I have a 1968 220D, w115 with the pneumatic M injection pump. I just replaced all my fuel lines as they were tired/cracked. I haven't had the car long, so this is only my second time installing the clear, plastic inline fuel filter (some call it the prefilter). You can tell I am not very handy, because I am having the darndest time not getting an air bubble in the filter. I cannot get the bubble out using the primer pump as it gets trapped in the plastic and doesn't fully leave. I tried prefilling the filter with fuel, but by the time I get the hose onto it, a little fuel always leaks out.

I used the old fuel lines as a guide for my new ones, so there is a long 180 degree bend before this filter, and only about 2 inches of line downstream from the filter. This results in the filter being horizontal. Would it somehow be better if the filter was oriented vertically?

The car still runs with the air bubble, but I worry that it will get sucked into the rest of the fuel system. A few other threads say to not worry about it. But I guess I still worry about it.

Thanks for your advice.

Dont worry, be happy !! The bubble will not cause you a problem. Most diesels have such a bubble & run fine.

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Grumpy Old Diesel Owners Club group

I no longer question authority, I annoy authority. More effect, less effort....

1967 230-6 auto parts car. rust bucket.
1980 300D now parts car 800k miles
1984 300D 500k miles
1987 250td 160k miles English import
2001 jeep turbo diesel 130k miles
1998 jeep tdi ~ followed me home. Needs a turbo.
1968 Ford F750 truck. 6-354 diesel conversion.
Other toys ~J.D.,Cat & GM ~ mainly earth moving

x2. As far as I know, this is normal.

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-1985 300TD - 324k mi on chassis. MB "Tauschaggregat" 617 motor + 4-speed conversion
Alaska Roadtrip 2009
-2002 E320 Wagon - 197k mi
-1998 E320 Wagon - 310k mi - retired

Mine also has a Bubble.

Even if you managed to get the Bubble out of the Filter the Bubble is still going to go to the Highest point and be somewhere in the Fuel Inlet line.

What you don't want to see is Bubbles passing through the Filter.

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84 300D, 82 Volvo 244Gl Diesel

Thanks guys for the reassurances. I have a loping/hunting idle and am trying to diagnose the cause(s). I will cross this off the list, and move onto the next candidate.

On the later engines a loping can be caused by the rack damper bolt being weak/not adjusted properly. Does that setup have anything similar?

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-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Unfortunately, no, the pneumatically governed IPs with the diaphragm do not have a rack damper bolt. I think that is what differentiated my old IP from the newer "mechanically governed" IPs.

It does, at idle and low speeds the poppet cam prevents a loping idle. It is at position 15b at idle and moves out of the way to position 15a when the accelerator pedal is pressed.