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Fuel Problem - '46 Merc


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My '46 Merc died on me.  mechanical fuel pump had a grass bowl and I could see there was no fuel.  Checked the fuel line and there was no obstruction.  Gas tank is about 1/2 full. I replaced the pump with a 6v Airtech pump wired directly to the ignition.  When I turn on the ignition without the engine running, I get about 5 PSI of pressure.  As soon as I start the engine, the pressure slowly drops to zero.  The engine continues to run for about a minute, then dies. Checked the carb.  It's clean and float seems to be ok.

 

Would sure appreciate some help.😔

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As Ben noted, mount electric pump as close to the tank, but also as low as possible. Electric pumps are a good "Pusher", BUT NOT SO MUCH AS PULLING.

Rather than bypass your mechanical pump, leave it as the primary, and wire the electric pump on a switch to use is as an addition when needed, such as priming the system after sitting for some time, and for when vapor lock affects your flathead which is prone to this condition.

Also, the Airtex you used is a cheaper type pulse pump, and in my years of experience is prone to failure, even in the short term.

You should strongly consider the rotary-vane type pump such as the Carter, available for a few bucks more through Amazon. This is a Free-flow design, and your mechanical puimp will pull fuel through it just fine. switch on the electric when your fuel system needs a temporary boost.

 

The rotary vane type pump is far better than the cheaper pulse (bullet) type, and will last many years longer.

My choice is the Carter P4259 - $78.11 & FREE Shipping at Amazon.com

I use this model on my

1930 Packard,

1937 Buick Roadmaster,

and 1941 Cadillac

https://www.amazon.com/Carter-P4259-Line-Electric-Fuel/dp/B000CIS4IU/ref=sr_1_4?dchild=1&gclid=Cj0KCQiAwMP9BRCzARIsAPWTJ_EVycjP6Y_TX2aDAx_qM6aEgNOYBimwB3t4kdE0jYQzVTnHNCcqun8aAkX4EALw_wcB&hvadid=177570362789&hvdev=c&hvlocphy=9025155&hvnetw=g&hvqmt=e&hvrand=7371372844440783517&hvtargid=kwd-3628355007&hydadcr=5739_9590425&keywords=6+volt+electric+fuel+pump&qid=1605486563&sr=8-4&tag=googhydr-20

 

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Sounds like there is an obstruction in the tank or line(maybe even a rubber hose is collapsing) somewhere.  When you checked for on obstruction did you blow back through the line?  If so you could have just pushed the obstruction out of the way.    

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I had a similar problem, it was garbage in the tank plugging the pickup.  You can put a light amount of air pressure by the mechanical fuel pump and blow the gas back into the tank, you should hear bubbles in the tank.  If you have a glass bowl, I have had the cork seal dry out but that would not explain the electric fuel pump problem.  I do not like electric fuel pumps, unless you also include an impact detector, they automatically turn off the pump in an accident, the race car guys use them all the time, most tracks require them.  I would guess they are required for vintage racing also?

 

image.thumb.png.155045da0a91276572d8cbebd9074b85.png

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6 hours ago, TerryB said:

Check to see if you really have gas in the tank, fuel gauge can go bad.  Sounds like fuel line or tank pickup might be blocked.

I don't use the fuel gauge.  I added fuel to the tank as soon s the problem came up.

I blew back the line with a compressor and also used a vacuum pull to pull fuel forward.

Both showed no obstruction.

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5 hours ago, Marty Roth said:

As Ben noted, mount electric pump as close to the tank, but also as low as possible. Electric pumps are a good "Pusher", BUT NOT SO MUCH AS PULLING.

Rather than bypass your mechanical pump, leave it as the primary, and wire the electric pump on a switch to use is as an addition when needed, such as priming the system after sitting for some time, and for when vapor lock affects your flathead which is prone to this condition.

Also, the Airtex you used is a cheaper type pulse pump, and in my years of experience is prone to failure, even in the short term.

You should strongly consider the rotary-vane type pump such as the Carter, available for a few bucks more through Amazon. This is a Free-flow design, and your mechanical puimp will pull fuel through it just fine. switch on the electric when your fuel system needs a temporary boost.

 

The rotary vane type pump is far better than the cheaper pulse (bullet) type, and will last many years longer.

My choice is the Carter P4259 - $78.11 & FREE Shipping at Amazon.com

I use this model on my

1930 Packard,

1937 Buick Roadmaster,

and 1941 Cadillac

https://www.amazon.com/Carter-P4259-Line-Electric-Fuel/dp/B000CIS4IU/ref=sr_1_4?dchild=1&gclid=Cj0KCQiAwMP9BRCzARIsAPWTJ_EVycjP6Y_TX2aDAx_qM6aEgNOYBimwB3t4kdE0jYQzVTnHNCcqun8aAkX4EALw_wcB&hvadid=177570362789&hvdev=c&hvlocphy=9025155&hvnetw=g&hvqmt=e&hvrand=7371372844440783517&hvtargid=kwd-3628355007&hydadcr=5739_9590425&keywords=6+volt+electric+fuel+pump&qid=1605486563&sr=8-4&tag=googhydr-20

 

 

Before reconnecting the fuel line to the carb, I ran the pump and got a good steady flow of fuel.  How could the pump be defective if it produces 5 psi of pressure before starting the engine?

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Just a thought but "As soon as I start the engine, the pressure slowly drops to zero" certainly sounds like a clogged vent. What happens if you leave the gas cap loose ?

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Are there any rubber hoses connecting the lines? I had a similar (not identical) and it turned out that there was hairline crack in one of the rubber lines sucking air in and causing the pressure to drop. I discovered this by blowing air backwards to the tank (with the cap loose) I then heard the air hissing out. Just a thought... 

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40 minutes ago, John348 said:

Are there any rubber hoses connecting the lines? I had a similar (not identical) and it turned out that there was hairline crack in one of the rubber lines sucking air in and causing the pressure to drop. I discovered this by blowing air backwards to the tank (with the cap loose) I then heard the air hissing out. Just a thought... 

Along those same lines, I had a 41 Olds that would idle and cruise around perfectly, but any kind of load it would die out.  Turns out there were holes in the metal fuel line as it went over the rear axle, never leaked just sucked air.   

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17 hours ago, padgett said:

Just a thought but "As soon as I start the engine, the pressure slowly drops to zero" certainly sounds like a clogged vent. What happens if you leave the gas cap loose ?

Just tried your suggestion and the car ran!  Not for long, but a significant improvement.  Ran for about 3 or 4 minutes with the pressure fluctuating between 2 and 4 psi.

I located the fill pipe vent and it was plugged shut with a long bolt with tape over the end to keep the bolt from falling out!  Looks to have been done a long time ago.  I removed the bolt and also cleaned out the gas cap vent.  Put the cap back on and the car started and ran, but again not for long.  Runs fairly good at high RMP, but will not idle.  Anyway, looks like you got me headed in the right direction.  Thanks very much. 

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17 hours ago, vermontboy said:

In the "probably not" category make certain the gas cap vent  isn't clogged just for kicks - doesn't sound like the right symptoms  but ....

Done, and I think that was part of the problem.  Thanks.

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1 hour ago, John348 said:

Are there any rubber hoses connecting the lines? I had a similar (not identical) and it turned out that there was hairline crack in one of the rubber lines sucking air in and causing the pressure to drop. I discovered this by blowing air backwards to the tank (with the cap loose) I then heard the air hissing out. Just a thought... 

There are the rubber hoses used to connect the fuel pump and also some under the hood.  The ones under the hood are fairly old and I'm going to replace them.  Thanks.

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  • Peter Gariepy changed the title to Fuel Problem - '46 Merc

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