vapor lock?

peter e mark · Post by **peter e mark** » Sun Jun 08, 2008 8:07 am

Well troopers, it seems Iv'e read so much about this aleady , but with all the otHer things I'm doing to this truck I have not yet addressed this frequent problem: While running the truck, even right after I filled the gas tank, the engine will "not get gas". It happened today, and luckily i was on an incline long road and just let it roll in gear and then it started again ! aND THEN IT WILL RUN OK .I do not have an original fuel puimp on this engine, and already changed the in-tank filter. At thid point I am willing to do anything to make this reliable, even go electric. ??????????

g741 · Post by **g741** » Wed Jun 11, 2008 5:04 pm

Look at your gas tank filler cap. If it is a correct one, it will have a valve inside it. On the outside of the cap it will say open for normal driving and closed for fording or vapor lock. If the vent on the cap is closed a little pressure will build up in the tank and help pump out the vapor lock, if that is the problem. Also, be sure you have a heat shield in place over the top of your fuel pump. Sid

hairpin151 · Post by **hairpin151** » Wed Jun 11, 2008 7:24 pm

do you have the correct cap?

peter e mark · Post by **peter e mark** » Sat Jun 14, 2008 4:51 am

Hello Everyone, And looking at my cap it says there is a vavle to adjust for fording or normal driving. I will look in my John Bizal TM and try and fing how to adjust this valve. Thank You..Peter

Lifer · Post by **Lifer** » Sat Jun 14, 2008 6:07 am

It's a little lever inside of the cap. You just flip it to one side or the other to block or open the vent hole.

peter e mark · Post by **peter e mark** » Sat Jun 14, 2008 11:42 am

Thank You Lifer, I did that and something else! I remembered I am an HVAC man. That means I know how to check fuel oil pumps on burners., so why not do the same procedure with a fuel pump for gas? I pulled out my drawer of fuel line fittings, you know that drawer, it's the one that wives refer to when they say we spend to much money on junk. Pulled out two 1/4 " brass tees, 4 barb adaptors for my 5/16" fuel line, and a pressure guage and a vacuum guage. Here's what I saw with the engine idling: Pump output gauge varies from 5.25 PSI to 3.75 PSI, and the vecuum guage oscillates between 0.0 " vacuum and 1.0" vacuum. A few times the vacuum went up to 2.25" VAC, and then dropped back to zero, and oscillating . My see thru fuel filter near the carb is only 1/4 full with all of this and I did see it full when I first bought the truck., This civilian fuel pump has a glass bowl that I cleaned a while back and it is not the original military pump. Any ideas ?

Lifer · Post by **Lifer** » Sat Jun 14, 2008 3:33 pm

There should be a gaasket between the glass bowl and the metal fuel pump body. If the bowl isn't seated properly or the gasket is cracked or broken you can be sucking air.

Another possibility is that the gas cap is defective and is not allowing air into the tank. When this happens, drawing fuel from the tank creates a vacuum in the tank and, eventually, the fuel pump will not be able to overcome the tank vacuum.

If you have ever noticed a "whoosh" sound when you remove the gas cap, this is very likely the problem.

peter e mark · Post by **peter e mark** » Sat Jun 14, 2008 6:30 pm

Thanks, Lifer. I shall listen for the sounds at the gas cap, but haven't heard it so far. I shall, as a matter of elimination process, buy another gas cap to be certain. I also may have a fuel pump from my WC52, and If i locate it tomorrowe I shall change it out and read the guages again, and drive the truck of course. If I can't have a reliable truck . it ain't worth having !

Lifer · Post by **Lifer** » Sat Jun 14, 2008 6:52 pm

Sure it is! It's a living piece of history. Besides, working on them is half the fun of owning them...unless you're doing it along the side of the road somewhere in the middle of nowhere. Then, it's an "adventure."

peter e mark · Post by **peter e mark** » Sat Jun 14, 2008 8:01 pm

Lifer, There are indeed things I own that don't work, but are worth having. Yes, such as my Mk5 Sten Gun, and Hotchkiss 13.2MM anit aircraft cannon, as used in Reenactment....

peter e mark · Post by **peter e mark** » Sun Jun 15, 2008 2:40 pm

Well I found what looked like a good fuel pump from my WC52, put it in, but now it won't draw gas into itself. Used alot of Hard Start and spun the egine, which ran for a few moments at a time, but the fuel pump glass bowl remains empty Opoerated the hand prime feature vthis pump ,has but it still dont prime . Tried priming it but can't seem to get gas into it thru that 5/16" inlet hose thru a funnel . Am going to next run a temp hose from the pump inlet to a small gas can and try to determine if it will prime that way. to eliminate a gas line restriction?

ron · Post by **ron** » Mon Jun 16, 2008 11:30 am

Hello Everyone, I'm alittle confused? I have a 1951 m37 with a newer type tank with the filter inside the tank. My gas cap says [ pressure cap ] on it. It doesn't have any vent on it. Should I get the vented cap or not. I'm not wanting any vapor lock problems .
Thanks Ron

g741 · Post by **g741** » Thu Jun 19, 2008 5:35 pm

Now here is a good story, regarding being broken down by the side of the road.

When I took my 1.5 ton Chevrolet (1941) to Normandy for the 60th of D day, we first landed the vehicles in England. We drove the Chevrolet over to Bovington to see the big tank museum there. At a rest stop on the four lane highway there was a 6X6 WWII Dodge parked, no one around. When we came back, the guy was there working on the Dodge. After some talk we decided we just had to stop and see if he needed help. I had to back in the exit of the rest stop. The guy was in denial, trying to say that his fuel lines were twisted and etc and fuel would not go through them. Just by pure dumb luck I had an exact replacement for his fuel pump with me. He installed it, the truck started and off he went. Now, how many times are you going to be stuck in England with a bad fuel pump and some poor old American pulls up with an exact replacement? It was a fun thing to do and I was glad I had the pump. Besides that, we needed a few Pounds Sterling! Sid

g741 · Post by **g741** » Thu Jun 19, 2008 5:42 pm

Did you check both the vacuum and discharge pressures with the WC Dodge fuel pump? If you have no suction pressure and or no discharge pressure the pump may be faulty. Your statement that with the original pump you are seeing an oscillating suction pressure that goes up to 1 inch, it means to me that you may have some kind of an obstruction. I think the suction pressure should stay lower than this. If it is OK, the WC pump hand primer should pull gas up right away. These pumps are easy to re-build and I have correct diaphragms (that will stand modern gas) and gaskets. Let me know if you are continuing to have problems. Sid

peter e mark · Post by **peter e mark** » Thu Jun 19, 2008 7:23 pm

Thank You G741. i AM quite sure there is a gas line rstriction now, and also I thought the dip tube in my tank was not low enough because I would "run out of gas" after using only 6 gallons from a full tank. The truth is the restriction would only allow the pump to pull upwards out of the tank till it reached that level. Now it makes sense. So saturday its a new 5/16" line for me all the way from the tank to the pump ! I will post results ..Thank You, Peter