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Old 07-28-2008, 10:47 PM
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Location: Texas
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Understanding dual fuel pump workings

I have an '88 gas 420sel with I think fuel issues. I have pulled the fuel tank and tank screen. Both are beautifully clean. My car has dual fuel pumps, but I do not understand how they work. The smaller line that goes to the fuel filter and then pump does not run gas out of it when disconnected. The larger line from the bottom of the tank just pours gas when disconnected from lines on fuel pump. But this line does not go through fuel filter, the smaller line does but does not allow gas to pour out when disconnected. Does it require fuel pump to be running to "pump gas"?
Why does this car have dual fuel pumps when earlier models appear to have only one?
The issue I was having when running was car will not go above about 25 miles per hour, as it then starts bucking like starving for fuel.
Before this issue, it was running great, and fuel pump relay went out. I jumpered connections and car ran again, but did not drive it far. Now with new pump relay, I am having the fuel starving issue. As a note, the new relay does not have all the pins the original does, but I was assured by shop that I think knows these cars well (mercedes their specialty) this would not make a difference. Does the extra pins run the second fuel pump and I am only running on one pump now? This is getting old real quick! Thanks if someone can explain the two pump issue and give me some insight. Steve
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  #2  
Old 07-29-2008, 03:30 AM
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Join Date: May 2001
Location: Northern Va.
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The dual fuel pump is also an issue for me. I overhauld the complete system on my 420SEL last year. My 300SE only has one pump but the steup is similar.

Fuel is gravity fed from the tank to the pump. From there is goes into the bottom pump and through the check valve to the upper pump and through its check valve to the filter and on to the CIS injection system. Attached to the pump is a fuel accumulator to keep the supply side pressurized and in case the internal diaphram ruptures, which is spring loaded, a return line to the 1st pump supply hose from the tank.

The reasons there are two pumps is the V8's require higher fuel pressure than the L6's, and higher fuel volume. The latter doesn't make sense because they are in series rather than parallel or one that is a higher volume pump.
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Warren

Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL

ENTER > = (HP RPN)

Not part of the in-crowd since 1952.
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Old 07-29-2008, 07:52 AM
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Join Date: Oct 2007
Location: Texas
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Do you know what happens if one pump is out? I thought I was having a fuel tank issue as the car has had minimal use for last few years, but fuel tank is great. Which now leads me to the fuel pumps. Will reinstall everything, and look for fuel pressure guage and see what that tells me I guess. Steve
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Old 07-30-2008, 12:30 AM
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I had a similar problem, one fuel pump was disconnected. The car didn't run well before I found this problem and it didn't run well after I fixed it. I kept checking the duty cycle on the diagnostic connector but it didn't have one, so I got a replacement ECU. It's wtill not running well and right now, the car is sitting.

As for the original question, it depends on which one. If the lower pump is not running (well), there won't be any fuel pumped to the second pump. If the second pump is not running, some fuel will make it past the second pump from the pressure of the first. It still won't run well.
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Warren

Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL

ENTER > = (HP RPN)

Not part of the in-crowd since 1952.
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  #5  
Old 07-30-2008, 11:43 AM
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Join Date: Apr 2000
Location: Falls Church, VA
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Are both pumps running when the engine is running (put your hand on them)?

Why don't you jumper the relay and see if that solves the problem?
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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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  #6  
Old 07-30-2008, 05:00 PM
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Join Date: Sep 2002
Location: Kingsport, TN
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When I replaced the fuel pumps, I had routed the fuel hose to the pump wrongly and it had a restriction/kink in the hose, which caused fuel starvation at about 3,800 rpms.
Might be worth checking the routing of the hoses.
Good luck
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Reinhard Kreutzer
1987 420 SEL (126)
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  #7  
Old 07-30-2008, 10:15 PM
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Join Date: Oct 2007
Location: Texas
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Car is Fixed!!!! That is until the next time I drive it Turns out the whole issue was one dead fuel pump. Pulled them out, put power to them, one would work, the other would not. Pulled two pumps out of the wrecking yard for $10.00 each, and they ran. Switched out the dead one, the car started right up, and drives wonderful now. So good now that the neighbors are wanting it. Steve
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