VCCA Home Forums GENERAL DISCUSSION New User Forum 50 Chevy air bubbles visable in glass fuel pump

My 50 Chevy Styleline Special, 216.5 is completely restored and has run great for many years. The car has been in the family since new. Lately I have had a problem that feels like an intermittent partial miss, like a mild bump during acceleration. It is noticeable primarily in easy load and coasting down. I know the ignition side is in excellent shape. Engine starts immediately and idles great. Today, I refueled from half tank and the intermittent partial miss was more prevalent. I noticed in viewing my clear glass fuel pump that a lot of air bubbles were in the fuel as it went through the bowl. The clear glass fuel filter at the carb was also bubbling. I am wondering if maybe I have an air leak at the glass bowl seal or similar. Any thoughts would be greatly appreciated.

A bad spark plug or spark plug wire.

Bill
Welcome to Chevy Chatter.
With air bubbles in the fuel glass you may have a pin hole in the fuel towards the tank but I dont think it is the cause of the misfire though. I tend to agree with Ray and would be closely inspecting the ignition system especially plugs and leads.
Tony

Some bubbling is normal in hot weather. Will be worse afer the engine is shut off.

I see you're from Georgia, and now that summer is upon us, I'm pretty sure those bubbles you are seeing are gasoline vapor bubbles due to the gasoline boiling. Today's gasoline, with its ethanol content, boils at a lower temperature than the gasoline of years ago. This causes problems in older vehicles with carburetors and mechanical fuel pumps. The vapor bubbles are in the carburetor float chamber, too, and cause the engine to miss as they work their way through the carburetor.

The problem is especially noticeable when you re-start the engine after a hot shutdown, such as coming down off the highway into a gas station and shutting down for ten or fifteen minutes while you fill the tank. When you shut the engine down, all cooling liquid and air flow through and across the engine stops, but the large heat sink that is the engine block and cylinder head continue to give off heat, causing the gas in the fuel pump, the fuel filter, and the carburetor to boil. When you go to re-start, you may notice that it takes considerably more cranking to get the engine started, which is due to flooding due to expanding and boiling gas in the carburetor, which percolates up the feed tube and drops down into the carburetor throat. Once you do get the engine running, it may run rough and not idle correctly, and when you start moving, the car may lurch and dive as the engine misses. However, once you get moving for a block or two, everything smoothes out, as the relatively cool gas from the fuel tank begins to work its way into the fuel pump and carburetor.

Often times, the addition of an electric fuel pump between the gas tank and the mechanical pump will alleviate the gas vapor problem. The safest way to hook an electric booster pump up is to wire it through a toggle switch located somewhere on or under the dash, so that you can turn it on when you notice the engine missing due to gas vaporization, then turn it off once the cooler gasoline has entered the system. If you hook up the electric fuel pump so that it runs whenever the ignition switch is on, there is a hazard potential if you were to get into a wreck and the fuel pump continued pumping through a fractured or broken fuel system component.

Mark

Thanks Mark. I believe you are right on target. It was a hot day yesterday and it is interesting that I had a half tank, filled it up and the random missing became worse. There was plenty of bubbling in the fuel pump, lots more than the occasional stray bubble. As far as the electrical side, I know for sure that it is solid with nothing that would contribute to a miss. Your answer is also great in helping to understand why one weekend it runs with very few misses and the next time it is worse. Yesterday was the worst and it was over 90 degrees. A cool start while in the garage is immediate and as you stated, to re-start after running is more difficult. Any other suggestions other than an electric fuel pump? What do others do with this issue who live in hot climates? Thanks again for the great explanation.

Suspect that the change from winter to summer gasoline blend is part of the problem. Takes a while to get the station's tanks to reflect the higher boiling blend not to mention what's in the car tank. Over time the more volatile gasoline components will vent from a vehicle tank particularly when the day to night temperature swings are greatest.

I suggest adding 10-15% diesel to the gas tank. It will not be a big problem problem in the older Chevys and reduce the tendency for boiling.

Mark, just reading some other posts concerning bubbles in fuel pumps and another suggestion is made that says it might be air bubbles but not actually boiling. Some suggestion was made that they say the sock in an old fuel tank blocking the line and the vacuum created by the pump caused bubbles to appear. It just makes me wonder why my random missing was worse just after I filled up the tank yesterday. Just considering all options especially since I have never replaced any sock or other filter in the tank.

The 1955 was the first year to have a "sock" filter in the gas tank. You could add some kerosene to the gas to lower its boiling point.

I wish I could tell you what specific things to do to correct your gasoline vaporization problem, but after ten years or so of working with the problem on my vintage Chevrolets, Buick, and Pontiac, I have yet to come up with a satisfactory solution, and have mostly learned to live with the problem in the summer months.

Regarding air bubbles in the fuel line, I think if there was a leak big enough to let air into the line when the fuel pump is operating, gasoline would seep out the leak when the pump is idle. A restriction in the fuel line between the fuel pump and the gas tank, if severe enough, will result in a lowering of the suction pressure in the line, which, in accordance with Charles' Law or Boyle's Law (Chipper can set us straight here), will cause the gasoline to boil at a lower temperature, resulting in vapor bubbles in the fuel line. Since the fuel pump can't pump vapor, delivery of liquid gasoline to the carburetor is interrupted and the engine misses badly or dies, commonly known as vapor lock. Having an electric pump mounted back near the fuel tank reduces the pressure drop in the suction line, thereby reducing (but not necessarily eliminating) vapor buildup in the fuel line. Modern cars avoid the problem all together by having the fuel pump submerged in the gas tank, and the entire fuel system up to the fuel injectors is pressurized at 40 or 50 psi or whatever pressure the fuel pump is set for, thereby eliminating any chance of the gasoline boiling.

Mark

Mark, thanks again for the good info. Several things have come to mind. I have a 76 Camaro with a straight 6 that is very similar to the 50-216.5 yet I have never seen this issue. I have also read the thought that I might have a partial blockage or restriction which causes a vacuum from the pumps operation, therefore lowering the temp at which fuel will bubble. Saturday the 50 started up immediately (kept in the garage) and idled and ran fine. I drove to the gas station and put in a half tank. It was after that when the occasional partial miss occurred. Makes me think that maybe the fuel entering the tank caused a stir in sediment that caused a partial blockage...lower pressure...lower bubbling point and so on. Seems it would be good to check out any restrictions at the tank. I agree that if there was a leak for air into the system that there should be a leak going out of fuel. I don't have any leaks. I am surprised to see all your experience and others with this very topic of bubbles in the fuel. First time I ever gave any thought to the changes in fuel today compared to back in the 50's. I am seeing lots of good ideas, now I just need to choose a direction and go. Still can't help but think about the events leading up to this random partial miss. I have had the partial miss occur in the past on and off from time to time and again wonder if there is stuff in the tank that moves around, allowing one day to be a good drive and the other to be spotty. It would be interesting to gently blow back to the tank to dislodge any blockage and then run and see what I see as far as bubbles in the fuel pump glass bowl. Driven this car for almost 50 years and never paid attention to the fuel bowl except for water or sediment, and now all this attention to the bubbles. The fun and joy of an antique car!