Vapor lock or other fuel issue - Mercedes-Benz Forum

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post #1 of 2 (permalink) Old 06-25-2013, 06:26 AM Thread Starter
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Date registered: Oct 2012
Vehicle: 1968 250S, 1991 300se, 1998 SL500, 2x 1977 450SEL 6.9. Past MB's; 300SD, 300E, 300TE, 190E, ML420
Location: Colorado
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Vapor lock or other fuel issue

A few weeks ago drove the 250S up the Colorado Mountains - Glenwood Springs and Aspen and had a fuel issue that at the time my best theory was vapor lock. 4 times after driving for over and hour and then stopping for a short time, then suffered a bogging down issue. either revving the engine to clear it, or letting car sit for a while then car ran fine. Had similar problem 4 years earlier up there. This was driving at 8-9000 ft elevation.

Carbs were rebuilt 1.5 years ago.

Just finished checking tank filter which was clean, and replace a few hoses near tank. I blew air through the lines from then engine back and when I did the return line, it seems like I encountered back pressure and then sort of a release. I found a small lg pea sized pile of very fine stuff on the ground that I am reasonable sure, though not 100% was not there earlier. I wish I used 2 people so I could catch what came out to be sure.

Any ideas? I have heard that ceramic coating inside and out of exhaust manifolds works well on older cars to eliminate heat.

Also does anyone know which way the counterweight on the exhaust riser flap is closed vs open. I want to wire it shut.

I have an inline fuel filter before the pump, that does not fill all the way up with gas, there is still air in there. Is that normal?

BTW found this interesting explanation of vapor lock:

Last edited by rumb; 06-25-2013 at 06:31 AM.
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post #2 of 2 (permalink) Old 06-25-2013, 08:12 AM
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Date registered: Nov 2012
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yes it is normal to see air in the filter .at the front cab , there is a fuel return valve , it should be open at idle / low trottle to allow gas to go back to tank , see if it is working .diconnect the return line and in a jar see if gas is coming out at idle and not when you step on accel .
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