What exactly is it? To me it sounds like a blowoff valve. Im not new to building cars but im just breaking into the diesel world so this is somewhat new to me

It's in the injection pump and has to do with fuel flow and filling the delivery valves.

A blow off valve vents intake manifold pressure on a forced induction car when the throttle plate shuts and there is a bunch of pressure in the intake and the charge piping. Diesels do not have throttle plates so generally they do not have, nor need a blow off valve.

I've seen incredibly high boost, highly modified diesels run blow off valves, like Duramaxes on 60+psi or some Merc diesels on 45psi. It may reach a point that there is no much pressure in the whole system they need to vent it. But in general you will never see a diesel with a BOV, and nothing we mess with on this forum would have come from Mercedes with one.

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Everything I read that was talking about the overflow spring turbos were brought up that's why I thought it might be associated.

I have been interested in gas cars my whole life but now that im in school t be a diesel tech I have realized diesel is the was to go

Is it possible that OP is referring to the spring in the fuel overflow valve? These springs get weak and need to be stretched to maintain adequate fuel pressure. Alternately performance springs are available. In mine the fuel pressure was down to 8 psi or so and after stretching the spring to spec, I now see teens to twenties on fuel pressure.

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The overflow valve is essentially the fuel pressure regulator on a mechanical injection pump. When the fuel pressure builds to a point, the valve opens slightly to release any excess pressure. In operation, it's usually pulsing open all the time because the fuel pump is mechanical and outputs fuel in "spurts".

I suspect the term "overflow valve" is a translation thing from German.

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250

Getting into MB Diesels, here is a good long thread talking about that Spring
and fuel pressure adjustment.

Fuel pressure relief valve adjustment


Charlie


sent from my pos computer

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The U.S. engineering term would be "back-pressure regulator". A very similar device is a "pressure relief valve". The difference is that the regulator ideally operates at a steady in-between position, whereas a relief valve is designed to hold open once it pops. As KarTek says, the "lift pump" outputs spurts so the valve probably doesn't sit at a steady position.

Some here have claimed a big improvement from upgrading the "overflow" spring, whereas others said no difference. As long the inlet pressure to the IP is high enough (factory spec ~12 psig), you should get enough inflow rate to completely fill the cylinders. You find the spring (and related parts) inside the banjo bolt fitting on the IP inlet.

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No difference should be investigated. There are a few reasons something else wrong can impact the spring stretch to increase base pressure. Weak lift pump, obstructed filters are a couple.

You cannot increase base pressure in an injection pump unless it is available to increase. Resistance to buying a 10.00 liquid dampened 0-30 pound gauge is common. Yet Mercedes specifies this as a periodic service item for these cars.