Fuel in crankcase - 1988 4.0L

[TerraWombat]

I can't seem to shake this problem - it started a couple of years ago when I noticed oil changes were getting increasingly thin and I was getting way more oil than I put in not to mention it smelled like gasoline. Parked the Jeep and it sat for a couple of years until I decided to figure out what exactly is wrong with it. Last month I sent the injectors out to be cleaned as someone told me they could be stuck open and allowing fuel to trickle past when the engine is off and it would seep past the rings and into the crankcase.

Unfortunately, that didn't seem to solve the problem as I drained all of the old gas today, put 5 gallons of fresh fuel in and ran the truck for a little while. Whether or not it's related, the truck idled a little rough, revved up pretty nice with no hesitation, but would backfire a bit coming back to idle. The truck is also running very rich as the electrodes on the spark plugs are very dark. My other suspect was the fuel pressure regulator thinking that the fuel is getting sucked past the diaphram and into the intake manifold, but I couldn't see any signs of that happening.

Anyone else have any ideas? I'm just about stumped at this point. The fuel system is so isolated from the rest of the engine that I can't fathom how I'm getting so much fuel into the crankcase. I literally changed the oil a couple months ago and have run the engine for maybe a total of an hour, if that, and the crankcase oil is already diluted to the point where it stinks of gasoline.

By the way, the truck is a 1988 Jeep Comanche SporTruck with a 4.0L engine and 5-speed manual transmission. There is 106k miles on the clock and the compression is 140-160 psi across the board. Seems a little high, but I suspect massive carbon buildup in the cylinders.

 

[Red90Laredo]

that is the scariest problem I have ever heard of. I have no idea man- but you need to do a fuel pressure test with and without the vac line on the regulator. post back with the results. whatever you do keep a fire extinguisher handy- that sounds like trouble waiting to happen to me.

 

[jneary]

with the early 4.0's i have seen a leaking fuel pressure regulator, where the diaphragm inside is bad and a lot of fuel gets sucked through the vacuum line, into the intake. then the raw gas goes past the rings and ends up in the pan

 

[old_man]

First off, do a complete wet and dry compression check,

 

[TerraWombat]

Fuel pressure w/vac line on - 30psi at idle and will jump to 40 psi when I give it gas and dip down to 25 psi when I let off the throttle.

Fuel pressure w/o vac line on - Solid 39-40psi

Dry compression test results on a warm engine, throttle full open, no spark plugs in any cylinders, 15 cranks per test:

1 - 140 psi
2 - 145 psi
3 - 155 psi
4 - 160 psi
5 - 140 psi
6 - 160 psi

I suspect my gauge reads a little high.

Cylinder leakdown and wet compression test results to follow shortly.

BTW - All sensors check out fine according to this website: http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm

Fuel pressure regulator does not appear to be leaking gas past the diaphram, but I'll do a closer inspection of that as well as seeing if the fuel pressure drops off after engine shutdown.

My suspect - piston rings are shot. Time will tell....

Edit: Mistyped fuel pressure results

 

[TerraWombat]

Engine has been off for roughly 10 minutes and the fuel pressure has been holding steady at 38-39psi. Right after I shut the engine down, I watched the fuel pressure climb from about 32 psi to 38 psi and it's been holding - no obvious signs of drop off to indicate a leaky injector(s) or fuel pressure regulator. I'll let it sit for another 20 minutes or so before I begin the leakdown test.

I also put in 4 new quarts of oil prior to all of this testing - purposely put in less than required to get a better idea of how much "extra" fluid I'm getting in the crankcase for a certain duration of time.

 

[Red90Laredo]

how does the gas mileage do on your rig?

 

[TerraWombat]

how does the gas mileage do on your rig?

Never checked. It was a farm vehicle since it's been in my possession so I didn't really care all too much. Since I discovered the gas in crankcase problem, I parked it in my shop and it's been sitting in here for two years and hasn't moved an inch - so it's getting 0 MPG at the moment.

About 15 minutes after engine shutdown and we're down to 26psi. Is fuel pressure drop off like this normal? I was under the impression the fuel gradually made its way into the return line and back to the tank or when the engine is off, is the fuel pressure regulator supposed to clamp shut?

30 minutes later and 2psi of fuel pressure.

Time to pull the injectors out of the intake and make a visual assessment.

 

[TerraWombat]

Injector leakage test in progress. I have all of the injectors pulled out from the intake manifold, but still securely attached to the fuel rail. I placed clean sheets of white paper under each of the injectors to give me an indication of how badly they are leaking. Turned the key to the on position and let the fuel pump pressurize the rail to 38 psi.

Now we wait...

...and wait...

I'll update the thread when the pressure drops to zero in about an hour or so. In the meantime, count your blessings you don't own the most problematic Jeep 4.0L engine on the planet.

 

[winterbeater]

Is the running temperature good, like 190-200 or does it run real cold?

 

[dinorl007]

fwiw, i have had this issuewith 3 vehicles, none of them was a jeep, but each time ended up replacing the ecm and solving the problem, the first 2 times i checked everything that was related to fuel pressure, new injectors, and checkede all electrical componets related to the fuel and air management it all showed within specs, so i swapped inanother ecm and it took care of it hth and good luck getting it fixed

 

[TerraWombat]

Is the running temperature good, like 190-200 or does it run real cold?

Seems to run a bit on the colder side. What could that be an indication of?

 

[winterbeater]

When you run cold, the ECM or PCM (whatever) compensates with a richer mixture. I guess that could put more gas in your oil by getting by the rings. Also, if you got good and hot like they are designed to, it would make the gas evaporate out of the oil and feed back in through the PCV system. Your plugs being dark is another sign of rich mixture. Rich mixture also makes the rings wear out by washing the oil off the cylinder walls more. You need to replace the thermostat with a standard thermostat (should be 195 degrees). Makes most Jeeps show about 200-210 on the gage. I betcha that will fix your problem. You already checked out everything else.

 

[TerraWombat]

When you run cold, the ECM or PCM (whatever) compensates with a richer mixture. I guess that could put more gas in your oil by getting by the rings. Also, if you got good and hot like they are designed to, it would make the gas evaporate out of the oil and feed back in through the PCV system. Your plugs being dark is another sign of rich mixture. Rich mixture also makes the rings wear out by washing the oil off the cylinder walls more. You need to replace the thermostat with a standard thermostat (should be 195 degrees). Makes most Jeeps show about 200-210 on the gage. I betcha that will fix your problem. You already checked out everything else.

Thanks for the info! I believe the thermostat was replaced sometime during my ownership - it's hard to say because it was used as a farm-only vehicle and my brother or my Dad may have changed out the t-stat when I was at college - I'll have to ask one of them if they ever remember swapping it out. I did realize that this Jeep did run a bit cold as the temperature gauge seemed to stay at the "1/4 tank" level (forget what the numbers are on it) - even the heater was only pushing out lukewarm air but I figured maybe it was because the truck is just idling in my shop and no real load has been put on it to warm it up.

Anyway, I'll swap it out with a 195 degree thermostat today for good measure.

As for the ECU being the culprit - I haven't yet ruled that out, but I did remove it, open the case and visually inspect the components. I read somewhere that the 2.5L ECUs have a diode that commonly fails and I know on a lot of Mitsubishi ECUs the capacitors leak causing all kinds of random, intermittent problems that make it very hard to diagnose the vehicle.

 

[TerraWombat]

I replaced the thermostat today - it had a 180 degree t-stat in there before. Put in a Stant 195 degree thermostat, started it up and coolant went spraying in every direction. Figured something wasn't seated right with the gasket, so I drained some coolant, removed the housing only to find it had cracked...back to the auto parts store. Luckily my local advance auto and auto zone tend to carry most of the parts I've ever needed for this truck - so between the two of them I'm set.

Replaced the thermostat housing and put a new gasket on - no leaks. Truck will warm up to 210 on the gauge - sometimes a little bit past, but if I turn the heater on, it'll drop down again. I don't have a fan shroud as my original is in about 10 pieces so that may have an impact on how well the engine is being cooled. Its been idling for about 20 minutes now as I'm testing it to see if I get any more fuel in the crankcase. My coolant temperature sensor reads about 375 ohms at 210 degrees, which is a little high, but not terribly out of spec. I have a new one coming to me that I can swap in if needed.

I'll probably let the truck idle until it runs out of fuel. There was only about 2.5-3 gallons left in the tank and I'd like to see how much of that ends up in the crankcase.