I just changed out all O-rings to viton, incl. del. valves, as well as a couple lines. I have air bubbles in the first line from the pre-heater/thermostat to the pre-filter. I also cut back the black rubber hose coming from the ridgid metal line going to the pre-heater. I did this to get a fresh area of rubber to clamp, before it was loose.
I believe that I'm getting the air bubbles from back further, maybe at the tank? Is this possible? I plan on changing the tank strainer soon anyway.
Also note ;Only bubbles in ,no fuel out now since the O-ring changes.

What year 606 are you driving? How many o-rings did you change out at the fuel heater. I've never done mine, but I thought someone mentioned there being more than one there.

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'95 E300D ("Tank") - 231,000 miles
'79 240D ("Biscuit") - 197,250 miles (Sold)
'83 240D ("Ding-Ding") - 217,000 miles (Death by deer)
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"Back off, man. I’m a scientist” ~ Peter Venkman

I would suggest changing the hose that you cut some off of. I had to replace mine on my 97 E300 to get rid of the air in my fuel system. also on the clear plastic lines sometimes changing the orings is not enough; if they are old enough they can suck air from between the clear part and the black connector.

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Keith Schuster
2006 E350 98K miles
2013 Ford Explorer 15K miles

I spent $700 at my former Indy's to find an air leak on my 98 E300. I took it home to try myself when they failed. Turned out they had STUFFED a wrong size O ring on the pre filter. BE SURE you use the MB O ring for this. It is a weird size and if it doesn't fit right will suck air. I found out the hard way.

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Muleears
'07 E320 Bluetec 133K my DD
'04 Jaguar XJ8 VDP, 34K
'10 Hyundai Accent 60K Grocery Getter
'02 VW Golf soon to be on the road again
'97 E300 Diesel Son's DD
'61 VERY tolerant wife

Hampton Roads, VA USA

Gone but not forgotten:
'67 250S 95K
'86 300SDL
'87 300D Turbo, 364K! R.I.P.
'98 E300 Turbodiesel, 213K
'02 S420, 164K
'01 Prius 138K

Is there any fuel leak ?
I would say check around ur IP c if u can find any fuel leak, I have the same problem but I don't c too much air in the lines since I change the tank Strainer, i found some fuel leaking around the fuel heater so I'm changing the o ring n the gasket.

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1999 W210 E300 Turbodiesel

I actually bought the gasket for the Pre-Heater, but that gasket seals the coolant at the mating surface between the PH and the head. That is from what I'm told, so that gasket will not solve any problem in reguard to air bubbles. I will be changing the black rubber hose between the hard metal line from the tank and the PH. Since the bubbles can be seen in that first clear plastic line (new) from the PH to the pre filter, air simply has to be entering before that point?