the time. The '88 MJ I was just given is currently stripped down to nothing and is ready to be painted with hopes and dreams that I can solve this engine problem and have a like new truck to cruise around in. Anyway, I digress. The point of this thread is for some of you guys to throw out some ideas for me. I've got some of my own, but I'd like to know what you all think and how I could go about trying to remedy this situation.

My one thought is fuel injectors that are stuck open and allowing fuel to enter the cylinder after the engine is off. The fuel then seeps down past the piston rings and into the crankcase. This would make sense for the '88 and the '90, but what about the '86 with a carb on it? I'm thinking the mechanical fuel pump is somehow leaking and allowing fuel to trickle into the crankcase. I would replace that and find out, but I've actually got the 2.8L engine out of the car and on the stand for a rebuild (or I may just replace it with another 4.0L).

Sorry for the long-winded post - just trying to be as precise and accurate as possible. Thanks for listening and I hope you can come up with some more ideas for me.

I forgot to add the following: The 1986 MJ has 126K miles on the clock. The 1988 MJ has 105K and the 1990 MJ has 115K on it.

the gas goes in the gas tank...not into the valve cover....




















j/k
anyway, i was going to say your running rich, but 3 vehicles.........

damn, either way, youve got to be running rich

Yep, running rich on all three of them - plugs are as black as any plug I've ever seen. I'm actually surprised the trucks still start up and run as well as they do, but then again, this 4.0L is bulletproof so it almost doesn't surprise me

Had an 87 XJ that had same problem,fixed by adding new fuel injectors and replacing vac line between throttle body and MAP sensor.With the carb motor its a chevy so the busted fuel pump is a possibality or the float in the carb has sunk and the fuel runs after stopping engine.
Wayne

TerraWombat said:

My one thought is fuel injectors that are stuck open and allowing fuel to enter the cylinder after the engine is off. The fuel then seeps down past the piston rings and into the crankcase. This would make sense for the '88 and the '90, but what about the '86 with a carb on it? I'm thinking the mechanical fuel pump is somehow leaking and allowing fuel to trickle into the crankcase.

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I think you're on the right track with the two 4.0's so replacing the injectors should fix it. Check the fuel pressure at the injector rail on each engine just to be sure that there isn't a problem with the fuel pressure regulator instead.
As for the 2.8, I think Wayne's hit the nail on the head.

Thanks for the suggestions guys. I did a quick search on fuel injectors at some local auto parts stores. ~$50 a piece - ouch. Is there a way to remove each injector and individually clean them or once they're broke, they're broke?

check into the ford mustang injectors. About half that price.
http://www.fiveomotorsport.com/

I got my set of injectors at fiveoclock auto parts on de interweb for $150 for set of 6 including O-rings.

I used a set off a 98 4.0,that was a junk yard motor,it was blown/cracked head,got them for free.
Wayne

Pulled the injectors off the '88 today. God I love these engines - oil in every vacuum line. Anyway, The injectors, although rusty and weathered, don't appear to be stuck open. I used my air compressor to blow air through each one and put my finger on the opposite end to see if I could feel any air leaking out - felt nothing on all six. Out of ideas now....I'm gonna try and replace the injectors just to see what'll happen, but I honestly don't think they're the problem. It's hard to fathom how so much fuel can mix in with this oil, yet all of the injectors appear to be fine. It made more sense to me with the 2.8L engine since there was a few places where I could make sense of gas mixing with the oil, but on these fuel injected trucks, the entire fuel system is so isolated from anything on the engine.

I'm stumped, anyone else got an idea?

TerraWombat said:

I'm stumped, anyone else got an idea?

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Your jeeps are spontaneously generating gasoline. You will be the savior of mankind when oil wells run dry and we frantically search for alternative fuel sources.


Also, is there anything a malicious individual could add to a gas tank that would destroy both injectors and carburetors?

TerraWombat said:

I'm stumped, anyone else got an idea?

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Is it possible for a fuel pressure regulator to leak through to it's vacuum line? The vac system could be getting filled with fuel, then draining back to the valve cover hose. Is there any sign of gas on your air filter?

scca28 said:

Is it possible for a fuel pressure regulator to leak through to it's vacuum line? The vac system could be getting filled with fuel, then draining back to the valve cover hose. Is there any sign of gas on your air filter?

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That's a good possibility. I'll have to pop the air filter off and give is a good wofting.

Something else I thought of today was the fuel pressure regulator is stuck closed and not letting the fuel exit through the return line, thus generating an enourmous amount of pressure in the fuel rail. So when an injector opens, it sprays entirely too much fuel into the intake. The problem with this theory is that the engine would most likely stall due to flooding. However, the truck still runs fine - a little on he rich side - but it still chugs away like nothing was wrong.

lilredwagn said:

Your jeeps are spontaneously generating gasoline. You will be the savior of mankind when oil wells run dry and we frantically search for alternative fuel sources.

Also, is there anything a malicious individual could add to a gas tank that would destroy both injectors and carburetors?

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Haha, good one. I'm actually doing research on biodiesel as an alternative fuel source. I'll have to tell my advisor about the three Jeeps I have that actually create fuel

i doubt its the injectors, pretty much every sensor on a renix jeep effets how much fuel is delervied.....

start ohming out sensors

Someone don't like you, putting gas in your THREE jeeps. The fact that this is happening to 3 make me think you need to tie a dog(s) to you jeeps at night.

badron said:

Someone don't like you, putting gas in your THREE jeeps. The fact that this is happening to 3 make me think you need to tie a dog(s) to you jeeps at night.

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What? Someone sabatoging his MJ's by dumping gas in his engine at $3.00+ bucks a gallon? I would sit on my porch with my shotgun instead. And take the gas they pour in his engines after getting inside and popping his hood. Sure....

TerraWombat said:

Something else I thought of today was the fuel pressure regulator is stuck closed and not letting the fuel exit through the return line, thus generating an enourmous amount of pressure in the fuel rail. So when an injector opens, it sprays entirely too much fuel into the intake.

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That's what I suggested in my first post so check the fuel pressure.

SORRY... I don't buy this. SOMEONE is fawking with your trucks.

ALL 3 with the SAME problem? Odds on that are pretty freakin high, I mean REALLY HIGH. Do you have a mechanic who works on all 3? These 3 trucks have SOMEONE in common other than you.