Vapor Lock

[Guest sndtubes]

Well, I just went on my first tour with the 1930 coupe. (Tour with the BDE). We went to French Lick, IN. It is about a 270 mile drive each way. All in all I put about 1200 miles on the old jalopy. It really ran well with the exception of a couple of instances of vapor lock. I've never experienced this before. Are they any "tricks" that anyone knows of to prevent this problem? Of course, it was pretty hot out; about 92-94 degrees. For those of you who have followed a different thread of my overheating problems, I am happy to report that problem is now very minor. Only a couple of times it boiled a little bit after stopping. While running it stayed between 160 and 185 degrees.

BTW, I did a search on vapor lock and didn't find anything.

[Clipper47]

Try here. The Packard Forum had quite a discussion about this just a few weeks ago

http://forums.aaca.org/ubbthreads.php/ubb/showflat/Number/427218/page/3#Post427218

[Mark Shaw]

I read the entire list of responses in the Packard forum, and concluded that nobody really completely understands how vapor lock occours. I sold positive displacement metering pumps for years and I think I understand the real problem. It is a combination of heat, long suction lines, low vapor pressure liquid, and old corroded components.

Although fuel can vaporize under pressure between the pump and the carburator, this would require much higher temperatures. Unlike centrifugel pumps, diaphragm fuel pumps have an intermittent flow characteristic; (flow starts and stops) so they only pump 50% of the time. The other 50% of the time they are drawing fuel into the displacement chamber. It is during the intermittent suction stroke that the atmospheric pressure in line from the tank to the pump is reduced to draw fuel from the tank. Therefore, they require about three times more net posidive suction head (NPSH) than continuous flowing pumps.

Suction flow stops during the discharge stoke, so each suction stroke must accellerate the fuel by reducing the pressure in the suction line to allow atmospheric pressure to push the fuel into the pump. The minimum pressure that must be supplied to the pump suction is determined by the displacement of the pump, suction line size, temperature, the specific gravity & vapor pressure of the liquid pumped.

Knowing this, you can conclude that the key variables are: Temperature, altitude, pump size, line size, and the specific gravity & vapor pressure of the gasoline. Increasing line size will help. Increasing any other variable will increase the probability of vaporization.

Check the screen in the tank and suction lines for corrosion, pinched lines, or anything that would restrict flow. Route pump suction lines well away from exhaust pipes, mufflers, etc.

Just give your original fuel pump a chance to work before you install an electric fuel pump.

[Guest sndtubes]

Mark,

Thanks for the info. I'll certainly look into all of those things. The tank has had the "tank re-nu" process, so shouldn't be a problem. The fuel pump has been rebuilt with a nitrile diaphram. I wonder if replacing the steel lines with rubber ones will help? The lines get pretty close to the engine down by the fuel pump, so I will try to re-route them some to get them as far away from the engine as possible. Also, another thing I tried is switching to premium fuel. In some states (and I'm not sure about here) all grades have 10 percent ethanol except for premium. Maybe it's the alcohol that is causing the problem? It hasn't vapor locked on me since I switched fuels but I really haven't given it a hard run on a hot day since I switched. Again, thanks for everyone's help.

[Guest brh]

You may find the premium fuel does not help. I had trouble with this on my 65 never experienced vapor lock. I will spare ya the entire story but my problem was a partially plugged radiator. When the car started to overheat, it simply would not start, car would begin to run like crud in bumper to bumper then when she cooled off like majic it ran O.K. Took me forever to figure this one out. She burns premium.

[Guest sndtubes]

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Bruce (aka) brh</div><div class="ubbcode-body">You may find the premium fuel does not help. I had trouble with this on my 65 never experienced vapor lock. I will spare ya the entire story but my problem was a partially plugged radiator. When the car started to overheat, it simply would not start, car would begin to run like crud in bumper to bumper then when she cooled off like majic it ran O.K. Took me forever to figure this one out. She burns premium. </div></div>

Very interesting. My car does exactly the same. However, the radiator has been replaced. I surely shouldn't be clogged. After driving the car hard for awhile in very hot weather, it will heat up some when it comes to a stop...... It is recently rebuilt and it really heated up on me for the first 1500 miles. Now it isn't too bad.

[Guest brh]

This may be a stupid question, but is it possible your engine was rebuilt real tight (look at your last response)? If she is getting better the more miles ya put on the better. Could be tolerances were real tight on the rebuild and all you need to do is drive her. In the meantime, I'd "rig" a fan on the radiator just to keep it cool, its never good to run an engine hot.

[garnetkid]

I have an article on vapor lock hard hot starting that was written by a friend of mine, Dave Botting of Summerland BC that he developed to correct the problem on his 1936 McLaughlin Buick. Hope this attachment works. You may want to consider this as it solved his problem.

http://i62.photobucket.com/albums/h97/garnetkid/img257.jpg

[Rooster]

I had vapour lock on my 1929 real bad when I first got it on the road. There are a few things you can do , but the only one that worked for me was to instal an electric fuel pump. I bolted it up on the chassis next to the fuel tank in line with the original fuel line. I only use it when first starting after a long layoff to fill carby bowl or when it does vapour lock. After it clears turn off. The original pump will still continue to work.

You can also try insulating the fuel line from tank to original pump. I tried this but made little difference. Another trick is to set up a return line from the original pump back to tank. What this does is have the fuel continually flowing , not held up in the line or pump getting hot and then eventually vapourizing. Most apparent to happen when you are stuck at traffic signals or in slow moving traffic, parades etc.

Another trick I've heard , but not tried is to add about a cup full of diesel to a tank of petrol.

This is supposed to stop vapourizing. As I said I've never tried it. Put in an electric pump with a remote underdash on / off switch and you won't go wrong.

Cheers, Ken.

1929 Tourer.