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Old 06-19-2009, 10:20 PM
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Locating Methods For Air Leaks In Mechanical Fuel Pump Fuel Lines

I Recently Rebuilt The A-b Combination Diaphragm Fuel Pump For My 1937 Cadillac Lasalle, Along With The Carter Carb. I Also Had The Fuel Tank Refurbished By A Tank Re-nu Facility. The Car Runs Exceptionally Well Until I Lose Prime At 40 M.p.h. On A Busy 4-lane Inter City Highway. (not A Good Thing)

I Recently Noticed, While Working On This Fuel Problem At My Home, That The Glass Fuel Bowl Started To Boil With Air Bubbles After The Car Was At Idle For 5-10 Minutes. I Know That These Air Bubbles Indicate A For Sure Air Leak, And I Assume That The Air Leak Would Have To Be On The Inlet Side Of The Fuel Pump, But I Cannot Seem To Find The Leak. There Seems To Be Only Four, (4), Places For The Leak To Occur, Unless I Am Mistaken. It Is Quite Possible That I Am Mistaken Since I Most Often Am.

(1) Fuel Line Fitting At Tank Attachment

(2) Fuel Line Fitting At The Reduction Of 3/8 To 1/4 Line

(3) 1/4 Line Fitting Into Inlet Of Fuel Pump

(4) Gasket At Glass Bowl

I Have Already Stripped The Threads Once, Since Repaired, Fuel Line Into The Carb. Trying To Stop Any And All Air Leaks, From Over Tightening The Fittings.

Any Suggestions As To An Easy Way To Locate And Remedy My Nemesis, Mr. Air Leak.

Thanks,

J.j. Lasalle

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Old 06-19-2009, 11:44 PM
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Boiling fuel

Boiling fuel does not mean you have an air leak. Boiling fuel will occur if you exceed the boiling point.

The boiling point will increase as pressure increases. What is your fuel pressure at idle?

The fuel will also boil if your block/pump is to warm. Was your engine/pump cold to the touch while fuel was "boiling"?

I have experienced fuel boiling with the new gasolines. A winter blend will boil very easy. In my case I took steps to reduce engine and underhood temps.

If fuel arrives to the pump under positive pressure then air can not leak into the fuel line. Some things that prevent positive pressure at the pump inlet:
1. Gas tank not vented.
2. Fuel line restricted or too small.
3. High pressure drop filter upstream of pump.
4. Fuel tank sock plugged.
5. Climbing a steep grade.
6. Fuel tank lower elevation than fuel pump.
7. Carb calling for more fuel than system can deliver.

If your system does not provide positive pressure to the pump suction then your pump may cavitate in severe cases then form bubbles and low flow. Your car should have positive pressure to the pump during idle, if your fuel level is higher than your pump and your tank is vented. Is your fuel level higher than the pump?

Your fuel was boiling at idle with a cold to the touch fuel pump?
In idle the fuel pump is not moving fuel most of the time; instead the pump is dead headed. If your fuel pump is cold to the touch and the fuel is "boiling" I would think it would be caused from the energy the pump is putting into the dead headed fuel; thus not the root cause of your engine stall.

That being said, you did not mention the possibility of a leaky diaphragm.
When you assembled the fuel pump was the pump spring in the correct tension state while installing the diaphragm? Was the diaphragm new or old stock?

Last edited by 001mustang; 06-20-2009 at 12:45 AM.
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Old 06-20-2009, 02:19 AM
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Locating Methods For Air Leaks In Fuel Line

Thanks For Your Response.

When I Wrote Boiling I Did Not Mean To Mislead Anyone By My Choice Of Words But Should Have Rather Said, "hundreds Of Bubbles Appearing In The Glass Bowl As If It Were Boiling.

The Fuel Line To The Pump Is Not In A Bad Area For Heat Transfer; Under The Frame. The Fuel Line From The Pump To The Carb. Is In A Rather Hot Area, Crossing Over The Exhaust Manifold. I Have Wrapped The Exhaust Manifolds With The Latest Composite Material And Sleeved The Fuel Line To The Carb. With This Heat, Shields Up To 950 Degrees, Protecting Material.

When The Car Is Up To Speed And Under Load, These Symptoms Appear With A Simultaneous Drop In Fuel Pressure.

When I Was Working On This Issue, At My Home, The Engine Was Freshly Started, Cold, And With Both Sides Of The Hood Fully Opened. When I Noticed The Bubbles Appearing In The Glass Bowl, I Also Had A Loss Of Fuel Pressure.

It Takes A Little While For The Air To Get In, Where Ever Its Entry Point Is, But It Does Happen, Along With The Pressure Loss.

J.j. Lasalle
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Old 06-20-2009, 03:40 AM
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Quote:
Originally Posted by J.J. LASALLE
I Recently Rebuilt The A-b Combination Diaphragm Fuel Pump For My 1937 Cadillac Lasalle
The problem lies in the fuel pump itself. Without a diagram, I hesitate to diagnose it further, but it should be considered suspect, IMO.

From the Cad/LaSalle CLUB , this guy is given as the contact for info on your year car:

Cadillac 1937-'48 ('41 best):
Marty Watkins
213 Fox Hollow Dr.
Langhorne, PA 19053
(215) 355-1519
martyhw@msn.com
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Old 06-20-2009, 09:01 AM
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How about the diaphram?

In my case the fuel was literally boiling. That's why I wanted to make sure your fuel pump was cold to the touch. It is not unusual for liquids to boil in a dead headed pump.

I don't suspect a vapor lock condition cruising at 40 mph.

In your case the air could be peculating around/thru a leaky pump diaphragm.
Also your pump check valves could be leaking thus not holding full pressure.

What is aprox diameter of the bubbles? Got any pics of your ride?

Quote:
Originally Posted by 001mustang
That being said, you did not mention the possibility of a leaky diaphragm.
When you assembled the fuel pump was the pump spring in the correct tension state while installing the diaphragm? Was the diaphragm new or old stock?

Last edited by 001mustang; 06-20-2009 at 09:26 AM.
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Old 06-20-2009, 03:41 PM
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Any rubber lines would also be suspect. They can get porous and suck air. Take the input line off the pump and feed the from a line going to a gas can and see if the issue goes away.
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Old 06-29-2009, 01:04 AM
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What was the problem?

Hello Lasalle. Did you solve your boiling issue?
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