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Fuel pump efficiency

Started by Jeff Trahan, August 11, 2022, 09:11:15 PM

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Jeff Trahan

I'm starting to think maybe the fuel pumps on my '38 V16 get less efficient when hot. If I let the car idle for a while, the level in the fuel pump bowls slowly goes down and the engine stalls. But if I rev the engine every few minutes, the bowls fill back up some and the engine keeps running. Has anyone seen this problem before (not necessarily on a flathead V16)? The strange thing is the fuel pump bowls fill up by themselves while the engine is sitting overnight. Why would that happen? Thanks.

Jeff
Jeff Trahan
#28564
1938 Cadillac V16 9033
1947 Cadillac 62 Convertible Coupe
1958 Cadillac Biarritz

Brad Ipsen CLC #737

Yes, I have seen the problem before in the V-8 pumps which are essentially the same mechanism.  When you stated your problem previously, I couldn't remember when I had this same problem.  But now I do.  It has to do with getting the stroke right on the pump.  When the engine is revved it just pumps enough but at idles it does not have enough volume to keep up.  It was professionally rebuilt.  I think I fixed by putting a different pump on and sending the pump back to the rebuilder.  Since the V-16 has two pumps they must both be set up the same since one pump should be able to keep up easily.  There is a volume test spec for these pumps to verify this. One spec I found in the later "Thoro-Check" book is 6 oz. should be pumped with 15 strokes.
Brad Ipsen
1940 Cadillac 60S
1938 Cadillac 9039
1940 Cadillac 6267
1940 LaSalle 5227
1949 Cadillac 6237X
1940 Cadillac 60S Limo

fishnjim

May also be related to the modern fuel.  The distillation curve is much different so it can more or less evaporate at running temperatures.
In order for the level to regain overnight, it's most likely due to the cooling down and creation of a slight vacuum to "suck" the fuel in. When that fuel that evaporated collapses it creates a vacuum.  There's no restriction in the fuel line per se.  Some of these early cars have much different fuel arrangements at the tank, so without a complete review, can only guess.  Not a prewaree.
It's why these "big motors" when the way of the buggy whip.  Too complicated to keep running.(my opinion)  Otherwise we'd seen V24 or larger by now.
Common solutions, add a electric pump at the tank, change to no ethanol fuel.
Seems like the checking of the delivery is also required, since not sure what's causing it.  I probably have some info in the period motors manual, but on request.

Lexi

#3
Quote from: Jeff Trahan on August 11, 2022, 09:11:15 PMIf I let the car idle for a while, the level in the fuel pump bowls slowly goes down and the engine stalls. But if I rev the engine every few minutes, the bowls fill back up some and the engine keeps running. The strange thing is the fuel pump bowls fill up by themselves while the engine is sitting overnight. Jeff

Although not 1930s, my '56 V8 does that as did my previous '56s. A buddy has had four of them and they all did the same. The former Administrator of the now defunct Mid-Century Cadillac CLC forum also noted the same fuel delivery issue with his '56 Eldo. He even recorded it and took temperature measurements which he posted on the site. I agree with Jim's observations regarding the different boiling point of modern fuel as a possible cause. I also agree that the vacuum created when it cools down can draw fuel back, hence the reason why some install a one way valve after the fuel pump. Also, the fuel pump may have lost some efficiency. There are a number of tests especially using a vacuum gauge, you can do to determine your fuel pump's efficiency. Even a PSI test. Get the specs on what a healthy fuel pump for your car should be as Brad noted, then conduct the necessary tests. Could be other issues, but at least this is a start. Clay/Lexi

James Landi

When the fuel is expanding in the site glass at the fuel pump and the fuel in the carburetor is boiling, the draw down from the engine will create a high static pressure causing the engine to stumble and stall, especially at idle. I think having an electric pump that;s activated by a momentary button would provide a suitable workaround, for a quick thinking driver.  I also BELIEVE (have no proof, though), that adding Marvel Oil to the gas ups the boiling point of the fuel.  James