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

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.

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

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.

I have real concerns about elevation of the relief valve pressure beyond about the twenty pounds you mentioned. We just do not know quite enough about this area yet and possible effects on the system. It may even be really benificial to go higher. There is probably a safe slight overpressure possible.

Some of my fears are overfuelling of one end of the element chain.Or the drip timing no longer corresponding to the actual timing. Plus getting off the injection pump fuel supply profile it was calibrated to. This all may be groundless or some of it.

Anyways for the curious mind. Establish the actual maximum delivery pressure to the base of the injection pump by clamping off the return line. If it is well above the relief valve setting drive the car around the block. If the sensation is enjoyable and engine does not sound like it is over advanced at idle before you leave we could look a little further into this.

The design of the lift pump is such it cannot be damaged by restriction of the return line. It will be pretty easy for example to determine if any type of fueling imballance is occuring at really elevated pressures.

I still feel this may have to be looked at before anything of a semi permanent nature is done. Again it is pretty simple to do using the milli volt method. Simple before and after comparisons of the cylinders running temperatures. We want the same cylinder temperature profile from one end of the engine to the other. The same way as it was before elevation of the presure. We deffinatly do not want to hole a piston. Dare I say increase stress on a given rod bearing either.

The other approach would be to check and make sure the calibration is equal on a test stand at say thirty pounds base pressure. If it is not then recalibrate the injection pump to the new curve that is present. Then I suspect we will have to work out a way to time the pump to the engine as the drip method will be off. The pulse thing might do well enough.

One last consideration. With the cigar hose eliminated from the setup if the return line is clamped off. This may not be quite the same as if the relief valve was operating at a higher pressure. Anytime I seriously consider the base of the injection pump those intense spikes flare up in my mind.

When the relief valve is normally open there is a chance the cigar hose deals with them somewhat. I am well aware that some respected members think the cigar hose is just to supress noise. I have never been sure of that. One gentleman I recommended to reinstall the missing hose. His problem at the time dissapeared when he did. There was some discussion it missing could not possibly effect what it did. I also know the european versions do not have them.

No other method is available???

What else is needed? Its a ball and spring check valve, it can't get much more simple.

I suppose you could build a computer that controls a solenoid to regulate pressure.

How about a replacement spring, as opposed to stretching the old one????

As I recall, this is a sealed assembly???

Why? Unless its actually broken, elongating the spring is good enough for Mercedes to put it right in the Factory Service Manual.

Very few are sealed. I have yet to see one myself. The few dozen I've pulled from 1975-1985 240D/300D/CD/TD/SD models have all been serviceable.
Its the same as with which cars got a KKK turbo, there is no pattern.

Interesting!!!!!

My 617.950 IP relief valve is a sealed assembly. I suppose obtaining one from Pik-N-Pull that is serviceable would be easy enough.

Why not shim the spring????

Again, why bother? It takes all of 30 seconds to stretch the spring and measure it.

I've never been an advocate for stretching springs...Would you stretch a worn tensioner or valve spring????

Seems like adding a shim, based on pressure would be even easier. You could start by using a shim that would give you the correct OAL and then go from there...Robert

Because its what the German engineers that designed the engine and had an exponentially higher R&D budget than myself decided was the "proper" way to fix the low fuel pressure problem.

A- Remove spring, pull it to 27mm, reinstall. Done.
B- Remove spring, make a shim to fit in the hole, reinstall spring, check pressure, remove spring, replace/add/alter shim, repeat until done.

Nope. Doesn't seem easier to me.

Hmmmmm...keep in mind that when you are performing a front end alignment under warranty, there is a 'correct' setting and a 'warranty' setting that is very wide....Robert

P.S. keep in mind those same engineers with unlimited R&D budgets won't hesitate to save a dollar or a man hour, even if quality is slightly compromised...