673 fuel questions, need me some edumacation

[ThaddeusW]

Okay. So two weeks ago I was working on my B61, letting it run for a while at idle. After about 5 minutes of running the engine at idle I began to slowly bring up the RPM's. It started to blow black smoke, sputter and then die. I freaked out for a second thinking the worst. I tried to crank it but nothing. It was as if it was getting no fuel.

Yesterday I spent a full day working on the truck and tackled the engine problem.

My first thought was to buy 5 fresh gallons of diesel, pull the fuel filter (I have two spin on filters), bang out any dirt, fill it with fresh diesel, prime and then start the truck. Nothing.

Next I thought it was a stuck rack in the injection pump. Pulled the cover, checked the rack movement and it looked fine. Not the pump.

I then opened the return line from the injectors, pump and secondary filter, put a cup under the lines and cranked it for 5 seconds. No fuel. Reconnect return lines.

Pumped the primer handle with the cup in place and fuel squirted out as it should. hmmmmmm.

Last thing, pull the feed line to the pump and put a cup under the hose. Saw a bit of fuel spurt out when I cracked the line and ....

And then things got interesting. I cranked the engine and the darn thing started WITHOUT the fuel supply line connected to the pump. I quickly shut it down and checked the cup which overflowed and spilled fuel all over. Well now, nothing was wrong, plenty of fuel! I'm sitting there scratching my head and it occurred to me, it must have been air lock. I reconnected the supply line, pumped the primer to flush out air, cranked it and she ran fine. Little rough at first because she must have sucked some air. But she smoothed out and ran fine after about a minute. Brought the RPM's up and it was fine.

I need some help understanding my fuel system. Two pictures:

I think I have everything worked out but here is what I know so far: (yes the injector line is unhooked, that was after my cup test)

1) Fuel supply to pump (from secondary filter?)

2) Return line from injectors

3) Return outlet from injector pump

4) Not sure, looks like it comes from the secondary filter.

5) Return line to tank

6) Lift pump, primer pump and fuel lines

1) supply from tank

2) Not sure where this lines runs best guess is to the lift pump?

3) Do these lines come from the lift pump and run back to the engine?

4) bypass/return line?

Here is what I think I have worked out so far: ( just spotted my glaring spelling mistake, lol)

1. Fuel is pumped from the tank, through the primary filter and into the lift pump.

2. From the lift pump, fuel then goes through the secondary filter, some returns to tank.

3. Fuel then runs from the secondary filter to the injection pump inlet.

4. Fuel is allowed to return from both the injection pump and the injectors back to the tank.

I'm no diesel tech but why is there a secondary fuel filter? My best guess is that "more filtering is better". And why does it allow some fuel to return to the tank, to prevent over pressure from a clogged filter?

[farmer52]

This is what I know about fuel filters.

Primary filters the "rocks and pebbles" and the secondary filter is for the "sand". The micron (pore size) of the filters are different with secondary being finer/smaller. Using a single filter may cause premature plugging.

[ThaddeusW]

This is what I know about fuel filters.

Primary filters the "rocks and pebbles" and the secondary filter is for the "sand". The micron (pore size) of the filters are different with secondary being finer/smaller. Using a single filter may cause premature plugging.

That makes sense! So used to the single filter on a gas four wheeler.