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Old 08-04-2001, 02:13 AM
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Join Date: Apr 1999
Posts: 2,632
screw-drive fuelpumps and rates

According to AllData, the screw-drive's high flow-rates allows much of the fuel to circulate thus reducing the chance of vaporlock by having "cool" fuel. Chris is also correct that not all of the fuel is being burned at that rate.

What is the rate and pressue of the 1988 CIS dual fuel-pump systems?
My 1995 C36 engine (AMG M104) supposedly uses a high-flow version of the 1995 screw-drive fuel pump which is rated at a minimum of 1.0L in 40 seconds.
Hence, is my CIS dual fuel-pump system providing too much pressure, and too little volume for the AMG M104 engine?

And again, why did MB go to the screw-drive fuel-pump?

Thanks again,
:-) neil
1988 360TE AMG

> Date: Fri, 03 Aug 2001 18:29:33 -0700
> From: "Christopher J. Pikus"
> At 06:04 PM 8/3/01 -0700, Dahlgren, Jack wrote:
> >
> >
> >
> >When I first looked at that 1.5 liters in 40 seconds and thought it was a
> >bit extreme, but it works out to be 4.2 MPG assuming you are travelling at
> >150 miles per hour. Or 1.8 MPG at freeway legal speeds. Similar to the
> >average SUV.
> >
> >This rate of fuel consumption would give you about 30 minutes of drive time
> >on an 18 US gallon tank.
> >
> >Sounds like an expensive engine to feed.
> >
> >-Jack Dahlgren
> >
> The spec is a bit misleading taken all by itself. Just because
> a car requires a fuel pump deliver 2 liters/min doesn't mean that it
> will actually burn fuel at that rate.
> First, fuel injection systems (and this includes diesels) like
> to have lots of fuel at their disposal at all times. This gives the
> injection pump latitude to instantaneously vary the amount of
> fuel delivered without having to worry about fluctuations in
> pressure on the supply rail. This way, the engineers of the FI
> system can assume that they have a nearly infinite (for their
> purposes) fuel supply with negligible pressure variation coming
> out of the pressure regulator -- the pressure regulator returns
> the excess to the fuel tank. In actual practice, most of the
> fuel delivered (I'm guessing 90%) ends up taking this return
> trip back to the tank.
> Second, the fuel delivery measurement specified above is
> at zero pressure: how fast fuel pours out of an open hose. When
> the pump actually has to deliver fuel at pressure, then the actual
> flow may be less. [Actually, this is only true for non positive
> displacement pumps (e.g. centrifugal water pumps). Since I suspect
> that the fuel pump is a positive displacement pump, then this
> argument would not apply.]
> chris pikus
> Date: Fri, 03 Aug 2001 15:52:51 -0700
> From: neilv
> Does anyone know why Mercedes went to screw-drive fuel-pumps after
> 1993/1994?
> Are they more reliable, quieter, or does their efficiency allow them to
> go away from two pumps (except for the wagons, they still have two
> pumps)?
> Also, I'd like to know if the fuel-pumps in a W124 300TE wagon can
> deliver approx 1.5 liters every 40 seconds (necessary to properly feed a
> AMG M104 engine) at 48-58 PSI.
> Thanks,
> :-) neil
> 1988 360TE AMG
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Old 08-04-2001, 09:28 PM
Registered User
Join Date: Mar 1999
Location: Suwanee, GA, USA
Posts: 4,712
What you have to do is monitor fuel pressure while driving...I don't think the dual pump will have inadequate flow unless one is faulty.

The problem is that your factory fuel pressure regulator is set to the other pump and is not adjustable. You may have to modify the fuel pressure regulator.

Drive the car with a gauge on it taped to the windshield.
Donnie Drummonds
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Old 08-05-2001, 12:46 AM
Registered User
Join Date: Apr 1999
Posts: 2,632
Thanks Donnie. I did that a couple of days ago,
and I get about 48 PSI on idle, and at full-throttle going up a decent grade I get between 56-58 PSI.

I've changed out the O2 sensors, but I still get CHECK ENGINE light, per OBDII scan, P0134.

Could going to a 3.07 limited-slip diff from the stock 2.87 of the 1995 C36 cause the CHECK ENGINE light to come on?

:-) neil
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Old 08-05-2001, 09:19 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The flow rate of the pumps bears no relationship to the amount of fuel used, unless the flow is too small. This can be verified by monitoring pressure while driving the car under full load, as Donnie has suggested.

I would suggest a couple other techniques other than taping to the windshield. The nicest is to use a pressure transducer and your trusty VOM. The way I have done it for years is to make a ten foot fuel line out of the aramid plastic fuel line used for VW etc injection line. Then you can set the gauge in your lap or on the dash.

The reason for the screw type pumps is cost IMHO.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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