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willysmjeeps.com :: View topic - Vapor lock
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Vapor lock

 
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weldernewt53
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Joined: Jun 28, 2011
Posts: 8

PostPosted: Fri May 25, 2012 10:02 am    Post subject: Vapor lock Reply with quote

I have a 1954 m38 a1 m170 that I have vapor lock problems with. Start's fine when cold but when the engine is warm I have to crank and crank to get it going. I feel it is vapor lock because when I crack the fuel line ,there are bubbles at the carb. Should I wrap the maifold with exhaust tape? Would an electric fuel pump help? If so what pressure should you use? any help would be appreciated.
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wesk
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Joined: Apr 04, 2005
Posts: 10037
Location: Wisconsin

PostPosted: Fri May 25, 2012 10:19 am    Post subject: Reply with quote

The M38A1 and M170 series have lasted over half a century without any inherent vapor look problems. With that said it is most likely a failure or degredation of performance of a component on your particular jeep that has caused the symptoms you now have. So making sudden engineering changes to your jeep's original design should be furthest from your mind. This will allow you to concentrate on the problem and the corrective fix needed.

What causes vapor lock? Heat normally is the main cause of vapor lock. However vapor lock like symptoms can occur when the real culprit is low fuel pump pressure, air induction through a loose plumbing fitting, a reduced fuel flow caused by a clogged filter or a fuel tank vent problem. So open the mind and thoroughly investigate.

1-Check all fittings and make sure they are tight. A slightly loose fitting before the pump can barely show a little wetness from leaking but will allow a tremendous amount of air to be sucked in.

2-"T" a line into the plumbing between the pump and the carb for a fuel pressure gauge and monitor fuel pressure on both cold and hot starts and while driving.

3-Measure fuel delivery by diverting fuel flow from the pump while the engine is running into a measured container.

4-Remove the filter and pick-up assembly from the tank and inspect the filter for debris and the tubes for pin holes from corrosion. These pin holes will allow the pump to suck air when the fule in the tank is below the level of the pin holes.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
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weldernewt53
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Joined: Jun 28, 2011
Posts: 8

PostPosted: Fri May 25, 2012 2:24 pm    Post subject: vapor lock Reply with quote

Thank you for your reply Wes. The fuel filter element in the tank has been removed and a new suck tube installed. John at Midwest Military suggested a inline filter and I went that route. He rebuilt the fuel pump last spring. The previous owner cut the line from the pump to the carb to install a inline filter. This maybe part of the problem. I will check my fuel pressure first and go from there. Thanks again Wes
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wesk
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Location: Wisconsin

PostPosted: Fri May 25, 2012 3:22 pm    Post subject: Reply with quote

I would use the original in tank filter. They have an excellent 62 year record. If for some reason this is not an option then I would use an inline or cartridge type filter between the pump and the tank not between the pump and the carb.
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Wes K
45 MB, 51 M38, 54 M37, 66 M101A1, 60 CJ5, 76 DJ5D, 47Bantam T3-C & 5? M100
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Texbillhs
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Joined: Apr 25, 2012
Posts: 110
Location: Pennsylvania -Avondale/Kennett Sq

PostPosted: Sun May 27, 2012 3:01 pm    Post subject: Ethenol Reply with quote

Ethanol is known to cause vapor locking in older vehicles that were designed in the good old days of pure gasoline - if you can find a non-ethanol station in your state try 5 gallons - the problem might go away. Search "Pure Gas" and click on your state. Not only vapor locking is an ethanol problem, so is diaphragm fuel pumps having the rubber destroyed, not to mention damage to older formula rubber gas hose, as well as the tank gauge floats destroyed. Also ethanol gas has a very short shelf life, which is a problem for the M38 with it's big tank - it sometimes takes months to use the gas. Bensinger (Narvon, PA) advised me to put an in-line fuel filter just before the carb to keep the dissolved rubber out of the jets - even though a M38 purest would say the M38 didn't have an in-line filter. If they force 15% it will only get worse. Here in PA antique tractor guys will run half way across the state for a 5 gallon can of gas.
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Cacti_Ken
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Joined: Apr 20, 2005
Posts: 1003
Location: Silsbee, Texas

PostPosted: Tue May 29, 2012 7:09 pm    Post subject: Reply with quote

The vehicle has been running for almost 5 years now. I have the original design filter in the tank and a inline filter before the pump. I also ran the pump out let tube far away from the exhaust manifold. I have never had a problem in hot weather or climbing steep hilly terrain in 4x4 low range. I have thought about putting a filter in the pump outlet for possible ethanol issues but haven't yet.

Added this: I have had to change the poor quality rubber bypass hose twice. I think I finally got a good one at Autozone recently.
I change the oil once a year, and flush the brake lines. Adjusted the brakes a few times. And Have changed the antifreeze a couple of times. That's about it.
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Last edited by Cacti_Ken on Wed Jun 06, 2012 7:29 am; edited 1 time in total
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maeserik
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Joined: Jun 14, 2006
Posts: 212
Location: Wijnegem Belgium Europe

PostPosted: Wed Jun 06, 2012 3:32 am    Post subject: Reply with quote

I have the same setup as cacti, and never had problems.

Erik
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1942 GPW 71336 my son's jeep restored now
1951 M38 MC11891
1953 VW beetle (genuine german Käfer )
1957 fantastic wife

http://www.surfacezero.com/g503/showgallery.php?cat=1865

and this webside
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