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  #1  
Old 07-27-2001, 09:57 PM
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Join Date: May 2000
Location: Georgetown, MA
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I have a 190E 1986 and I just replaced the fuel pump.
Since I replaced the fuel pump the engine is no longer running smoothly and have developed a valve tick. I checked the fuel pump and the MB number on the new pump is different from the number on the old pump. Both are MB pumps. Although they look the same, do the different MB pumps have different flow rates and could I be washing down the oil with a higher flow rate.
John
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Old 07-28-2001, 12:47 AM
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The flow rate of the pump bears no relation to the amount of fuel you use unless it is insufficient.

The fuel supply system is a loop. The pump supplies much more fuel than is used so that the pressure can be precisely maintained. If you put ten pumps on the system there would just be more fuel returning to the tank.

I don't know about your number problem but the running problem is probably a coincidence unless disturbing the fuel circuit dislodged some dirt which is now causing a problem.
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Steve Brotherton
Continental Imports
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Old 07-28-2001, 05:31 AM
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Steve
Thanks for the reply. You confirmed what I suspected in the closed loop aspect. However, one of the fuel pump tests in the MB manual indicates that a too low pressure rate calls for pump replacement. Too High pressure also calls for replacement of the pump. Getting back to the pump model number differences, do some pumps generate more pressure? My real question is the 190E and the 300E and the 380 SEL all have different part numbers even though dimensionally they all appear alike? What then is the difference? What happens when the pump pressure is too high? What are the symptoms?
John
86 300E
82 380 SEL
84 300 SD
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Old 07-28-2001, 10:47 AM
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A common misconception is that the pump controls pressure. It DOES NOT. The pressure regulator is part of the fuel distributor in K-jet and K-jet w/lambda. The KE cars have an external separate fuel pressure regulator.

The only way the pump can affect this pressure is if it doesn't pump enough to be regulated.

There are a couple differences in pumps that could make a difference. The pumps have an internal by-pass (safety valve) to keep them from blowing up if the system is plugged. Since the systems work at different pressures the blow-off pressures will be different which could mess things up if say a pump used on an EFI systems (running 45psi or less) were used on a K-jet at 70psi or a KE-jet running 90psi. I think the blow-off pressure would be set high enough that a K-jet pump probably could be used on the higher pressure KE systems (not that I would recommend it).

The other factor in pump design is volume. Different pumps do produce different flow rates. Not as much fuel is needed to mantain pressure on a 4cyl as opposed to an 8cyl. The factor of safety in design here is probably large enough that a 4cyl KE pump would probably do good enough for a 8cyl but why take it to the edge.

The proper test for fuel pumps is volume at pressure. This is easy to measure by taking the return line off the fuel pressure reg. This fuel would be returning to the tank. The fuel in the system is regulated and the excess is what you measure.
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