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  #16  
Old 12-16-2019, 04:41 PM
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In my opinion that is not worth that much. Your test indicates the valves in the lift pump are okay. It could indicate as well that your relief valve is not very good. You could be back sucking air in through it as you pump. If it is either too leaky or just very weak. The more you learn about that system the easier it is to deal with. That is why I have always recommended checking it.

As an example. If normal pressure is present from the lift pump. As soon as you start the engine the relief valve opens quickly. Dumping any pressure above the set point. At some time in the cars history the deteriorated fuel pressure can be pretty close to the threshold of it. So the ball bearing spends a lot of time moving off the seat and back on. Wear can occur that is not detectable to the human eye but allows leakage. One open and closure per two engine cycles is possible to me. This was never the design intent. In that condition on the road in high load situations. Even just highway driving your supply pressure can drop off.

We get all kinds of start issues at the first part of winter reported. The engine will not start until there is adequate fuel injected for the cold conditions. If I was to use a 123 round the year in my climate.

I would try to make the system as tight as it was new. To promote quicker cold starts. Ideally if it would retain some fuel pressure in the off condition. Like a good tight vacuum system does.I suspected they might have been tight when new.

As I have a new Mercedes drop in engine in one of mine With a supplied injection pump and all accessories. If that engine turns over it is started. At room temperatures it does not need a glow plug cycle at all. It just lights off almost instantly to the starter rolling over.

A cold engine needs a full blast injection far more than a warm one does. I also suspect a leaky old never maintained system. May fall down too far under load.

There are subjective tests of these fuel systems. Quick starting and good acceleration when passing is a yardstick I use. For example my 1984 turbo model lights off very easily and is really strong when passing on the highway as needed.

So I suspect it is getting quick fuel on starting and enough fuel at really high load situations. It would not be as good with a poor primary fuel supply system. It has 169K miles indicated on it that are possibily authentic. You never really know in most acquisitions for certain.

There is no absolute proof yet but I suspect your relief valve is too leaky. When the primer pump was retracting it may have been sucking fuel or air back into the injection pump. Through the relief valve. Possibly just the air your pumping down stroke was putting through it. When you depressed it was pushing fuel and air back through it or a percentage of it.

Now a lift pump in operation can overcome this. Because it has two check valves. One on intake and one on the output of the pump. It cannot generate reverse or line suction pressure on the output. The lift pump can. I would remove and test the relief valve. If it tests less than solid it needs replaced. Test a used one before installing it. Hook a line to the output of it and apply some air pressure with it submerged. There should be no bubbles. When you test yours I expect there will be bubbles. Or the spring inside is broken.

Look at the upside I am not going to describe what was probably occurring when the return line was closed. It is too close to Christmas and I have to have a little mercy. Plus I could be wrong.


Last edited by barry12345; 12-17-2019 at 11:26 AM.
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  #17  
Old 12-16-2019, 05:06 PM
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Well much cranking and sweating and swearing later, I got it going.

Fwiw, the slight advanced timing has actually improved my idle a touch. And there does seem to be a hair more power pulling hills, but what the hell can I tell with the seat of my pants being what they are. It does sound like it is idling slightly faster, however.

That was a real drag, I wish I understood the issue.

Barry, the check valve bearing is actually something I will be replacing this week.
Mine is fairly scored, which may have contributed to this situation.

Thanks all for the encouragement. Don't think I'll be doing pump timing again for a while.
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  #18  
Old 12-16-2019, 05:43 PM
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Glad you got it running.

I expected it could run. Simplifying to the extreme. What you did when clamping and releasing the relief valve line. Was just a substitute for one of the relief valves functions. You stopped or reduced the back suction through it. That should not exist.
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  #19  
Old 12-16-2019, 10:47 PM
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It should not take much cranking to bleed air out of the injector hard lines. First you work the primer pump to fill the IP till you feel resistance in the plunger. Next crack open one injector line nut ((NOT all 4) 1/4 turn. Wrap a rag around it to catch spitting fuel. Activate the glow plugs for 30 seconds then start cranking. Air + fuel will bleed out of the open injector line and it should start within a few seconds of cranking. The idle will be a little rough because of the open line. Have a 17 mm wrench ready to tighten the nut - idle will smooth out. Go for a drive to bleed the rest of the air out
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  #20  
Old 12-17-2019, 12:32 AM
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Originally Posted by funola View Post
It should not take much cranking to bleed air out of the injector hard lines. First you work the primer pump to fill the IP till you feel resistance in the plunger. Next crack open one injector line nut ((NOT all 4) 1/4 turn. Wrap a rag around it to catch spitting fuel. Activate the glow plugs for 30 seconds then start cranking. Air + fuel will bleed out of the open injector line and it should start within a few seconds of cranking. The idle will be a little rough because of the open line. Have a 17 mm wrench ready to tighten the nut - idle will smooth out. Go for a drive to bleed the rest of the air out
And yet it did. Two and half batteries worth.

To be honest, I've never seen the benefit in cracking the lines at all. Nor have I ever had to, and I've had them off countless times. I cracked them today to be able to confirm there was in fact fuel dribbling out but mainly out of desperation.

Lines off to do injector work: 20 seconds of cranking.
Plastic lines off the pump to change o-rings: priming and then 10 seconds of cranking.
The combination: near defeat.

I have two theories at the moment. I put a level on the valve cover and found I was 8 degrees nose up due to the incline of my driveway.
I usually park with the nose down when I remove the plastic lines/ofv. I do this because I read that fueling starts from the number 1 pump element. Nose down and it becomes the low point in the pump. I also do it because it seems to require the least amount of cranking when I'm done working. With the nose up, and the number 1 pump element at the high point, it's possible I could prime forever without pushing the air out. I'm not attached to this, it's just an idea.
The second idea is that the check valve in my return line is scored enough to leak pressure which makes it very hard for me to build enough to displace the bubble. Or some combination of the two.

***EDIT*** Another thing I'd like to know, what happens to all the fuel that's being injected into prechambers that are not combusting? Am I flooding them the longer I crank without ignition? Is that an issue?
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Last edited by Shern; 12-17-2019 at 12:46 AM.
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  #21  
Old 12-17-2019, 09:19 AM
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If fuel is injected into a prechamber, it'll combust if your compression is good. Partial combustion is the white smoke you get when low compression, low fuel, or airbound. Too much air in the lines acts like a spring, you'll never pop the pintle in the injector, thus no fuel injected. This is why it's helpful to crack a hard line or two to "burp" the fuel rail. Air is compressible, fuel is not.
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  #22  
Old 12-17-2019, 02:15 PM
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Quote:
Originally Posted by Shern View Post
And yet it did. Two and half batteries worth.

To be honest, I've never seen the benefit in cracking the lines at all. Nor have I ever had to, and I've had them off countless times. I cracked them today to be able to confirm there was in fact fuel dribbling out but mainly out of desperation.

Lines off to do injector work: 20 seconds of cranking.
Plastic lines off the pump to change o-rings: priming and then 10 seconds of cranking.
The combination: near defeat.

I have two theories at the moment. I put a level on the valve cover and found I was 8 degrees nose up due to the incline of my driveway.
I usually park with the nose down when I remove the plastic lines/ofv. I do this because I read that fueling starts from the number 1 pump element. Nose down and it becomes the low point in the pump. I also do it because it seems to require the least amount of cranking when I'm done working. With the nose up, and the number 1 pump element at the high point, it's possible I could prime forever without pushing the air out. I'm not attached to this, it's just an idea.
The second idea is that the check valve in my return line is scored enough to leak pressure which makes it very hard for me to build enough to displace the bubble. Or some combination of the two.

***EDIT*** Another thing I'd like to know, what happens to all the fuel that's being injected into prechambers that are not combusting? Am I flooding them the longer I crank without ignition? Is that an issue?
The procedure I described is pretty much what the FSM says to do. It shouldn't take two and half batteries worth and 30 minutes of cranking. Shouldn't matter if the car is nose up or down.
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  #23  
Old 12-17-2019, 02:31 PM
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Originally Posted by funola View Post
The procedure I described is pretty much what the FSM says to do. It shouldn't take two and half batteries worth and 30 minutes of cranking. Shouldn't matter if the car is nose up or down.
Copy that. I have an FSM myself. Any ideas what might have caused the issue?
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  #24  
Old 12-17-2019, 02:42 PM
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Copy that. I have an FSM myself. Any ideas what might have caused the issue?
You got air into the IP. The primer pump can only do so much. You have to have an outlet for the air that's still in the IP. The most expeditious way is crack an injector hard line nut then crank the starter after a pre glow.
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  #25  
Old 12-17-2019, 02:49 PM
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Correction to my post #16. It is the primer pump that can create a back or reverse suction force on the relief valve in operation. It actuallty does even when the relief valve is normal.It is the lift pump that cannot .

Unless perhaps if the output valve has failed in it. I think when that happens would have some effect. No way to determine the levels of leakages in this system to allow to remain once detected.

I must have been tired when I reversed the two last night. Important distinction.

If anyone runs across an identical situation in the future. Running the engine on wd 40 should get it self primed up and started fairly easy.

Last edited by barry12345; 12-17-2019 at 03:00 PM.
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  #26  
Old 12-17-2019, 04:17 PM
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Originally Posted by Shern View Post
Copy that. I have an FSM myself. Any ideas what might have caused the issue?


When you were pushing the primer pump down the contents of the primer pump were looking for the easiest way out or path of least resistance. It was through the leaky relief valve.

When the pump was retracting it was sucking the air back into the injection pump through it . That had been driven out initially. The leakage was so bad that the pumping had a substantial back and forth component, Rather than directional to build pressure and expel the air in the injection pump enough.

What really got fuel up was when you were depleting the batteries. The lift pump has a check valve on the output side as well. It was working as you were cranking. It provided your real elimination of the air. As in operation it cannot support reverse suction.

Do not expect to find much literature on this. If the maintenance plan Mercedes wanted on these cars is followed. Issues like this are detected far earlier than when they become problems. There are many simple tests that can be conducted on this supply system to detect what the problem is.

I seriously toyed with the ideal of getting the system in shape to retain the fuel pressure at least until the next use of the car. As a subject on the site. First I would have to get members interested in buying a pressure gauge.

It was not happening so I put the concept on the back burner. After a lot of thought I suspect that rebuilt engine drop in the 240d may do this. I also suspect when brand new they all did it for a percentage of their early lives.

I see no reason that with a tight relief valve and tight lift pump valves. Primary fuel pressure cannot remain after shutdown. At those low pressures it probably cannot escape past the injection pump elements and even if it does. The older delivery valves may have leaked down enough fuel over time. It blocks the loss of pressure. It may even increase it a little and that would open the relief valve to restore the residual pressure.

All the clamping off of the return line really achieved was a positive identification the relief valve had far too much leakage.

We have over time seen some members examples start in temperatures most will not even consider doing. I assumed they just did not have internally leaky fuel system issues. Compression checks on these older engines remain generally very good for their age and miles. Some are substandard but the majority are still decent.

As I have mentioned many times doing a real maintenance run of these fuel supply systems is cheap . Plus it gives you a better understanding. So if some issue arises it is far easier for you to deal with. Most problems in that fuel system designed in the 1930s do give warnings well before the problem erupts.

I almost cringe when a new member comes on board. Quoting I just acquired a thousand dollar old diesel Mercedes. What should I check before leaving on a five thousand mile trip. It is amazing that so many make it in my mind.

A percentage do not and a proper maintenance run on the car plus a few things that have been learnt over time ..Could avoid many of the expensive road breakdowns that do occur. Generally it is pretty inexpensive to do this yourself

Last edited by barry12345; 12-17-2019 at 04:39 PM.
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  #27  
Old 12-17-2019, 04:36 PM
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The priming pump has a check valve, if working correctly, should not "suck".

If you can feel resistance in the lift pump plunger when you got most of the air out of the IP, the overflow valve is probably good enough to enable the engine to start
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  #28  
Old 12-17-2019, 05:59 PM
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Originally Posted by funola View Post
The priming pump has a check valve, if working correctly, should not "suck".

If you can feel resistance in the lift pump plunger when you got most of the air out of the IP, the overflow valve is probably good enough to enable the engine to start


A the primer pump comes up it takes in fuel by over coming the valves in the lift pump. Or provides it by creating a vacuum. When you press back down the lift pump valves stop the reverse flow, They actually stop it when you are not even pumping. There is no need for a check valve in the primer pump.


If it cannot discharge from some source it will not work. As far as I am aware it Is a piston with a sealing ring in a bore. The lift pump valves perform the systems valve function for it.


If you think about it for a moment. It has to aquire air or something to pump out. It has but one hole in the bottom. If there was a valve it either would not take in anything or not pump it out.


It is not like say an air pump that secures air from the outside to pump. It is important to understand this distinction. It is a pump with out a contained check valve. It is dependant on the lift pumps valves.


Why is the distinction important? As a test if you are pumping the lift pump with the valves in the lift pump leaking badly. You will see the back and forth surge in the pre filter sometimes. As I mentioned in another post. It is a simple system. Anything not understood can cause issues when dealing with it. Also that is why it is easy to maintain properly.

Last edited by barry12345; 12-17-2019 at 06:12 PM.
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  #29  
Old 12-17-2019, 06:12 PM
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Barry I think you've got it.

Funola- you may be describing the world as it should be, rather than the way it is. Cracking the injector lines did nothing. I was not getting fuel to the number one line no matter what I did. Cracking lines was the second thing I did after cranking far longer than usual -and this is on the first battery charge. Clearly, I had trapped air. Why I was unable to expel it is the question.
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  #30  
Old 12-17-2019, 06:35 PM
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I want all members to get information on this simple system understood. Then they will understand the true importance of it.


On a periodic maintenance program you can in a lot of cases. Eventually work it back to as it was when new. It is pretty obvious if you have it in good shape and there are still issues.


With miles those pesky and not cheap or easily available delivery check valves are going to need replacement. I have been trying to mentally form up the easiest test possible in place.

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