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  #1  
Old 11-04-2003, 07:31 PM
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Location: Boston, MA - USA
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Failing Fuel Pressure Regulator - M103

What are the sysmptoms of a failing Fuel Pressure Regulator?

My 1989 300SE - M103 motor - has fuel which makes it past the vaccum diaphragm and ends up in the clear plastic vacuum line going from the rear of the FPR to the breather hose (which runs from the VC to the Air Cleaner housing).

Aside from this, the car runs great. I am wondering what the motor will do next as the Fuel Pressure Regulator becomes progressively more damaged.
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1979 450SLC
1989 300SE
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  #2  
Old 11-04-2003, 11:05 PM
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For me on a 104 motor the symptom was that it would not start right away - had to turn the key several and/or pump the gas pedal. On a 104 the part is about $35 and installs in about five minutes. This is a very rare case of a 103 part costing more than a 104 part - I think your's is about $100 or something like that, and more difficult to put in.
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  #3  
Old 11-05-2003, 09:32 AM
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Check this out , especially MB Doc's response. Also search for 'regulator' under my username. There are numerous posts there. Fuel pressure regulator leaking.
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  #4  
Old 11-05-2003, 10:30 AM
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Thank you, Deanyel and Cap'n

Today was a cold morning and the motor stumbled through the cold start. It caught in a lumpy sort-of way, in about 4-5 seconds of cranking, but evened out to the high warm up idle speed without problems.

Still, the lumpy cold start behaviour is a relatively new development, and now that I think about it, the car began to gradually develop that condition over the last few months. I originally thought: "plugs, cap, rotor."

I guess the progressively lumpier starts condition coincides with the progressively failing Fuel Pressure Regulator.

Aside from this, I am curious about one more thing - the vacuum line from the FPR and the diaphragm it controls: I've read posts where people say the vaccum line is merely a vent. From my limited knowledge of MB Fuel systems, I would have concluded that the vaccum line controls a diaphragm, under which a spring mounted valve of some sort operates inside the FPR.

One thing is clear - fuel should not make its way into that vaccum line, but if it does, the FPR is failing (or failed) and the fuel might end up in the crankcase.

Finally, I am wondering if a high Fuel Tank pressure (you know - a nice big pressure puff when the gas cap is removed) can cause or contribute to the failed diapragm in the FPR.
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1979 450SLC
1989 300SE
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  #5  
Old 11-05-2003, 10:42 AM
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There is no vacuum to speak of on that line. It just LOOKS like a vacuum line. It vents the back side of the diaphram to atmospheric pressure, necessary for proper regulator operation. It is connected to the breather line just so there is someplace relatively safe for any leaked fumes or raw gas to go.

Steve
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