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  #1  
Old 10-29-2011, 06:49 PM
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1980 300D I suspect a bad lift pump

A few weeks ago I was driving my 1980 300D sedan to work and it stalled out about 1/2 mile from home. The car was running OK and just stalled out. It kind of felt like a plugged fuel filter. First was to change the primary filter, no luck... Walked home, got into another of my cars and drove to work. Had the car towed home and went through a series of items over the next few weeks to no avail. I'll list them out: new tank strainer although the one I pulled out wasn't too bad... no where to the point of being non operable, new primary and secondary filters, tried multiple times to reprime the system but never could get fuel to the front four injectors just the rear injector. I also tried flipping return and supply lines. I also installed a new Bosch primer pump but that did not help either.

I don't have access to a spare lift pump and for awhile now have been thinking about hooking up an auxillary electric pump to aid in priming.

Whats the process in adding the electric fuel pump? Do I bypass the oem lift pump or does the fuel still run through the lift pump assembly? A picture would be great

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  #2  
Old 10-29-2011, 08:01 PM
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Is the hand pump making enough pressure to trip the IP pressure relief valve? (You should be able to hear the relief valve actuating as you pump.) If not, the suction valve on the fuel pump could be stuck open.
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  #3  
Old 10-29-2011, 08:10 PM
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Yep, assuming that you have a good working primer pump, pump the heck out if it until you hear the little buzzing noise, then start it up. If it runs for a couple of minutes and the falls flat, good chance the lift pump is the problem.

There is a lift pump repair kit available, not too expensive. I am told it is not too difficult to do, but, a bit messy.
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  #4  
Old 10-29-2011, 08:17 PM
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Quote:
Originally Posted by Palangi View Post
Yep, assuming that you have a good working primer pump, pump the heck out if it until you hear the little buzzing noise, then start it up.
Thing is, the hand pump relies on the exact same valves as the mechanical pump.
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  #5  
Old 10-29-2011, 08:22 PM
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Quote:
Originally Posted by tangofox007 View Post
Thing is, the hand pump relies on the exact same valves as the mechanical pump.
I ran into one where an inline fuel filter with a paper element came apart. You could pump it up easily, but it would only run for a minute or so. Lift pump kit fixed it.
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  #6  
Old 10-29-2011, 09:32 PM
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I can pump until the cows come home and nothing comes through to the injectors. I even removed the injector lines to see if the lines were clogged and to no avail. No fuel comes out of the top of the injection pump, just a little comes through the port that corresponds to the injector closest to the firewall.

I'll rebuild or find another lift pump later but in the mean time.....

Whats the process in adding the electric fuel pump? Do I bypass the oem lift pump or does the fuel still run through the lift pump assembly? A picture would be great.
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  #7  
Old 10-29-2011, 09:38 PM
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Quote:
Originally Posted by neumann View Post
No fuel comes out of the top of the injection pump, just a little comes through the port that corresponds to the injector closest to the firewall.
Is that while cranking the engine?
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Old 10-29-2011, 10:08 PM
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Quote:
Originally Posted by tangofox007 View Post
Is that while cranking the engine?
yes while cranking the engine
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  #9  
Old 10-29-2011, 10:23 PM
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You can install the Electric Fuel Pump between the Plastic Primary Filter and where the Fuel from the Tank comes off of the Fender Well.

If you installed the Electric Pump direct to the Seconday Filter you would have to do something about the stock Fuel Supply Pump because the Piston inside would still be moving and it is lubed by the Fuel going through it. With No Fuel going through it you would eventually ruin it.

You might simply ask your local Bosch Fuel Injection Shop how much they would charge you for the Fuel Supply Pump rebuild kit.

Anoter possible issue:
FUEL PRESSURE RELIEF VALVE THREAD/ OVERFLOW VALVE THREAD
Fuel pressure relief valve adjustment - PeachParts Mercedes ShopForum
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  #10  
Old 10-30-2011, 01:57 AM
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It is a fairly common problem with this system that when the lift pump has been completely drained and dry that it must be primed before the primer pump will work. In old pumps the valves often do not seal well enough to pump air and provide sufficient suction to lift fuel from a low tank and open the fuel pump relief valve. This does not mean that you need a new lift pump, it will work perfectly well once primed!

To Prime the lift pump follow these steps:

1) Fill the main spin-on filter with fuel before you install it.

2) Remove the fuel hose at the steel supply line from the tank and, using a small funnel, fill the pre-filter and hose to the top and all air is out. Before reconnecting give the Primer Pump 2 or 3 strokes to check it is drawing fuel then top up the pipe again and reconnect. If it is not drawing fuel you must strip the pump - may be just full of crud.

3) Keep pumping primer for at least a minute after you hear the pump relief valve buzzing.

Quote:
Originally Posted by neumann View Post
I also tried flipping return and supply lines. I also installed a new Bosch primer pump but that did not help either.
Double check you are connected to the supply line.
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Last edited by Beagle; 10-30-2011 at 02:23 AM.
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  #11  
Old 10-31-2011, 10:35 PM
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The car was running fine for weeks, drove it one morning for about 1/2 mile and then it stalled out.

I went through all the priming procedures in previous posts/attempts and still no go with fuel to four of the injectors. Supply and return are in correct locations.

I did get my hands on a spare lift pump yesterday and hope to install it later this week and also install an electric pump for backup.
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  #12  
Old 11-01-2011, 12:16 AM
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Have you thoroughly flushed out the steel supply line with compressed air and fuel? Crud can collect and block at bend near the fire wall.

Connect lengths of hose to both inlet and outlet of lift pump and pump from one small container to another by both hand primer pump and cranking. This way you can either confirm or eliminate lift pump as problem.
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Last edited by Beagle; 11-01-2011 at 12:38 AM.
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  #13  
Old 11-01-2011, 12:03 PM
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I did use compressed air going into the return line and fuel freely flowed through the supply line and through the primary filter. Also tried the separate fuel container as well. All to no avail. Going with the used lift pump install later this week.
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  #14  
Old 11-01-2011, 12:10 PM
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For a simple test of the pump, you might check for the presence of suction on the intake line while the hand pump is on the upstroke (a good thing) and pressure on the downstroke (a bad thing.) Reverse the concept for the discharge side. There you want to see pressure on the downstroke and no suction on the upstroke. Your problem could be as simple as a gummed-up valve in the pump, which is a quick and easy fix.
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  #15  
Old 11-01-2011, 03:27 PM
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similar issue

I had a priming issue once.
I disconnected the fuel line from the Lift pump.
Installed a 2-3 foot section of Clear hose.
Then using a small squezze bottle of diesel I proceeded to fill the clear hose with fluid.
Then I would be able to see fluid movement when I manually worked the primer pump. It would move like a couple 3 inches at a stroke as I recall.
If for anyreason you do not see any movement you could useing a hand pressure/vacuum pump force slightly the diesel fluid into the lift pump as you are manually working the primer pump.
I found that it took a bit of time and effort to get the IP properly filled with Fluid prior to attempting a good startup. I would then get the fluid level below the hose pump connection and remove the clear line. Then use the vacuum pump to draw diesel from the tank to the end of the normal Fuel hose and then put a hose clamp on it so that it would help hold the prime while I re-installed the normal fuel hose on the lift pump. Then ensure all clamps are tight with no leaks. Then with a charged battery give it a turn over. Just some thoughts on how it worked for me. Air can be a *****!

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