I am having intermittent starting problems with my C220. Occasionally it will crank but doesn't start. I am leaning towards a fuel delivery problem.

My questions:
o Once I turn the ignition on prior to cranking should the fuel pump be running? Will it go up to pressure and then turn off?
o What is the purpose of the Fuel Cutoff Relay?

My feeling is that I should be able to tell if the pump is operating by feeling it once the ignition is on.

When you turn the ignition switch on, the pump should run for only a few seconds.

It should run whilst cranking.

Might be best to check this out with two people, one at the switch, the other observing the pump.

Jim

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85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles

Thanks Jim for the quick response. Are you saying that the pump will run until the pressure is built up. What turns it off? Does the Fuel Cutoff relay play a part here?

There is a test port on the fuel rail for checking pressure and flow...

Next time it fails to start, give the OVP a few taps..

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=2AO123CS42EB0RZRTB&year=1995&make=MB&model=C-220-001&category=P&part=Overload+Relay

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A Dalton

Can anyone explain to me how the fuel system works? I know there is a FP relay that turns the pump on and there is a Fuel Pressure Regulator that keeps the pressure constant for the injectors. It was mentioned that the pump only stays on for a few seconds when you turn the ignition on and goes back on during cranking and thereafter. I would assume that the Engine Management system tells the pump to turn off after a few seconds. I also assume that the Fuel cutoff valve in not in the loop and is only there for emergencies.

What is your question ?

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A Dalton

What causes the fuel pump to shut off after a few seconds when you go to position 2 prior to cranking?

What is the function of the Fuel Cutoff Valve?

The fuel is cut off after 2 secs by the HFM/SFI modules mapping.. it needs a starter activation to resume and then it need an RPM and MAF input to continue running.
The 2 secs is just to get fuel rail pressure pre-sart.
If you are having fuel problems , there is a test port on the fuel rail

here are the specs:
FP @ idle with vac to regulator..3.2-3.6 Bar
FP @ idle w/o vac , 3.7 -4.2 Bar
FP > 2.5 Bar after 30 min rest.

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A Dalton

Thanks Arthur
What is the function of the Fuel Cutoff Valve?

Does the Mercedes Fuel rail use the standard Schrader fitting for Fuel pressure testing?

Yes..you can use an old style R12 hose if you do not have a FP Tester Kit...fuel shut off is for RPM limit and Converter protection in N or P ....

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A Dalton

Thanks Arthur, I really appreciate the help. I hope I am not being too much of a nuisance.

Not at all..
For those reading this post, if one suspects no fuel on these type systems and they have no proper test tools, you can get a general idea of fuel at least reaching the engine if you just take the cap off the test port and push the valve in like on any bike tube...use a rag and if you get a decent presure squirt of fuel at that valve , you can assume you do have fuel pressure...a gauge will tell if it is to spec, but that quick test will tell you that at least you have something at the rail ....which is a good diagnostic start.
Specially if you are stuck somewhere...........................

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A Dalton

What a great tip. Thanks again Art.

The problem seems to be correcting itself. It does not seem to happen very frequently anymore. I seem to notice alot of activity (noise in right rear gas cap area of the car for the first five miles of car travel). I assume that this is normal and not related to the problem.