How to prime A3 fuel system.

my69camaro · Jun 14, 2015

I've got a 3116 powered a3 that has been sitting in my parents back yard for 3yrs. Trying to get it up and going now. Installed good batts, cranks good but I'm not getting fuel. Little to no trickle loosening the fuel filter. I keep reading of priming with compressed air. Where/which line am I doing this to?

porkysplace · Jun 14, 2015

A few similar threads might help.

M35A3 - No Start - Lost Prime

M35a3 fuel problem

M35a3 lost power and now nothing

wreckerman893 · Jun 14, 2015

You do not want to put a lot of air pressure on the system. You just want to push the fuel to the IP. Too much pressure and something will blow out. 5 PSI or less will do it.

Monkeyboyarmy · Jun 14, 2015

You may have to disconnect the return line to the tank and plug the tank side. This will pressurize the feed side without pressurizing the return side. The flow of fuel is from the tank to the piston pump, through the filter to the back of the head, through the head to a pressure check valve on the front, and through the return back to the tank.

Warthog · Jun 15, 2015

Also check the check valve on top of the tank. They are known to stick and cause fuel flow issues.

M35A2-AZ · Jun 15, 2015

If I remember when I had my A3 that the TM said to hold the throttle paddle down when your cranking it and it would take a little of that before it would prime.

Scar59 · Jun 15, 2015

Warthog said:
Also check the check valve on top of the tank. They are known to stick and cause fuel flow issues.

Warthog is spot on. Sitting that long the check valve at the top of the tank connecting to the fuel line is stuck closed, need to remove and clean it real well. Prime the short fuel line at the IP, reconnect, shot of ether, and fire her up. This has fixed several A3 no start problems.

David.1000 · Feb 18, 2025

After my M35A3 sits for a couple of weeks, it loses prime in the fuel system. My fix is to disconnect the fuel return line at the fuel pump fitting, take a rubber tip air gun and slowly add air to the return hose. The fuel tank valve limits the amount of air pressure you can put on the system so that's the fail-safe in my process, just don't go crazy. You can hear the air and fuel moving through the system and when the diesel starts to come out of the fuel pump return line fitting, quickly reattach the return line to the pump fitting. After a few cranks the 3116 comes to life and all is good.

HDN · Feb 19, 2025

David.1000 said:
After my M35A3 sits for a couple of weeks, it loses prime in the fuel system. My fix is to disconnect the fuel return line at the fuel pump fitting, take a rubber tip air gun and slowly add air to the return hose. The fuel tank valve limits the amount of air pressure you can put on the system so that's the fail-safe in my process, just don't go crazy. You can hear the air and fuel moving through the system and when the diesel starts to come out of the fuel pump return line fitting, quickly reattach the return line to the pump fitting. After a few cranks the 3116 comes to life and all is good.

Sounds like your fuel system needs to be tightened up. My truck sits all winter (several months) and has never lost its prime like that. The only time priming is an issue is when I replace the fuel filter. Then I gently crank it for some time before it fires right back up.

There might be a dirty check valve somewhere too that could be causing the problem. I'm not quite sure though as I've never had my fuel system apart. But I heard about a return line check valve getting gunked up.

How to prime A3 fuel system.

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my69camaro

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porkysplace

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M35A2-AZ

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