The lift pump/fuel feed pressure going to the injection pump is regulated by the relief valve on the IP.
RE: changing the spring in the lift pump.

Note:
Many lift pump springs are getting old.
Twenty + years of faithful service from a spring is nothing to complain about...

Isn't the only function of the spring in the lift pump to return the plunger to its rest position??? Seems to me, the real advantage would be putting a heavier spring in the relief valve. However, IIRC, the relief valve is a sealed assembly...No???...Robert

Agreed...but then again, no benefit in fuel pressure...Robert

>>Isn't the only function of the spring in the lift pump to return the plunger to its rest position???

At the great risk of upsetting FI

No, it's actually the other way round. The plunger / rod is what returns the piston to its rest position, and it is the lift pump spring which actually pumps the fuel.

The fuel pressure in this circuit is governed by whichever of the lift pump spring operating on the lift pump piston or IP relief valve spring acting upon the IP relief valve produces the lower pressure.

In standard form, without any tweaking of springs or messing about, the IP relief valve is the lower pressure, and so, yes, it should be the IP relief valve which governs the pressure.

Thanks for the clarification. I didn't know that!!!!!

Some relief valves are sealed and others not. If pressure delivery from the lift pump is found adaquate yet the internal pump pressure is low. (the safe squeezing off of the return line test) Either an obstructed filter or something else could be the cause. A tired or broken relief valve spring possibility enters the lineup. Only after the other items are checked though.

All we want to do is make sure the base of the injection pump has a reasonable pressure at highway cruise especially. You may have a reasonable pressure at idle for example or even under static test to 3000 rpm. Restrictions or the lift pump unable to continue proper pressure when engine is under demand load is not good in my mind.

The cycle of the lift pump I believe just loads the internal spring to apply almost constant fuel pressure. That is until the next recharge of the spring occures after all the elements have been loaded once. . A weak lift pump spring I imagine would have some kind of effect. Perhaps lower initial pressure or pressure dropping faster than originally intended. Or even a combination of the two. Members may soon cross reference that lift pump spring if not a cheap part available from mercedes.

Most age deterioration of the lift pump probably occurs from tired valves. What is nice is that lift pump kit without the pump spring is only ten dollars. At the age and milage of these vehicles it just might be good maintenance to check the lift pump out to see if it has deteriorated. In my opinion a lot of of sub standard situations exist out there. Low injection pump base pressure does impact several things in my opinion.

It has always been an interest of mine why some examples seem to run better than others. I see no downside at all to a permanent pressure gauge installed for this area. It should provide some advanced warning pressure is dropping off well before car becomes troublesome. Usually will just be a filter getting plugged up and reducing flow. This is overall a very manageable area for the vast majority of site users.

Specifically to split hairs. The fuel filter system presents a variable restriction to the fuel fed from the lift pump. The relief valve spring regulates the base pressure present in the injection pump.

Of course to preform its function a reasonable pressure supply must always be present after the filters. That supply pressure because of the varience in flow will not be constant. As I said kind of splitting hairs yet indicates why static tests are not conclusive. Flow rate is low so pressure may be high.

Another way to look at this. If the relief valve was moved to the exit of the lift pump. The base pressure in the injection pump would decline as flow increased. The percentage of decline would depend on the restriction present in the fuel filters.

Interesting...so around 20 psi under load is what we would like to see between the lift pump and the IP????

the safe way to run fuel pressure into the passenger compartment is with a gauge isolator, a very expensive acessory available at gage supplies in towns with chemical plants and refineries. Another safe way is put an electrical transducer in the engine compartment and an electric gage in the passenger compartment. I have found no cheap way to do this in cars.
After eliminating filter (used 1 quart primary filter) as source of my starving on my 240D, pulled the fuel tank. It had 1/8" of metal rocks and gunk at bottom of tank, and 1980 240d drains straight from the bottom of tank with no strainer. No previous WVO on my car. Scraped them out with a scraper on a 2 foot handle through gage hole. Blew out line with an air hose, too. I doubt if your previous owners fuel supplier was anymore careful than here-My PO lived in the fancy pants east end of Louisville.

__________________
'80 240D biege "Mercey Bird"
'80 240D light blue "Slug"
'80 300SD dark blue "Theodin"

Do like your MBZ professionals do at the dealer and duct tape the gauge to the outside of the windscreen...Robert

About 19 pounds in the base of the injection pump would be ideal. With a system in good enough condition to almost maintain this at cruising speed.

At speed the pressure between the lift pump and filters may be higher. Actually it would have to be as the filters are obstructive to some degree.

Now reverse what he told you and it will be right.

I cannot ever recall checking lift pump pressure because we never had an issue that wasn't corrected by IP timing, camshaft timing, boost pressure, filters, etc. This is a great resource.

Now then, does anyone make a higher pressure relief valve assembly????

The procedure is to stretch the spring to 27mm.

No other method is available???