Hi. I have just finished rebuilding a 606. Have had problems over the years getting 'air' out of the fuel system.

I imagine most diesel Mercedes have similar small bore fuel tank outlet inner id.
This one proved a little more problematic than usual in the fuel department.

On the 606 the i/p pump is larger, pumps more volume. Until this pump, the earlier i/p's pulled fuel too but the 606 pump really sucks. I wasn't getting the engine to rev up like I knew it could so I checked fuel delivery .

Zero air leaks but small vapor bubbles at 4000 rpm . Checked and rechecked and no air leaks anywhere. This may have been covered but it appears that diesel will boil off vapor if sucked on hard. Ergo: Bubbles in the fuel lines and rough running engine. Bubbles will not collapse back into fuel again though.

I thought about a 12 vdc pusher pump near the tank but that introduced another 'weak link' plus is would be subject to unfiltered fuel and new owner wants to run 50/50 wvo to diesel.

I pulled the strainer out of the tank and found 1/2 inch of black sludge and 50% restricted tank strainer. Also the outlet fitting was very small...about 6 to 7 mm into the flex fuel hose. Couples with the 6mm flex hose restriction it seemed too small especially considering the long steel fuel line to the fuel pump.

So, I drilled out the tank strainer to just smaller than the threads for the outlet hose. Discarded the OE flex hose and found a 12mm silicone hose to marry the now 12mm outlet with a 1/2 barb fitting screwed into the tank strainer outlet.

Now have no bubbles now even at 5500 rpm and engine ramps up smartly to 5k..

So if you having reduced power in your 606, look to opening up the tank strainer.

I also squeezed the blanking ball in the i/p relief banjo in just a hair and found 8 psi blow off now, instead of 4 psi before, with a pressure fitting test. Should they test higher ?( FYI : Early W123 and 126 relief valves are smaller thread. So no super duper relief valves for 606's.)



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