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I have a R 686 dump truck with a300 Mack engine. After sitting for a few days you have to spin it over for a while to get it to start. It’s like it loses its prime on the fuel. Is there a check valve in the line going back to the tank that I could take a look at? 

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3 hours ago, NLC said:

I have a R 686 dump truck with a300 Mack engine. After sitting for a few days you have to spin it over for a while to get it to start. It’s like it loses its prime on the fuel. Is there a check valve in the line going back to the tank that I could take a look at? 

No clue i'm not a wrench😎 other than the simple things first.  Check all the fuel line fittings to make sure they all are tight. I had unbolted fuel filter bracket to get easier access to steering box and in the process of constantly moving it back and forth to keep it out of my way, I  must have loosened fittings.  I was looking for fuel leaks but it was a suction line that was getting air!

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You have a leak in the return side of fuel system

Pressurize the tank slightly to show the leak 

A hose clamp and a piece of old inner tube over the filler will show the leak pretty quickly

Wind the air regulator on your shop compressor down as low as it will go, once the fuel system is at 10 or 15 psi have a look all around the fuel lines for the fuel leak 

 

Paul

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I had a similar problem with a 350 in a superliner always had to crank it a long time after sitting for a couple of days finally found a tiny crack in the fuel filter head where the fitting for the line to the pump screwed in gravity was allowing the fuel to return to the tank  and partially emptying the filter

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23 hours ago, D-Day said:

I had a similar problem with a 350 in a superliner always had to crank it a long time after sitting for a couple of days finally found a tiny crack in the fuel filter head where the fitting for the line to the pump screwed in gravity was allowing the fuel to return to the tank  and partially emptying the filter

No matter were the leak is I think putting some pressure in the system is by far the quickest way to find a pesky leak 

 

Paul

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On 7/21/2021 at 4:34 PM, mrsmackpaul said:

No matter were the leak is I think putting some pressure in the system is by far the quickest way to find a pesky leak 

 

Paul

Paul, 

Point of interest-in my case I was looking for leaks-nothing.  I pulled filter and level was below the pick  up.  so  I would refill filter, crank and it would  fire up and  run for a minute or two then quit.  I finally noticed bright brass threads on filter  head on what I  thought was the suction  line-tightened those  and end of problem.  So isn't the suction line a negative pressure and it will not show fluid???

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Red Horse, Even though the suction line is normally a vacuum line, if you put 5 PSI of pressure in the tank the suction line "should" leak/seep??

Brocky

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17 minutes ago, Brocky said:

Red Horse, Even though the suction line is normally a vacuum line, if you put 5 PSI of pressure in the tank the suction line "should" leak/seep??

This is what I would expect to happen, the only leak it wont show up is the pipe inside the tank that returns the the fuel to the bottom of the tank

 

Paul 

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On 7/27/2021 at 4:35 PM, mrsmackpaul said:

This is what I would expect to happen, the only leak it wont show up is the pipe inside the tank that returns the the fuel to the bottom of the tank

 

Paul 

Brocky and Paul- I guess my problem was I did not pressurize tank and for sure once level in filter dropped below the "drop" in the nipple that is  on the bottom of the filter head,  it would quit running

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