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  #1  
Old 09-10-2010, 11:02 PM
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Fuel Pump Suction/Delivery Valve

The engine on my '83 300D recently, and suddenly, became slow to start on the first start of the day. A bit of investigation seems to suggest that the fuel is flowing back to the tank as the vehicle sits. Given that there are no signs of external leaks, I am wondering if the problem isn't the suction/delivery valve in the fuel pump. Does it act as a check valve? I note that service manual recommends that the valve be either cleaned or replaced whenever the pump is removed. Does anyone have experience with this valve?

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Old 09-11-2010, 12:05 AM
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The 2 valves in the lift pump can fail. If you have more than 1/2 a tank it wont drain back as the return line will be below the fuel level in the tank. Unless you have an air leak in your fuel system.
If you park the car facing down hill over night & the problem is not there the next morning then it strongly suggests an air leak.
Have you changed your filters recently?
Are you only using regular diesel?
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  #3  
Old 09-11-2010, 12:32 AM
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Air leaks can also be responsiable for fuel draining back.

I do not know if an Air leak at the Hand Primer can do that but one in the hoses can.


Less likely but any sort of Vacuum or caused by a restricted Fule tank vent I suppose might also pull fuel back. When you park the Car for the Night loosen the cap and leave it loose until morning. Try starting the next day and see if it improves.

Fastlane sells a new valve Fuel Supply/Lift Pump Valve kit for about $10; don't know what the shipping would be.

If you get one of those 1 or 2.5 liter plastic spare Fuel container; disconnect the Fuel Inlet hose and replace it with a longer one run into the Spare Fuel container and drive it it might give you some info.
I believe on US models the spare Fuel Container will fit nicely behind the Head Lights; under the Hood.
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Old 09-11-2010, 11:49 AM
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Quote:
Originally Posted by layback40 View Post
If you have more than 1/2 a tank it wont drain back as the return line will be below the fuel level in the tank.
The problem did originally occur with the fuel tank substantially empty. I will see if a full tank changes anything.


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Originally Posted by layback40 View Post
Unless you have an air leak in your fuel system.
It would seem that any air leak would have a corresponding fuel leak. No fuel leak found.


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Originally Posted by layback40 View Post
Have you changed your filters recently?
The engine performs great once started, reasonably excluding a clogged filter.

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Originally Posted by layback40 View Post
Are you only using regular diesel?
Yes.

Last edited by qwerty; 09-11-2010 at 11:06 PM.
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  #5  
Old 09-11-2010, 11:51 AM
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Quote:
Originally Posted by Diesel911 View Post

Fastlane sells a new valve Fuel Supply/Lift Pump Valve kit for about $10; don't know what the shipping would be.
That sounds like what I need. But I am having a hard time finding it.
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  #6  
Old 09-11-2010, 01:27 PM
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Upon further investigation, I can actually see the fuel flow reversing in the clear pump-to-filter fuel line when the primer pump plunger is released. And the hand pump won't develop its usual level of pressure. The problem almost has to be the valve in the fuel pump.
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  #7  
Old 09-11-2010, 08:47 PM
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I think I found one of the threads with the part number:

OM617.951 and .952 Lift pump Repair kit W126.120 300SD W123.133 300D

Other Info:
Lift Pump Rebuild...
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Old 09-11-2010, 10:42 PM
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That's very helpful;, thanks!!!

The problem persists even with a full fuel tank.
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  #9  
Old 09-12-2010, 02:40 PM
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The problem appears to be solved. I removed the delivery valve, hoping to find an obvious cause to my problem, but nothing jumped out. The only possible issue was a bit of "gum" in the nipple bore where the valve plunger rides. In addition to cleaning the valve, I stretched the spring slightly to give it a bit more enthusiasm when it comes to holding the valve closed. When reassembled, the hand pump developed substantially more pressure, easily activating the IP relief valve with every stroke, which had not been possible before.

Diesel911, the link to the thread describing your was very helpful. Prior to seeing it, I was set to order a new lift pump. Thanks, you probably saved me $140.

Last edited by qwerty; 09-12-2010 at 03:07 PM.
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Old 09-12-2010, 04:45 PM
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[QUOTE=qwerty;2543153]The problem appears to be solved. I removed the delivery valve, hoping to find an obvious cause to my problem, but nothing jumped out. The only possible issue was a bit of "gum" in the nipple bore where the valve plunger rides. In addition to cleaning the valve, I stretched the spring slightly to give it a bit more enthusiasm when it comes to holding the valve closed. When reassembled, the hand pump developed substantially more pressure, easily activating the IP relief valve with every stroke, which had not been possible before.

I must be half asleep. Can you describe what the delivery valve is? Are we talking the primer pumps valve?? Or what exactly.

If the relief valve into the return line it sounds like it was semi open and never closing completely. Therefore allowing back drainage of fuel over time. This would also account for the soft feeling primer pump of course as well.

If the above was so you should get a gauge on that base system and get an accurate pressure reading to manipulte whatever is required to see about 19 pounds sustained at idle. To change anything other than perhaps rekit the lift pump on speculation is far less cost effective than buying a gauge and dealing with this type of problem.
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  #11  
Old 09-12-2010, 05:38 PM
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[QUOTE=barry123400;2543215]
Quote:
Originally Posted by qwerty View Post
I must be half asleep. Can you describe what the delivery valve is? Are we talking the primer pumps valve?? Or what exactly.
The links in post #7 should clear up any confusion.
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Old 09-12-2010, 09:43 PM
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[QUOTE=qwerty;2543230]
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Originally Posted by barry123400 View Post

The links in post #7 should clear up any confusion.
Thanks. got it. The partially open lift pumps check or delivery valve if you wish was the way of less resistance for the fuel you were pumping with the lift pump. There has been a visual back and forth surging of the fuel effect observed before on several posts in the primary filter. Thats with the car running. The valve possibly was not quite as open as yours in those cases. Or perhaps more so. They exhibited some questionable running but all would start after sitting for periods I think I remember.

For your no fuel available after the car sits overnight there may still be someplace air is entering the system. This would have allowed the fuel to escape back towards the tank with the poorly fitting lift pump check valve. A truly sealed system cannot back drain even with a very poor check valve in general. My guess at these cars ages most those valves and seats are in less than stellar condition.

The air that displaces the fuel can come from even before the lift pump in my opinion rising through a leaky check valve. Over time moving upward and enabling the fuel to back down by gravity.

What happens then I suppose is instead of the lift pump building presssure instantly it has to compress air first. Either internally or somewhere ahead of it. Generally speaking at reasonable temperatures all my diesels catch almost instantly with the key after sitting for prolonged periods.

This indicates to me on those engines their fuel supply systems are tight and in reasonable condition. I know they are getting good sustained fuel at pressure as soon as the engine turns or very quickly if not. The level of fuel in the injection pump has not been depleted in some of the elements by back drainage over time. Even in one extreme example of sitting for four years.

Since any place past the lift pump is under positive pressure it would vsuially leak under loading. So the remaining sources if any might be gunk in the relief valve on the injection pump or a point of air ingress before the lift pump. This relief valve gunking possibility is based on your finding gunk in the area of the check valve increasing chances of the remote possibility.

Although your car will probably run and start okay after long periods of sitting. A slight problem may still remain. I have been wanting to establish how long a good system retains base fuel pressure in the injection pump for some time now. Or how quick the pressure rises on the first crank or two of the engine. This may be important for quick and clean starting.

Most lift pumps in service probably have a little valve leakage at their age on this series of cars. We just never have bothered to quantify where the acceptable level of retention is. Instead I personally have done the clamp off test to make sure the lift pump can supply at least 30 lbs presssure as a general indication of lift pump performance.

We are basically restoring functions on cars of this age as well as general repairs to get them back to what they were at one time. Or as close as economically practical. This whole area is a cheap one to recondition. The bottom line is get this system right and overall the car will thank you for it.

Your proof all is okay for air leakage is simple. After sitting for at least overnight. In reasonable ambient temperatures. How much does the engine crank before it catches. Any more than two turns of the engine is probably too much if compression and glow plugs are in reasonable shape.

Now you are just left with the unknown of how much sustained pressure is present when running. If this pressure is also very low starting may be delayed a little. Simple to check by closing off the return fuel line temporarily. If improvement in starting ability occurs under test. Chances are that return line valve on the injection pump is either gunked up or weak.

The fuel supply system is actually fairly simple on these cars. At the same time it needs to be in reasonable condition overall. This general area makes a difference
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Old 09-13-2010, 12:19 AM
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[QUOTE=barry123400;2543340]
Quote:
Originally Posted by qwerty View Post



For your no fuel available after the car sits overnight there may still be someplace air is entering the system. This would have allowed the fuel to escape back towards the tank with the poorly fitting lift pump check valve. A truly sealed system cannot back drain even with a very poor check valve in general.
In retrospect, I believe that my original suspicion that fuel was draining back to the tank was not valid. From all indications, the slow starting condition was caused by the lift pump's inability to build pressure rapidly, due to the open delivery valve. Cleaning the delivery valve, along with stretching the coil spring slightly, immediately restored a "firm" feel to the hand pump and reduced the required cranking time from seconds to milliseconds.

At this point, the only mystery is how the engine ran as well as it did after it was started.
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Old 09-13-2010, 08:41 PM
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One of our members observed he had no fuel pressure with a gauge in the base of the injection pump. His engine still ran. Most will not. Our member may have had a high fuel level in the tank helping out a little.
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  #15  
Old 09-21-2010, 11:53 PM
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A secondary benefit of cleaning up the fuel pump valves is a greatly improved idle quality. Most of the previous idle shake is gone.

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