Re: M342 (M35A2)fuel delivery problem

From: Bill & Bonnie (bprestin@chartermi.net)
Date: Wed Jun 28 2006 - 22:25:00 PDT


    After last falls trail ride my truck sat all winter and spring awaiting
further attention. I was finally spurred to figure it out so as not to miss
this years forth of July parade here in Birch Run, MI.

 You can read the previous message below this one to see what my engine
problems were.

The fuel system looks very complicated but is not too bad once it is
understood.

Here is how the system works:
     There is an in tank electric fuel boost pump that sends 5-7 lbs of fuel
pressure through the frame mounted primary fuel filter housing to the fuel
supply pump mounted on the side of the fuel injector pump. This fuel supply
pump is gear driven from the injector pump and supplies fuel under
pressure(30lbs at idle-75lbs at speed) depending on engine RPM to the engine
mounted fuel filter housing.
     In the top of the engine mounted fuel filter housing is a pressure

regulator valve that starts to open at approximately 60 lbs and regulates
the engine fuel pressure between 65 and 75 lbs psi. at 2000 RPM and above.
Any excess fuel allowed to escape by the opening of the fuel pressure
regulating valve is sent back to the fuel tank. This 65-75 lbs. pressurized
fuel is sent through the engine mounted secondary fuel filter housings and

to the fuel density compensator mounted on top of the fuel injector pump.
After passing through the fuel density compensator the fuel enters the fuel
injector pumps hydraulic head and is pressurized to a high level and sent to
the fuel injectors. Excess fuel returning from the fuel injectors and flame
heater assy returns to the injector pump fuel overflow valve assy on top of
the front of the fuel injector pump and is sent back to the fuel tank via
the same fuel line coming from the fuel pressure regulator valve.

 I measured the pressure at the primary fuel filter and found I had the
necessary 7lbs of pressure. I measured it at the top bleeder valve on the
secondary fuel filter housings mounted on the engine and found 50lbs of
pressure at 1200 RPM. This was within the specified 48-57lbs for that engine
speed. I measured it after the secondary fuel filter housing but before it

re-entered the injector pump and found the same 50lbs of pressure.
    Then a funny thing started to happen. As the engine warmed up(about 10
min.of running) the 50 lbs of pressure dropped off to 28lbs. After really
reading into the manuals I figured out the above system and thought it might
be the fuel pressure regulating valve. If it was the fuel supply pump the
engine would have to be jacked up to remove it from the side of the injector
pump as there is less than one inch between the boost pump and the steering
gear box. I am not sure the engine could be raised enough as there is only a
couple of inches between the top of the flywheel bell housing and the cabs
firewall.

    On top of the engine mounted secondary fuel filter housing there is what
looks like an extra bolt, its really a plug with an O-ring. When I unscrewed
it a spring flew out and almost got lost under the truck. I used a pair of
needle nose pliers and pulled out the valve plunger and under it was a piece
of crud holding it up off its seat. I cleaned the plunger and its seat and
reinstalled the parts.
    After this I ran the engine for almost a half an hour at 1200 RPM. The
pressure stayed above 50 lbs. at all times.

I'll take it for a test drive tomorrow but I am sure it is fixed.

Here are the pressures necessary at the top of the secondary fuel filter
housing.

RPM Min press lbs Max press lbs
700 30 35
1200 48 57
1600 57 64
2000 63 69
2400 65 73
2600& above65 75

Bill
Michigan

----- Original Message -----
From: "Bill & Bonnie Prestin" <bprestin@chartermi.net>
To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
Sent: Sunday, October 09, 2005 4:03 AM
Subject: [MV] M342 (M35A2)fuel delivery problem

> Hi,
> I have an M342 dump truck with an LDS465-1C engine, I am struggling
> with fuel delivery problems. It will drive down the road great for a

> couple of miles (accelerates fine) then just runs out of fuel. After
> sitting for a few minutes with the fuel pump turned on, it will start back
> up and drive a couple of miles more. It will also idle fine forever, so a
> little fuel is getting through. I changed all the fuel filters, still no
> luck. I also tried bleeding the valve above the fuel filters on the engine
> while the truck is idling. Plenty of fuel squirts out, so the problem must
> be beyond there. The problem developed while I was up in northern Michigan
> on our clubs fall trail ride. The 120 mile trip home was pretty rough.
> Anyone have any ideas where the system might be clogged? My engine has the
> sticker on the side that says
> "use diesel fuel only. Fuel compensator has been bypassed to prevent
> engine oil dilution"
> But there is still a maze of fuel lines around the injector pump.
>
>
>
> Bill
> Michigan
>
>
>
> ===Mil-Veh is a member-supported mailing list===
> To unsubscribe, send e-mail to: <mil-veh-off@mil-veh.org>
> To switch to the DIGEST mode, send e-mail to <mil-veh-digest@mil-veh.org>
> To reach a human, contact <ack@mil-veh.org>
>



This archive was generated by hypermail 2.1.4 : Tue Jul 18 2006 - 21:47:28 PDT