The truck is a Mitsubishi fuso fe-sd tow truck. it has a 4-cylinder diesel engine. i know its not a Mercedes but you guys seem to be pretty knowledgeable about diesels.

The truck will crank and idle just fine but when you get going down the road it seems to lose all power and die.

The truck has a fuel filter on the driver (left) side towards the back of the engine. there is a bleeder bolt on top of this filter. If we open this bleeder valve and let air into the system the truck seems to run fine. what could be causing this? maybe a clogged filter somewhere in the fuel tank?

My 99 Fuso FG has a separate water separating filter right near the fuel tank. I'm guessing yours has the same. Check it.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Yes it does have the water separation filter.

Is that filter letting air in?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I opened the drain on the bottom of that filter and a rusty looking mixture of fuel and water poured out. How can I tell if its letting air in? Is it supposed to let air in?

No it's not supposed to let air in. But if you're bleeding air out of the secondary filter, air is getting in somewhere, either thru a leaky fuel line, bad connection or something wrong with that primary filter. You could rig up a small tank and feed it directly to the secondary filter and see if the problem goes away. If it does, it proves you have a problem with the fuel supply between the tank and the secondary filter.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Okay, we took the water separation filter off and cleaned it up. Put back together and the truck runs great now.

But there's a problem with it shifting into overdrive. If we take out the a/t tranny fuse (f23) it will shift into overdrive but will not downshift without pulling the gear shifter down manually. With the fuse in place it downshifts fine but won't go into overdrive or 4th gear.

Mine's a standard so I can't help with that. Try the Mitsubishi Heavy Truck forum on Expedition Portal. Lots of knowledgeable Fuso people on there, many from Australia.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

Is it possible whey you turned the Bleed Screw on the Filter up front you were hearing a Vacuum being relieved; or did some Fuel spit out showing that there was pressure inside?

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84 300D, 82 Volvo 244Gl Diesel