I like the ideal of time /use/fatigue. I think the injection pump should retain 14 lbs pressure before the relief return valve opens.

My understanding is that if you tee in to the line from the lift pump you should see 14 lbs. What you are really seeing is that return valve when it is open. This may vary depending on the model of pump on 123s but I doubt it. Too simular in general design.

So a lift pump pressure test in itself is suspect if you get a low reading. I guess you should pinch off your return from the pump to see if you exceed the 14lbs to be safe. Some pumps are not designed to have a positive output restriction in place though. So the mentioned volume flow approach with time probably makes more initial sense. It is not absolutly conclusive though it just proves you are getting volume really.

Probably the best way to retension the spring is to insert an adapter into the input or output of the injection pump and read the operating pressure. A new spring from the pump manufacturer may be pre calibrated and pretty cheap as an alternative solution.

If so better than trying to adjust the calibration ourselves. It would not surprise me if that spring is changed out at every major pump service or the pressure retention calibration of the pump at least checked. I hope they do not miss it. A pump guy could quote on that or anyone with specific knowledge.

This kind of sounds like it should be a tune up item on a 123 diesel. If your spring has gotten weak with time and age it will not be the only one. I have to wonder what your pressure is now though. Plus it might be subjecting the lift pump to new dynamics that may not keep it happy long. Thats if you are overpressured. Not sure either why the car is harder starting. Theoretically it should be easier unless you are way over pressure perhaps. You in effect might be well advancing the timing if the elements are taking on more fuel than designed

I also feel that if the pressure is too low inside the pump some elements act like they are starving a little. Or to be correct just not taking on their design volume of fuel. I have noticed lots of examples of this on poor injector pump supply problems posted over time. Since you noticed an improvememnt perhaps that was the reason.

I had wondered for a long time why nobody ever really expressed or checked this item for proper function. It may be because the car still runs but not as good as it should and some other possible unknown is just blamed like lower compression. Or the injectors are old.

I have been kind of watching and hoping someday the reason for inferior milage on some 240ds in comparison to others was discovered. The same effect is seen on 300ds but not as bad a percentage spread. . Low retention pressure in the injection pump could in my opinion result in uneven injector element loading and feeding on some cylinders. This would cause an imbalanced firing at speed but likely not noticed by the driver. Its probably like trying to hear your watch tick while riding in a rotating cement mixers drum. Probably could reduce the general milage as well.

Remember some people have claimed it appeared marginal fuel filters hurt milage in their experience. This may have been the injection pump was not getting enough fuel to balance off the injector elements equally. That could also cause enough less fuel flowing that the injection pump would not reach the 14 lb operational pressure. The pump was consuming fuel faster than a designed operational pressure could build up or accumulate.

Originally I did not discount people that claimed a partially obstructed fuel filter hurt milage. Yet I could not figure out why this was occuring. In my opinion lesser performance levels because of lack of enough fuel should be equal to running lean. Or since it is a diesel and I have no flame suit handy underfuelled on elements. If instead it causes an unequal element loading in the pump though that would do it. This return pressure calibration might even be critical to economical operation.

Now it looks as though what you have dived into may be a factor even with a good fuel filter. I hope others will jump on this and express opinions and thoughts. Maybe we can get all our 240ds on an equal miles per gallon basis yet someday. I still feel it is a generally unknown common denominator that causes the milage spread that is greater than just trying to brush it off by stating it is driver related.

Generally when highway cruising the operator does not have enough influence to cause a 22 to say 30 mpg spread on different yet the same cars. For a long time I felt it may be a tired centrifical advance mechanisim on some cars but could find nothing concrete to indicate it.


Top speed on the 240s would have been non existant for example. Pump problems like element timing drifting off sequence drifted across my mind. . Though that did not really gell either as indirect old fashioned engines have great tollerance compared to sophisticated systems. This type pump if properly supplying fuel at idle is going to generally supply at higher rpms by it's general design. If you undersupply the elements at idle the problem should be worse at higher revs. That is kind of a new consideration of mine since you posted.

Cervan could you keep an open mind and notice if the milage on your car seems better or worse. I do not think it will be the same.

That would indicate this area should really be investigated. Even though I am longwinded I have been watching for anything that even had the potential of being the common denominator. This area might fill the bill. Really glad you posted on this. If there is anything positive that results what a cheap and easy fix it would be for many. If the springs are pre calibrated and cheap I will change mine out if milage is inferior when doing other jobs around the cars.

There has been constant indicators that the amount of pressure applied to the general fuel waiting to be loaded into the elements is important. I suspect it prevents forming of air bubbles at the time of element lloading. The 14 lbs pressure may be very critical. It may be one of the considerations involved in how much fuel is loaded in the element as well. I have not decided if the elements piston at the bottom of its stroke is still filled with fuel left from the last compression stroke. Since fluid in neither compressable or mallable by vacuum. Does a vaccum induced bubble present itself when the piston decends to the bootom where a slot allows fuel to enter as in a two stroke engine?

Cervan if you have more residual pressure in the general fuel supply now you might be intaking a little more fuel into the element. Thats especially if you are now higher than the 14lbs design. The notacable increased torque is a slight change in the effective timing if the idle sounds about the same as it was in my opinion. If the residual pressure was say just 8 lbs and yoiur stretching the spring has raised it to about fourteen pounds then perhaps the increase torque is because all elements are now able to get a balanced loading. The new milage you get will tell one way or another I think.