Vapor lock question

1941 - 1945, MB, GPW Technical questions and discussions, regarding anything related to the WWII jeep.

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Vapor lock question

Postby pvtcook » Sun Jun 05, 2011 8:01 pm

Would an electric fuel pump eliminate a vapor lock issue? I dont have any trouble w/ this until summer. I was told that electric would stop this. I thought that I could maybe mount it under the jeep so that its not in sight. I havent tried anything preventive yet. Im thinking of covering the metal line w a rubber one to insulate it. Also going to check that I have a good thick gasket between the carb/manifold. If anyone has tried the electric route I would appreciate your advice. I really would like to keep it original looking but would also like to eliminate this problem! Its embarrassing to have such a nice jeep and then it wont start. It never fails when someone is admiring it and telling me how good that it looks that it wont start!!
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Re: Vapor lock question

Postby Cuz » Sun Jun 05, 2011 8:53 pm

Well, believe it or not even electric fuel pump systems can suffer from vapor lock.

Instead of modifying the jeep check the common causes first.

1-Is your fuel line from the pump to the carb too close to the exhaust manifold?

2-Do you have the correct very thick spacer with metal "V" installed between the carb base and the intake manifold?

3-Are you sure your heat riser is in the correct position when the engine is hot?
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Re: Vapor lock question

Postby Ben Dover » Mon Jun 06, 2011 3:51 am

Also try using a different brand gasoline. These Jeeps were designed for all climates. Do you have an in line fuel filter that is installed at the LH site of the engine? That can also create a problem.
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Re: Vapor lock question

Postby pvtcook » Mon Jun 06, 2011 4:05 am

Cuz yes to questions 2 and 3 I have checked the routing of my fuel line and it is correct, and no I haven't tried different fuel and it only has the original firewall fuel filter.
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Re: Vapor lock question

Postby Cuz » Mon Jun 06, 2011 5:56 am

I didn't ask if your fuel line routing was correct. I asked if it is too close to your exhaust manifold. Different regional fuel compositions will behave differently when plumbed in an easily heat soaked area. The original correct position of the fuel line is not always the best position in the field. The M38 had an MWO issued to reposition the original factory positioned fuel line further from the exhaust manifold for that very reason. You would be surprised how much difference 1" further away from that heat source makes. You can also wrap the tube with aluminum foil to help.

On the heat riser there were two different spring set-ups. Either can be installed incorrectly and cause the valve in the exhaust manifold to be in the wrong "heat on" position after the engine is warmed up.
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Re: Vapor lock question

Postby Ben Dover » Mon Jun 06, 2011 7:18 am

And if changing the brand does not help, try upping the grade during the period when the temperature is in the mid 70's and up. Today's fuels are not as agreeable as the leaded gasolines of 30 years ago. I have had it happen in my MB being driven by a volunteer from the local Air Guard in a July 4 Parade, he let it stall and could not restart it, thankfully I was there and got it going in a few seconds.
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Re: Vapor lock question

Postby Chuck Lutz » Mon Jun 06, 2011 8:14 am

1) Does your line from the tank go to the firewall filter, across the front of the motor to the pump and up to the carb?
2) Does your line from the tank go along the frame rail straight forward to the fuel pump?

The close proximity to the exhaust system may be the problem if the routing to the fuel pump is #2.
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Re: Vapor lock question

Postby pvtcook » Mon Jun 06, 2011 9:05 am

fuel line 009.JPG
fuel line 009.JPG (125.82 KiB) Viewed 764 times
Chuck its routed like No1. Here are some pics of the fuel lines.
Also a pic of the jeep after the winter refreshing!!
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Attachments
fuel line 010.JPG
fuel line 010.JPG (131.47 KiB) Viewed 756 times
fuel line 007.JPG
fuel line 007.JPG (110.9 KiB) Viewed 755 times
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Re: Vapor lock question

Postby Marty, SoCal » Mon Jun 06, 2011 9:57 am

The fuel line from the pump to the carb is incorrectly bent. Looks home made. The line should come straight out sideways from the fuel pump, then 90* straight up in line with the carb fitting, then 90* straight back to the carb. It will be farther away from the exhaust manifold that way.
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Re: Vapor lock question

Postby Chuck Lutz » Mon Jun 06, 2011 10:08 am

I think the routing of the Pump-to-Carb line is the problem as mentioned....
Here is a crated motor:
MB512XXX side.jpg
MB512XXX side.jpg (77.51 KiB) Viewed 756 times


Here is yours:
fuel%20line%20007.JPG
fuel%20line%20007.JPG (110.9 KiB) Viewed 755 times
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Re: Vapor lock question

Postby 41jeeps » Mon Jun 06, 2011 10:13 am

I would first check if it does not flood after shut down.
Many jeeps suffer from leaky floater needles and flood a few minutes after shut down.
You can easilly check that out by releaving the fuel pump pressure by unscrewing the fuel line at the carb right after shut down.
A good squirt of fuel will come out, be carefull for fire :!:
If the jeep start easilly, you found the culprit.
The way your fuel line is bend will not give any problems.
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Re: Vapor lock question

Postby pvtcook » Mon Jun 06, 2011 11:47 am

All I know is when it does this that when I work the throttle no fuel squirts into the carb. When I take a oil can and squirt gas into the carb it will crank but won't keep running.
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Re: Vapor lock question

Postby 41jeeps » Mon Jun 06, 2011 12:19 pm

If no air leak in the suction side from the fuel system it most likely is vapour lock.
The small flex hose between engine and body is usually the culprit.
A vacuum gauge is handy to trouble shoot these issues.
I have driven my jeep for a week daily in 38°C temperatures and never got a vapour lock problem.
Actually I have no experience with vapour lock at all. :|
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Re: Vapor lock question

Postby pvtcook » Mon Jun 06, 2011 12:37 pm

Yeah some seem more likely to do it than others. This is my third jeep and the first to give me this problem. I'm going to try getting a fuel line w the correct bend and insulating it first. If that doesn't solve the problem I'll pull a vacuum on the suction side. I believe it's vapor lock because this winter and in the morning I never have a problem. Only when it's hot and I shut it off and try to recrank within a minute or two.
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Re: Vapor lock question

Postby Marty, SoCal » Mon Jun 06, 2011 12:50 pm

Another possible cause is the needle sticking in the carb. Try this the next time it happens: Tap the carb near the inlet fitting with the plastic handle of a screwdriver. Sometimes this will unstick a binding needle and allow the carb to refill.

Still no fuel? Remove the fuel line from the carb fitting, using a short length of rubber fuel hose connect it to a fuel safe conainer, an old water bottle works well. Crank the engine with the key off, if fuel will flow then, then the needle may still be stuck. Should be able to supply a half a cup in about 15 seconds of cranking. No fuel, maybe the fuel pump itself is defective, or it's vapro locked on the supply side of the pump.

A length of clear hose, (vinyl works well), spliced into the fuel pump inlet, between it and the supply line will show if the fuel pump is pulling air or vapor into the inlet. Air bubbles in the stream of fuel on the supply side indicates a pinhole in a fitting or hose. Pure air, it may be vapor locking on the supply to the pump side. Solid fuel supply to the pump, but it still doesn't work may mean sticky or inop check valve.

Inspect the supply tube that goes behind the timing cover, it is normally mounted with two "P" clamps with insulation between the "P" clamp and the tube. If it is solidly mounted with a metal clip, and/or it's rubbing on the block, it may be picking up heat directly from the engine.

Hope this helps!
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