I have just replaced the FPR. If anyone is wondering what the performance difference is, I can report that the car has gone back to near instantaneous cold starts. As the FPR progressively deteriorated, the cold start behavior went from being almost instant to needing to crank for a second or two longer than before, to an occasional stall, and at all times, a few lumpy seconds after the motor caught. With the new FPR, instant and smooth starts in 30F to 40F temps.

I detected fuel in the FPR vent tube (clear plastic vacuum-type line) and am grateful for such an easy pinpoint of a faulty part - a leaky FPR diaphragm. I could have been chasing the deteriorating cold start condition all over the place for a long time, before discovering the real culprit.

Also on my mind: why the FPR failure? On the one hand, the motor has 142K miles, on the other, the car has always been well maintained. There is an area of suspect - high pressure in the fuel tank ( a big exhale when the gas cap is removed for a fill up). I've installed a new gas cap (the old one said "unventilated" [?]) and also a new Regeneration Valve - AKA Fuel Tank Vent Valve. The RV is located in the engine compartment, inner left fender,
on my car (89 300SE) - within easy reach, but I know that at least one more vent or purge valve is somewhere near the rear axle. Anyone know of a procedure for testing the fuel tank vent system on a W126? I don't know if I am connecting my supposition points correctly, but could constant and seemingly excessive fuel tank pressure exert itself on the FPR diaphragm and hasten its demise?

Aside from this, if you're still here, let me endorse the replacement of old rubber vacuum fittings, at every point you can find them; also the plastic vacuum lines in the motor compartment. I've gone from a very nice running M103, with no evidence of any vacuum leaks whatsoever, to a silky smooth motor, simply great(!) - evidence that perhaps some fitting somewhere did have a minute air leak and some vacuum line probably did as well. The effect of any leak could have been amplified by altering the behavior of a thermo valve here or an actuator here, etc., etc. until the motor gradually lost the smoothness it once had, while retaining the solid and even feel.

Anyway, I am very happy with the results.

Cheers

__________________
Michael Stoic

1979 450SLC
1989 300SE