Currently getting 8 mpg from my 280 SE 4.5. Pulled plugs and they have a lot of black soot. Car runs well and smooth with good power, but is obviously running rich. Steady vacuum reading just above 17" at idle. No vacuum leaks that I can find, believe I have checked all possibilities including installing new hose from intake to MAP. Door locks work as new. Suspected a leaky cold start valve dumping excess fuel into the intake. I ran the fuel pressure diagnostic in the 117 engine manual and the pressure was steady to spec @ 28psi, however the system is supposed to hold pressure after engine shutdown, and my pressure dropped to 0 almost immediately after shutdown. Additional tests are called for to isolate the leaking component in this order: Cold Start Valve - Fuel Pressure Regulator - Fuel Pump Check Valve - Fuel Injectors. So I started the procedure and the cold start valve passed, but the next test showed the fuel pressure regulator as the leaky component.

Here is my question: Is it possible for the fuel pressure regulator to leak excess fuel into the intake causing the rich running condition? If no, then based on the information above, what else should I check for this rich running condition?

Thanks for any opinions and help,

Tom Drew

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1972 280SE 4.5

The pressure regulator is external and would not leak into the intake. It might be leaking and wasting fuel, but you could see that.

You might want to check your fuel pressure and adjust it if necessary - see my recent "Waking Up D-Jet" thread for details.

But the most common culprits in rich running seem to be the air intake and coolant temp sensors. They should have 260-340 ohms and 2100-3100 ohms resistance, at 68 degrees, respectively.

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Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe