M923A2 Fuel Filter change

[InTheMitten74]

Okay, so I have 5 separate PDF TM's for the M939 series trucks and I'm trying to find where it covers changing the fuel filter.

Is it as easy as taking the old one off and putting a new one on or, are there other steps that need to be followed?

If anyone can, please point me to the correct chapter.... or, tell me if it's as easy as old off, new on.

 

[98G]

A2s frequently have two filters. One on the block that's as simple as it looks. The second one on the firewall is a cartridge cannister filter and is also straightforward. Not all A2s have the one on the firewall.

 

[M35A2-AZ]

If you have the big one under the front left finder, remove it with a 3/4 wrench and remove the filter housing , you will have to pour the diesel into something, clean the housing and install new filter.
I would add some diesel to the housing before you install it.
There is a o-ring under the 3/4" bolt washer be sure to put that back in place or you will lose your prime all the time.

 

[Scrounger]

M35A2-AZ covered the large filter perfectly. The A2 trucks have a screw on filter on the left side of the engine above the fuel pump. It helps but isn’t necessary to fill the second filter with fuel before priming if the first filter has fuel in it.

After all the new filter(s) are installed, pump the primer on the fuel pump 10-20 times. Then crack open the bleeder on the bottom of the second filter. You should hear some air escaping. Close the bleeder and repeat the pumping of the primer pump, then release the air. It can take a while to prime the filters. Once there is a stream of fuel with little or no air when bleeding it should be good enough. When most of the air is clear when pumping the primer, one should hear the fuel going thru the injection pump to the bypass valve. The engine should start with minimal cranking at that point.

After the engine starts, idle for 5-10 minutes to clear any remaining air.

 

[74M35A2]

There is also a bleed screw on the injection pump that can be used in conjunction with the push button primer on the lift pump. Makes quick work of all of it. Can even fill the frame mounted canister filter (if you have one) reasonably quick.

Pre-filling filters reduces the work, just be careful to keep it a clean operation. 1/2 of the filter top side will go directly to the injection pump and injectors and not through the filter. For the newer common rail engines, service manuals specifically specify against pre-filling any filters, to be sure all fuel reaching the high pressure pump and injectors is filtered.

For the frame mounted fuel filter, if you have one, be sure the square bolt washer atop of it seats correctly in the recess of the filter head, as mentioned.