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A friend of mine has a 85? 6.9 diesel and today he had an air bubble that of course caused the truck not to start. This is not the first time this has happened. My question is, is there a trick or simple way to find the leak in the fuel system that is sucking in the air and hydrolocking the system?
 

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easiest way is to look for leaks
then look at the return lines, between the injectors, then the one form the pump to the filter head.
Then start looking down wards towards the frame.
 

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A friend of mine has a 85? 6.9 diesel and today he had an air bubble that of course caused the truck not to start. This is not the first time this has happened. My question is, is there a trick or simple way to find the leak in the fuel system that is sucking in the air and hydrolocking the system?
First of all its not hydrolocking. that would be caused by too much liquid in the cylinders. If anything you could say its vapor locking.

Since its not leaking fuel out then you have a fuel line that has a tear/hole in it and is allowing air to enter.
 

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Common place is the return lines. Either the o-rings under the return T's wear out after a while or the rubber return lines from one injector to the next dries out and cracks, and then the leaks occur. Another common problem is simply screwing the fuel filter on wrong. Yeah people think they are the shit and they can get a damn fuel bowl on right, no they can't. It just kinda happens every now and then.

But seeing its an 85, a leak in the return lines wouldn't suprise me at all.

Edit: J-rod kinda covered all of that.
If you need new return lines, you can get it all from www.mwfi.com, or Midwest Fuel Injection. They are good people.
Also, I have a leak in mine from when I switched out the stock mech lift pump for an electric Holley fuel pump... I just bleed the fuel bowl everytime. That depends on how lazy you are though.
 

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If you don't have any fuel leaks then your issue is on the suction side not the pressure or return side. But like was said considering the age I would not be surprised to find leaks. I would use an air line regulator like for a paint sprayer have it set to 2-4 psi and put some pressure in the tank. Then go looking for leaks on the suction side. But remember no more pressure than 2-4 psi the tank is not designed as a pressure vessel. If this air lock issue happens first thing in the AM then I would suspect a bad check valve (I can't remember if IDI engines have them, I know the PSD do.) or bad lift pump allowing the fuel to drain back into the tank. Try priming the pump before cranking to see if it has prime or not. Loose fuel filters are not uncommon like was said.
 

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Discussion Starter #6
Good info guys, I'll forward this along to him. Hopefully he can find it and get it fixed. Thanks for the help
 
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