My 79 300TD has a bad Injection Pump seal somewhere and I was wondering whether there where any easier/common problems I had not yet thought of. Im sure someone reading this is already starting to type LIFT PUMP but...
After cranking and cranking to get fuel up to the injectors I can get it to fire. It runs like a dog for a while, wont idle well, then starts to smooth out. I can see a large amount of air running through the Injection pump to Main filter return connection at High RPM. It seems to "cloud" the fuel with air. My first thought was the inline filter or lift pump so I installed clear vinyl hose between Inline filter, lift pump, lift pump to main filter, and main filter to injection pump and ran it again. The thing is i don't see ANY air entering before the IP/after the LP, SO... is my IP shot?? Could the gasket between the Lift Pump and Injection Pump be letting air into the Injection pump? Could the Injection pump to Engine gasket be letting air into the IP? Is there anywhere else I could look? What i really dont understand is: if the air is not entering BEFORE the Lift Pump(suction side), wouldn't I have an obvious FUEL LEAK (on pressure side) rather than air coming in? All the pressure of the lift pump pushing the fuel out? OR am i mistaken and the Injection Pump also has suction?

Air can only enter the system where it is under vacuum, or before the lift pump. Any leaks past the lift pump will just leak fuel. Do you have the old (white) style primer pump? I would try replacing that first.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine

The fuel tank on the TD's is much lower than the tanks on the other models so they are more susceptible to air leak problems. Start by rigging up a separate fuel supply from a soda bottle in the engine compartment which is higher than the lift pump. If it runs ok on that (feeding right into the primary filter) then you know the problem is upstream, back towards the tank.

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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

Replace the hand pump, it's leaking.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

I still don't understand why I don't see the air coming thru the clear lines before or after the lift pump...??? But at least Im not being told my IP is junk... So I don't think Ill REPLACE the lift pump but I COULD order the REBUILD KIT from GERMANY...Still confused on the science.. One thing I was wondering...Suppose my IP was full of air, how long should it really take to bleed this thing out with the following method: Crank and crank till fuel is at opened injector connections, fire it up, rev it up and crack the injectors back open, one at a time and let the fuel/air mix out. Cause Ive been up to that a while and the air just keeps coming. (Awhile=3+ minutes at each injector). Thanks all.

Oh, and the Lift Pump has the original body, but the updated hand pump.

if you deside you need a fuel pump or the injection pump
I know where to get some fairly cheap used ones

My td had the same problem. I solved the problem by cleaning the tank screen. Search my post on a cheap td screen tool.

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a few mb diesels

Try the separate fuel source. It would eliminate all the line from the primary filter back to the tank and would confirm whether Bruce's solution might be yours.

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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

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Hello Everyone

I have had the old rubber fuel lines near the tank let in air.
The cracks were just small enough that they did not leak fuel while sitting,
but when under vacuum from the ip they leaked air into the system.

I have had a system also that had the bypass fuel line looped back into the intake fuel line.
When a system setup like this gets air into it, the air continues to circle and circle.
I reconnected the bypass line back to the tank only, and poof the problem goes away, no more air.
It would be possible to purge the air by stopping and restarting the flow thru the looped pipe, while bleeding the air off, but it would take a long time.

Hope this helps
Have Fun
RichC

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