Stretching the return valve spring only increases the base pressure if the system is capable of delivering enough pressure to overcome it. Both before the stretch and after. Knowing the actual pressure is important especially for general maintenance items like fuel filter changes.

Leaving a fuel filter in service till a problem is noticable may cause damage plus your fuel milage will be less for quite a period before if the blockage of the filter is slow to occur.

Anyone installing a gauge should consider it as a permanent addition as it aids troubleshooting if the need arises with the fuel system or at least gives warning of impending problems.

I am still not sure if the contributing cause of the number one rod failure is low fuel pressure in the base of the injection pump on the 616 engines. Yet there is some evidence. Installing a ten dollar fluid filled gauge seems a pretty sound and possibly cheap alternative. Although it may not be conclusive as I suspect the rod problem occurs from years of running at substandard pressure.

A gauge and the right pressure may just deffer the failure if too much wear has already occured. In my mind too many things are possible with low pressure. Slower starting, smoothness of idle, power, milage, maybe the number one rod from the first cylinder taking on too much of the load with unequal element loading? Anyway the engine will be better at 19 pounds operational pressure than lower. Some have virtually none or very little and still run.

I suspect the poster took apart the sealed relief valve type when he mentions restaking it. If so he may be the first known case to do this. There are two types of relief valves. One sealed and the other serviceable.

It was the serviceable one. Bad choice of words on the staking it. Reseated the check ball is what I did. I used rubber jaw inserts in my vice as well so I didn't damage the face or threads on the check valve / banjo bolt.

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Experience : what you receive 3 seconds after you really needed it !!



86 300SDL 387,000? Motor committed suicide
81 300SD 214,000 "new" 132,000 motor
83 300SD 212,000 parts car
83 300SD 147,000

91 F700 5.9 cummins 5spd eaton 298,000
66 AMC rambler American 2dr auto 108,000
95 Chevy 3/4 ton auto 160,000
03 Toyota 4runner 180,000 wifes

Barry could you give us info on how and where to install a gauge?

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

Nope. Just have a good vibration at lowest idle in gear, especially with ac on.

Valves ok, mounts ok, injectors???

Putting speed up 1/4 gives super smooth idle in neutral, going halfway is good only except when in drive with ac on. Turning the knob all the way makes it fine. Without a tach it is tough to know if perhaps the base idle is just too slow.

The car only has 73k or so, and is very peppy for it's output...

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Current Diesels:
1981 240D (73K)
1982 300CD (169k)
1985 190D (169k)
1991 350SD (113k)
1991 350SD (206k)
1991 300D (228k)
1993 300SD (291k)
1993 300D 2.5T (338k)
1996 Dodge Ram CTD (442k)
1996 Dodge Ram CTD (265k)

Past Diesels:
1983 300D (228K)
1985 300D (233K)

The gauge supply point has to be somewhere between the secondary filter output and before the output of the injection pumps relief valve. There has not been much discussion about the best way to accomplish this to my knowledge. Someone may develop the best way yet.

At the current time I would buy a new short plastic fuel line that goes between the secondary filters output and the injection pump. Just because the new hose is more plyable and should be cheap. One could cut into the original line but it may be too stiff as plastic hardens over time.

Install a barbed tee in it and feed the output to the gauge. Make sure the gauge is a thirty pound scale fluid dampened type. Around ten dollars or less at harbour freight and simular outlets. It will not be the first chinese item most members buy anyways.

It is in reality so simple It amazes me more have not done it. I intend to route the output hose to the cabin interior if I get around to it. The location of the gauge troubles me so I might just install it in the glove compartment. Others will develop better ideals.

This way you can monitor the pump pressure at full load on the highway. Or if you pick up a load of really dirty fuel from a station the pressure will sag as the dirt obstructs the filter. Can obstruct the tank, primary or secondary filter. If any the pressure will sag. Also tells you when you need a new filter as the pressure will slowly sag down with time. This alone more than pays for the effort in my opinion.

If for example initially I had nineteen pounds with a known good filter and it sags down to ten pounds over time I would change the filter. The car is still running well but operating at lower than ten pounds is probably false economy. The filter acts as a choke between the lift pump and injection pump base when dirty lowering the pressure in the injection pump base. Less pressure from a dirty filter lowers fuel milage on these cars is proven by people quoting on site that they changed their fuel filter and observed that fuel milage increased.

By simply closing off the return line the condition of the lift pump can be checked once a year as well for example. With reasonable filters the gauge should spike to about thirty pounds. Thats if the lift pump is still in good condition.

Always remember you always want more pressure than that required to keep the relief valve open. This cools the injection pump by flowing the fuel back to the tank taking the residual core heat in the injection pump away.

The original pump manufacturer was well aware of these issues but said little. I assume that under warranty the filters were routinely changed by milage and the aging of components was not a factor during that short warranty period. Yet mercedes still hinted they wanted that pressure checked at dealerships.

Again until proven otherwise I almost see that gauge setup as perhaps critical to the engines well being over the long term. Especially the 616 engines. More important perhaps even than that is the engine will just run better and have more power with good pressure. This has more or less been absolutly proven.

I also see it as a major learning tool in relation to the fuel supply circuit on these cars for many. The more you understand it the less intiminating it is to deal with. Plus the gauge is an excellent diagnostic tool.

It is my belief that as people start to install these and post the results on site at some point they will become a common modification to these cars.

Hi Barry,

Thanks for the detailed response!.....any chance you could post a digital pic of your guage setup?

Thanks in advance!

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1987 560SL - 95K
1996 E300D - 183K
1997 E300D - 162K
1982 240D - 160K

No I cannot produce a picture as I have not had the time to install it. The gauge presently sits on the shelf. I did have a good look some time ago and the method I mentioned is what I will do if I get the time. Or I start driving the 240d.

Actually it is probably the simplest as well. I might get more motivated to do it as soon as somebody finds a good location for the gauge in the cabin as well.

For the time being I do not have time to drive the old mercedes or work on them. Hopefully that will change soon. The fundemental installation is about as straight forward and easy to acomplish as anything can be I think. That is the inherent beauty of it.

Berry, Thank you very much for the great run down.

Several years ago on my first MB I swapped out that clear hose with some from Ace, there was allot of discussion at the time but its still doing fine, I'll use that again. Thanks, I also am planing on changing fuel filter more often

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"

From time to time I try to stir the pot a little. So thanks for your quote.. If it results in people at least considering something is the best we can hope for.

My ideal is to only change the filter when the need is indicated. Waiting till there are performance issues is far too long. Without a gauge in the circuit you will never get the filter changes right. Too much depends on the variable quantity of junk in the fuel where we buy it.

I would very much like to spend the time to prove some of my more serious contentions one way or another. I have a few ideals reserved to look at when and if I get the time. To me they represent puzzles to solve.

changing fuel filters on a schedule like lub oil, never made sense to me, clean fuel= many miles of motoring, a load of nasty fuel may mean changing filters a couple times before you get home. A gauge to let you know the condition of your filters seems like a no brainer. Many years ago a tug I decked on had a gauge in the wheel house that measured the vacuum between the filters and the engine, thus letting you know when the filters needed changing. I have worked on many boats since then and have never seen another such gauge, its a mystery, why not. Anyway thanks for the info. I'll be interested in how the mounting of the gauge in the cabin works out

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1985 Euro 240D 5 spd 140K
1979 240D 5 spd, 40K on engine rebuild
1994 Dodge/Cummins, 5 spd, 121K
1964 Allice Chalmers D15 tractor
2014 Kubota L3800 tractor
1964 VW bug

"Lifes too short to drive a boring car"