Vapor Lock

Admiral

Admiral

Well-known member
What are the symptoms of vapor lock? Can it happen while you're driving?

Today was the first really nice day of the year here in Michigan with temps in the low 70s. I took my car out for a drive of about 80 miles. The first half it drove beautifully but after that it started to lose power at certain points intermittently (seemingly around 45 miles an hour in high gear), which necessitated me hitting the momentary switch that turns on my in-line Airtex electric fuel pump. Doing this brought the engine back to life so to speak. When the car lost power it also made a faint moaning sound, which was very strange.

While driving the engine temp fluctuated between about 150 and 170 degrees on my aftermarket temp gauge. Does this sound like vapor lock or something else? I'm puzzled :confused:. Also, the car has been fully restored with all-new fuel line, a fresh filter, a rebuilt mechanical pump, etc. The engine is a 59L at about 251 cubic inches with a stock camshaft and a Stromberg 97 with stock jets.



Thanks!

- Craig
 
Vapor lock or not, if running the auxiliary pump restored the power I'd say it was a fuel delivery problem. You didn't mention the fuel pump push rod as new. MAYBE it is the problem, or worn camshaft lobe, or bad rebuilt pump. Just thinking out loud.
 
Craig, the term vapor lock is anathema to many people, so I'll just confirm that yes, you had a lack of fuel situation that was improved by the use of your electric fuel pump. But of course you already knew that! Many things can cause a lack of fuel, from dirt in the lines; air in the lines; mechanical pump issues; and yes, vapor lock. As your air temperature wasn't hot today, I'd suggest you go through your system from tank to carburetor bowl in search of the culprit... And stay away from oxygenated and/or Ethanol laced fuel, which is the elephant in the vapor lock issue in these modern times.
 
Admiral said:
This thread was valuable. And come to think of it, when stopped I could hear the fuel percolating in the carburetor. I've ordered one of Bob Drake's phenolic spacers and will check out the rest of the fuel system later today. I'm surprised this is an issue in such mild temperatures. How do guys get anywhere down in Florida? :eek:
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I'm down here in Florida. For the past month or two 88 to 90 every day
and summer isn't here yet. That's why a year or two back I did extensive
testing every day for a fairly long period of time and posted the results
on this forum. The only sure fire fix is a SMALL fuel return line after the
pump back to the tank. When going very slow or at idle the fuel stays in
the pump to long and at about 130 degrees it boils and turns to vapor.
The small amount of cool fuel running through the pump and back to the
the tank is enough to keep the pump cool and prevent VL. Don't forget
there is a .098 inlet hole at the needle valve of the carb so very little fuel
is flowing through the pump and the 1/4" line. You can only have a
return line less than .125 or the carb will starve at higher RPM's. A little
higher pump pressure, 4 lbs cold helps as the pressure drops as the pump
gets hot and down around 1 1/2 to 2 lbs when hot the problem starts.
G.M.
 
had a problemwith vapor lock in my 1985 s-15.the fuel line routed from front of 2.8 v6,to rear of carb.line was getting heated from the head,and problem stopped when I put a section of foil tube between line and head.Yes,it happened while driving,had to keep a fast idle,and it was backfiring.
 
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Vapor lock does not need extreme ambient temperatures to happen. I get it regularly even at 70-80°. You've already got the best solution - your Airtex electric back up pump. That's what I use regularly.
 
With Temperatures in the low 70s I don't think you have vapour lock ,you possible have a faulty pump or its sucking air .You should also check the temp of your coil .My car runs 180 to 190 water Temperature so under hood is up there I have never experienced vapour lock .high Under hood temperatures should = a boiling Radiator .
 
It's not the AMBIENT temps that matter. It is the temperature of the FUEL in the pump and carburetor (under the hood......on top of that HOT engine) that is critical. Many of these newer fuels have a relatively low boiling point, at which temp the liquid begins to boil and change state into vapor state. The real answer is to control the fuel temp. DD
 
V8COOPMAN said:
It's not the AMBIENT temps that matter. It is the temperature of the FUEL in the pump and carburetor (under the hood......on top of that HOT engine) that is critical. Many of these newer fuels have a relatively low boiling point, at which temp the liquid begins to boil and change state into vapor state. The real answer is to control the fuel temp. DD
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You got it. Most guessperts don't understand. G.M.
 
lol, the real experts on here say there is no such thing as vapor lock, so no matter how much evidence you present that there is they all will say you don't know what your talking about
 
WestCoast said:
lol, the real experts on here say there is no such thing as vapor lock, so no matter how much evidence you present that there is they all will say you don't know what your talking about
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I've always rankled at G.M.'s word "guesspert", however, I'd say the quote above may well prove the existence of same.
 
Sorry that I am not towing the party line ! I think the Admiral is entitled to a alternate view ,I am sure vapour lock does exist but in his case I think he should consider other options as well .Ted
 
Ha ha! Ted, We agree! My comment about G.M.'s Guessperts was not addressed to you!
I knew the subject "Vapor~Lock" would get the blood pumping!
 
With a new fuel line and associated parts, I'd check the ignition coil. When an original coil gets hot, it mimics the infamous "vapor lock". If it hasn't been rebuilt, send it to Skip Haney. Another suspect is the ignition switch. The Bakelite base usually wears grooves where the contact rides, reducing the contact pressure. The base and the "brush" (the moving part) should be changed as a pair. Finally, be sure the gas cap is vented properly.
 
Ted, I agree he needs other options, but if the engine starts to die and an electric fuel pump cures that problem, what do you think caused the problem?, fuel boiling in the pump causes it to loose suction, thus no fuel to carb, (vapor lock.) a hot coil will not get any better with an electric fuel pump operating in the system, by all means, check all posibilties, bad electrical connections included
 
Fuel boiling in the float bowl is a pretty good clue that it's fuel related, but only a true expert can say for sure.
 
OK Alan I get your point .The electric pumps making up for the Mechanical one not doing its job ,set up a vacuum gauge at the tank end and check for leaks .Also the reduction in performance of a mechanical pump pulling fuel through a Artex pump has to be considered ,Tests ? Ted
 
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With the tiles on the space shuttle being able to keep the heat on one side oNly,you think there would be some modern material that could be used to insulate the fuel pump and carb from the engine and manifold,
It's easy to insulate the fuel line ,but no so the pump and carb,
I have made a small air scoop that fits in beside the radiator and takes air from in front of the radiator via a 1-1/4 hose to the fuel pump,this helps ,also have a spacer under the carb,
But in 39c the fuel will still vaporise.
Any one work for NASA who get get us one of those tiles?
Lawrie
 
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