Gas in Oil- Resolved

[WarrenD]

About a month ago, I tried to start the truck. It would crank, but not fire. I looked at the in-line fuel filter between the pump and the carb and it was dry, even after several bouts of cranking. I pulled the dipstick and thought I smelled gas in the engine oil. I figured the pump let go, so I put in a rebuilt one (it was rebuilt by Then and Now and sent to Charles for a check after he said T&N sometimes uses civie parts in the military pumps. According to Charles, the pump was fine). I changed the oil and filter (the stuff in there had less than 100 miles on it...) along with the fuel filter. When I went to start it, it was similar, cranked alot but I figured it would take a while to get gas up to the carb. Eventually it fired and idled well. I let it idle for quite a while and then took it for a drive of about 15 miles during which it ran fine, just as it has all along. Plenty of power, pulled hills well, etc.
When I got home, I popped the hood and noticed that there were some air bubbles coming up into the gas filter. Not constant, but every 10 seconds or so, and just a few, maybe half-dozen at a time. I shut the truck down and had a devil of a time getting it restarted. I eventually did, and again, once running, it seemed fine. After it cooled down, I went to check the oil and it smells like gas again.
This morning, I took the old fuel pump apart and I can't see any tears in the diaphragm, it looks almost new. I'm starting to wonder if there could be another source for gas getting into the engine oil? It seems unlikely that the newly rebuilt pump would go bad right off, especially when the old pump didn't appear to be leaking gas. The only other component I can think of is the carb, but I can't wrap my brain around how gas would get from there into the oil without the truck running very badly. During the last test run, I didn't see any smoke out the exhaust.
Ideas appreciated.

[ZGjethro]

My truck did that terribly when I first bought it, but it was flooding. It was barely running, and finally it would not start at all. I pulled the carb and the intake manifold was full of liquid gasoline. I think the gas was getting past my rings and into the case. I had a faulty pressure regulator and an electric pump. The engine in a good state of tune now (head is pulled though) and I still get a tiny odor of gas, but nothing like before. Hopefully the problem for you and me is not a cracked block.

[ZGjethro]

No other replies on this one? I am curious also about causes of gas in the oil. I have deleted the mechanical pump, so that is not an issue with my truck. I think my carb is set a little on the rich side, but nothing like before. The truck had over 110 psi on all cylinders when I tested it a few years ago. I am not sure if gas is getting by the rings.

[Hemiman]

90% of the time, a crankcase full of gas is caused by a faulty fuel pump. The fact that you see bubbles in the pressure side of the pump adds to this diagnosis. Sometimes you can get a out of box failure on brand new parts. Somewhat rare on NOS M37 parts, as they had good QC back then. Notoriously bad on current Chinese made parts.

A simple test would be to bypass your mechanical pump with an electric pump. Use an inexpensive in-line 12V unit, wired to one of your batteries (If you still have the 24V system). Make sure you include a kill switch in this circuit.

Carb could be the problem. Bad needle and seat on the float can flood the engine with gas, but this is almost always accompanied by a very poor (RICH) running engine.

Good luck!

[WarrenD]

I spoke with Bob Stahl this afternoon, and we both agree that it really does sound like the fuel pump. The only other way to get gas in the crankcase is past the rings and while it does appear possible that too much choke during start-up can cause this, it doesn't explain the apparent quantity of gas getting in there. If it were to build gradually over time, the choke could be the cause but not so quickly. As Hemiman points out, if it were a carb problem the truck would be running very bad and probably belching smoke. It's not, it ran fine with plenty of power (eliminates a cylinder with a broken ring as I'd surely notice a loss of what little power it has! Also, after just a few miles it wouldn't likely have the gas build up that is there, again, it would accumulate over time.)
So, it looks like we are left with the fuel pump. I have a hard time believing that a reputable rebuild that was checked over by a third party would fail and so does Bob, but strange things happen (often to me!) and I guess it's time to replace the replacement pump.

[RMS]

The only other way to get gas in the crankcase is past the rings

gas never has to enter the combustion chamber to enter the crank case. from what I've seen a typical "L head" will pull a meager .5 to 2 pounds of vacuum when cranking. the top of the valve guide sits about 1 inch below the deck. see were im going with this?

I tried to start the truck. It would crank, but not fire. even after several bouts of cranking. I pulled the dipstick and thought I smelled gas in the engine oil.

it seem to me that it takes very little vacuum to operate the throttle pump and that it would be very easy to fill the intake with raw gas and never have it bridge the inch from the top of the guide to the combustion chamber.

Eventually it fired and idled well. I let it idle for quite a while and then took it for a drive

as the manifold heats so does the gas, it heats to the point that it atomizes then is burnt off.

when the choke is pulled closed every pulse of vacuum causes the throttle pump to inject gas into the manifold.so with a very low cranking speed, negligible spark and a carter that spills gas rather than injects starting can be hard. the gas is not atomized and quickly pools in the manifold. it runs down the guides past the lifters then into the pan.

[Hemiman]

I've seen that before on L heads. Usually your discover it with an engine that has a history of flooding upon starting. However, if the vehicle is run at operating temp for any amount of time, the gas in the oil will vaporize and burn off via the PCV. If it's a fuel pump failure, continually introducing more fuel to the crankcase, you end up with the diluted oil I believe we're talking about here, but I may be wrong.

I once repaired a neighbors mower that was running rich. So rich, it would run and cut the lawn with the gas line completely shut off! WOW, a perpetually running Briggs & Stratton! (Must have been one of those experimental ones that the oil companies don't want us to know about)!..... Nah! The needle and seat were bad, and being gravity feed, it drained the tank into to crankcase. Was running of pure blow-by! Not the same issue, but funny!

[ZGjethro]

The only other way to get gas in the crankcase is past the rings

gas never has to enter the combustion chamber to enter the crank case. from what I've seen a typical "L head" will pull a meager .5 to 2 pounds of vacuum when cranking. the top of the valve guide sits about 1 inch below the deck. see were im going with this?

I tried to start the truck. It would crank, but not fire. even after several bouts of cranking. I pulled the dipstick and thought I smelled gas in the engine oil.

it seem to me that it takes very little vacuum to operate the throttle pump and that it would be very easy to fill the intake with raw gas and never have it bridge the inch from the top of the guide to the combustion chamber.

Eventually it fired and idled well. I let it idle for quite a while and then took it for a drive

as the manifold heats so does the gas, it heats to the point that it atomizes then is burnt off.

when the choke is pulled closed every pulse of vacuum causes the throttle pump to inject gas into the manifold.so with a very low cranking speed, negligible spark and a carter that spills gas rather than injects starting can be hard. the gas is not atomized and quickly pools in the manifold. it runs down the guides past the lifters then into the pan.

Thanks for this info. I was not sure how gas was getting in my crankcase since I have no mechanical fuel pump, and to my knowledge the rings/compression is good. I will consider the valves as a path for the gas to trickle down. I also need to check or replace my pcv valve.

[m37jarhead]

Warren, you had the new pump checked by Charles.
Have you called him to get some ideas on this problem?
Jerry

[RMS]

Thanks for this info. I was not sure how gas was getting in my crankcase since I have no mechanical fuel pump, and to my knowledge the rings/compression is good. I will consider the valves as a path for the gas to trickle down. I also need to check or replace my pcv valve.

when you start your truck does it stumble to life, slowly picking up rpm's ?
I did some testing last night: first I started the truck by pulling the hand throttle out two notches depressed the starter for a count of two then quickly blipped the choke and she fired and idled smooth. I turned her off and tried a different method of starting. I pulled the choke all the way out with the hand throttle in, cranked for a count of three and she stumbled to life taking a good two minutes to even out and idle nice. this morning i went out to see how quickly the manifold would fill. choke all the way out, coil disconnected, cranked for a count of 1, 2, 3, 4. then pulled the plug at the back of the manifold and inserted a thin strip of cardboard to measure how much fuel was sitting in the manifold . there was a quarter inch of gas on my cardboard dipstick

[WarrenD]

I haven't spoken to Charles. I pulled the pump yesterday and I think I have the proverbial smoking gun. In the recessed area below the lever, inside the pump casting, where there should be oil there was mostly gas with a touch of oil. I was careful removing the pump to keep it upright and there is no chance any gas from the lines got to that area. I'm moving forward with replacing the pump and when that's done, I'll change the oil and filter again and take it from there.

[WarrenD]

Problem appears to be resolved with the replacement of the fuel pump. I've made a couple of runs now and things seem to be back to normal.