My wife’s S420 has 112K miles and developed a problem a few weeks ago. When the car was cold it started easily, and she could drive it without any problems. But when she stopped and turned the engine off for a few minutes, and the engine was still warm, it would start with hesitation then immediately stall when put into gear. After starting and stalling a few times, it would not start at all until the engine cooled down for a half-hour or so.

In trying to diagnose the problem I noticed that there was a lot of pressure built up in the fuel tank and there was a loud “whoosh” when I removed the gas cap. With that as a clue, I started searching for these symptoms and found some links on the mercedesshop forums:

98 S420 (W140) Hard starting

http://www.peachparts.com/shopforum/tech-help/135594-how-remove-fuel-pressure-regulator-frm-98-s420-post997557.html?highlight=s420+fuel+pressure#post997557

According to these postings, the Fuel Pressure Regulator was the suspected cause of the problem. I found a drawing of the assembly and I am including it as an attachment to this message.

I purchased a replacement part online

To replace the regulator I removed the right-side air filter assembly. I don’t know if it matters but I also removed the gas cap to release the air pressure in the fuel tank.

Facing the front of the car, the Fuel Pressure Regulator is located at the right rear of the engine compartment. I removed the rubber hose. Then I used snap-ring pliers and a metal pick to remove the circle clip. I had to use regular pliers to ease the old regulator out.

I cleaned the surfaces with a Q-tip, then inserted the new regulator. I had trouble seating the new regulator into the housing so I gently applied pressure with a C-clamp, cushioning the C-clamp where it touched the regulator with a rubber washer. Then I replaced the circle clip using the snap-ring pliers. I reconnected the rubber hose.

Before replacing the air filter assembly I started the engine and checked for fuel leaks or other problems. Everything looked OK so I let it run for 20 minutes until it was warm. I turned the engine off and let it sit for 10 minutes and then restarted it. There was a slight hesitation but it did not stall when I put it in gear. I replaced the air filter assembly and cleaned up.

Later I drove the car on several short errands and it never stalled or hesitated when restarting. My wife has also driven it without any problems. It seems like the problem has been resolved.

Thanks to smk_texas, JimF, A. Rosich, and M.B.DOC for their prior postings on this problem.

Attached are photos showing the location of the Fuel Pressure Regulator and the tools used to replace it.

Attached Thumbnails