I am going to be changing the Fuel Pressure Regulator on my 1989 300SE, M103. This is the FPR which has four connections: #1 & #2 - the High Pressure Line (in/out) - both connect to the threaded fitting metal fuel lines; #3 - the fuel return line, smaller diameter threaded fitting metal fuel line; and #4 - a vent hose, connected through push-on rubber fitting.

1) My main question concerns how best to relieve fuel pressure to minimize the amount of fuel I get out of the #1 & #2 high pressure line connections. I do not have means to raise the car and get to the fuel pump, so anything I do, I'll have to do from the motor.

2) In case the pressure is not relieved, how much fuel can I expect to spill when the high pressure connections #1 & #2 are released?

3) After the reassembly, is there a proceedure to purge the air from the fuel system? Is this a must?

4) Finally, does removing the FPR on an M103 give one access to any other air passage hoses or vacuum connection fittings that are easiest reached with the FPR removed? Nothing there is "broke," that I know of, but if access to them is otherwise obstructed and the car having 142K miles...

Many thanks for your comments!

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Michael Stoic

1979 450SLC
1989 300SE

It's a really simple procedure. You'll need a couple of 17mm open end wrenches, 10mm and 12 mm open ends and a rag to catch the gas. There won't be a whole lot of pressure on the return line so crack it open 1st, remove and plug with a golf tee. The rest is straightforward. You can get a good view of the front idle control valve hose while you have it off. Wipe up the gas, let it dry and you're ready. Have someone else start the car while you watch for leaks.