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

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Michael Stoic

1979 450SLC
1989 300SE

Thanks for posting your information. I just bought a 1989 300SE so I'll be checking these same things on my car.

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Regards

Warren

Currently 1965 220Sb, 2002 FORD Crown Vic Police Interceptor

Had 1965 220SEb, 1967 230S, 280SE 4.5, 300SE (W126), 420SEL

ENTER > = (HP RPN)

Not part of the in-crowd since 1952.

[QUOTE]Originally posted by Michael Stoic
[B]
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?

Hello Michael Stoic,

I believe your 300SE is close to my 260E 1988, I changed both valves you mentioned, the one in the engine compartment and the one on the right rear side and it goes to the tank. It is connected with two times 5cm of fuel hoses.
Very easy job and the part is very cheap.

What were your reasons for changing the Fuel Tank Purge/Vent Valves?

What was the car doing before you changed them?

Did the new FT Vent Valves change what the car was doing?

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Michael Stoic

1979 450SLC
1989 300SE

My car was having unstable and irregular idle, plus loosing power when driving or riding like new, absolutely impossible to produce/simulate the default.

See my post "loss of power while running"

I realised that there is a very complex vacuum management system. I thought my problem could be vaccum related, then I started changing these valves (which fortunately are cheap), the hoses to the Idle control valve as well.
Now I have a very smooth idle, less stalling but the problem is so erratic that I do not dare to say that it is over.

I was hoping you found a fix for the excessive FT pressure (assuming your car had that condition) by replacing the FT Purge/Vent Valves.

Were you also trying to diagnose that specific problem? It sounds like you replaced some items because they were old etc., while chasing another condition.

I'm still after the Cause/Effect in the excessive (sounds excessive to me) FT pressure department and whether this condition might cause the Fuel Pressure Regulator diaphragm damage.

I'd like to find the procedure to test the FT Vent system (FT, Purge Valves, Charcola canister, vaccum interplay, etc.).

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Michael Stoic

1979 450SLC
1989 300SE

1990 300E

Where is the fuel pressure regulator located? Any picture?