On top of the fuel filter in the CDI (and later) there is a valve that can be loosened and tightened. I've read/watched a few tutorials on how to change the fuel filer, but have seen no mention made of using that valve. PO of my car changed the filter many times and never touched that valve.

Is it for bleeding air from the fuel system and under what circumstances might that be needed?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Nothing said about it in the service manual CDROM either.

John

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Certified Registered Nurse Anesthetist (CRNA)
2002 F250 powerstroke with Plantdrive WVO conversion
1983 300SD 190K miles ,sold
2006 E320 CDI

picture??

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2004 F150 4.6L -My Daily
2007 Volvo XC70 -Wife's Daily
1998 Ford F150 -Rear ended
1989 J-spec 420SEL -passed onto its new keeper
1982 BMW 733i -fixed and traded for the 420SEL
2003 Volvo V70 5 Speed -scrapped
1997 E290 Turbo Diesel Wagon -traded for above
1992 BMW 525i -traded in
1990 Silver 300TE -hated the M103
1985 Grey 380SE Diesel Conversion, 2.47 rear end, ABS -Sold, really should have kept this one
1979 Silver 300D "The Silver Slug" -Sold

It will let you drain off the water collected in the bottom of the filter. Not that I've ever seen water in the fuel be a problem, at least where I am. The electrical connector on the filter is for contacts that can sense water in the filter and set a fault code. FWIW bleeding of the CDI fuel system isn't necessary as they all have low pressure electric pumps.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

So let's say that the fault code went off. What would I do to drain the water?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 157k miles
06 E320 CDI "Rutherford", Black on Tan, 175k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 70k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Open the valve a bit, put a hose on the nipple in the center, and turn the key on. Fuel pump will run for a couple of seconds which should be enough to push out any water. Catch it in a clear container and if necessary cycle the key a few more times until straight fuel comes out.

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90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff

so its a sediment trap for water.

I recall them on old japanese diesel engines, they had a screw type job on the bottom - just unscrew it, catch all the drain in a pan - prime the filter again and start it up.

some even had a basic switch on the tap. once shorted with water a lamp lit up on the dashboard.

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2012 BMW X5 (Beef + Granite suspension model)

1995 E300D - The original humming machine (consumed by Flood 2017)
2000 E320 - The evolution (consumed by flood 2017)

Yes they do have the alarm. Under the black cap is an electrical probe assembly. It dips down into the well at the center of the filter. Water shorts the probes and triggers an alarm on the console.

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The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech

if you turn the key with the valve open and without a hose hooked up, fuel will hit the ceiling......

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

I can personally atest to this!

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1959 Gravely LI, 1963 Gravely L8, 1973 Gravely C12
1982 380SL
1978 450 SEL 6.9 euro restoration at 63% and climbing
1987 300 D
2005 CDI European Delivery
2006 CDI Handed down to daughter
2007 GL CDI. Wifes

A few decades ago I owned an Isuzu I6 5.9 liter diesel, that had an aftermarket Racor brand fuel filter / water separator installed at the fuel tank. Worked great!

I've got a water separator as standard equipment on my 2002 Ford Power stroke diesel 7.3 liter V8. I've never had any residue come out of the manual release valve, nor, water-in-fuel instrument cluster indicator light come in 14-years now.

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'06 E320 CDI
'17 Corvette Stingray Vert

The warning system is great. most times if you have water it was pumped in when you brought fuel. Typically you might drive away from a station and some miles later the warning goes on.

I really try to stay away from low volume diesel fuel sellers. I even had to change a frozen fuel filter and thaw a frozen blockage in a low spot on the fuel line one winter.

Never ever buy fuel at a place that has a heater running at their pump in the winter. In most cases it is summer fuel that has gelled in their pump.