When you see no change after the spring change. If there also is no overflow from the relief valve. Or none existing before the spring change even.

It is probable there is not enough fuel pressure being applied to the base of the injection pump to start with. Possibilities are tired lift pump, Dirty tank filter or even the fuel filters up front.

Constant overflow to me is an indication that the pressure regulation change or the original configuration is regulating the pressure. Plus the applied pressure does exceed what is needed for the relief valve to function properly.

If you use a fluid dampened gauge. Put a close off valve in the line to it. Those strong pulses the injection pump elements generate can and probably will destroy any typical gauge. So you want to sample the pressure. Not monitor it over a period of time constantly. The harbor freight 0-60 pound cheap liquid filled gauge is more than adequate. It was around ten dollars for a very long time and may still be in America.

One thing not discussed at all so far is a pressure reading at speed. Under loaded conditions. To me this is also important. So I guess the best ideal is a long hose to get the gauge back into viewing range at speed.

Too many people think they do not have enough boost. Or just do not think it is safe to pass. When the reality is the engine is fuel starved for all practical purposes. Really degrading the potential performance.

My suspicion is when the operational pressure drops too low at speed. The engine is out of good power balance as well. Simply because a decent pressure makes the engine a little quieter at speed. This has been reported far too many times to ignore.

I also feel like a broken record. Until I can or someone else can come up with an alternative reasonable answer. Low fuel supply pressure on the 616 may be the cause of the number one rod bearing failure.

In fact if I did buy another 616 engine and it had very low fuel pressure. The engine is noisy enough to cover a wearing rod bearing. I would plasti gauge that bearing. Easy enough to change bearing shells on the first cylinder rod. Before the crankshaft gets scored up.

I suspected that damage may be created over a very long time of running low fuel pressure. Readers probably are aware that the first rod bearing is usually the one to fail. Especially on the 616 engines. Less so on the 617 engines. Yet still happens more than not. To me it trumps having to change an engine out down the road.

Also the 616 lift pump can be upgraded to a higher output pressure just by changing the lift pumps regulation spring from a 617 turbo engine. Leaving the lift pump as original makes the fuel filters restricting effect greater as it ages. Plus it will help to keep that higher pressure relief valve functioning better.

In manufacturing the 616 engines lift pump. It was designed to have a lower original output pressure. Same as the naturally aspired 5 cylinder I think.

Before I entered the picture it was assumed the first rod bearing failed because it was the last bearing on the crankshaft to see oil pressure.

At the time I was asked why they failed. After a lot of thought. I felt the old answer just did not hold water. Both engines have about the same oil pressure. The number one bearing on the 5 cylinder is even farther away.

Plus the loading of the cylinder is greater on the turbo five cylinders. So technically in my mind it should fail more than the four cylinder 616 engines experience.

Make no misteak though at the same time if a rod bearing fails even on the 617. Chances are very high it will still be the number one or two rod bearing on them.

When most of us acquired these cars they were old already. So I also considered the possibility of sludge build up in the oil passages from the lack of oil changes. Then again if this were the case. The oil has to travel farther to get to the number one rod bearing on the 5 cylinder. Leaving me with the only other difference being there are more power strokes per revolution on the 5 cylinder than the 4 cylinder . At the same time the five cylinder turbo power strokes are much stronger.

To remove sludge an old engine. I thought that a quart of miracle mystery oil in the base. Say every second oil change for awhile. Would slowly reduce it. It seems a bad ideal to get it all loose faster. That can block oil passages you are trying to clean. With resulting destructive results. Although people switching over to synthetic oils have not reported the problem either. It does have a cleaning action as well.

There also where some other considerations. Operational RPMs are higher on the 616 on the highway for example. So my only certain conclusion was it is better to have good operational fuel pressure than not. With one perhaps still remote possibility. That otherwise it might also shorten the engines life substantially by not having it.

I designed a test to perhaps prove the concept. It is an involved test and could cause me pain in also indicating something else to investigate. So I just made sure I had good operational fuel pressure on my cars.

So I leave this post with if a spring is changed and there is no difference. Either you had high operational pressure before. Far more likely though is you have another problem present in the same system as well.

This area in general is about the cheapest area to restore. Yet there has been massive resistance to even checking it out properly. In the majority of cases it is also great preventative maintenance to reduce episodes of quitting on the road.

Things that are becoming sub standard in the fuel supply system generate gradually lower and lower than desired fuel pressure readings first. Usually the fuel supply system just does not suddenly pack it in from being a good normal one on these cars

Plus as a fuel filter builds resistance to flow. The pressure differential across it increases because of the lack of changing it. This could force more dirt through it than intended by the filter designers. I could write a manual perhaps on what preventative maintenance should be done. To stop all too many cases of premature engine failures.

I am very lucky in that all four of my 616 and 617 engines are in very good condition overall. One original engine in a car with a perhaps honest 165 thousand miles and another being a total drop in by a Mercedes dealer not long before I aquired it. Years ago now. Both those cars indicate to me just what they were like when pretty new performance wise.