W123 • Fuel Pressure Relief Valve

Final check to insure a smooth idle... The Fuel Pressure Relief Valve. It's simple, quick and just might make all the difference in the world. 

It's hard to believe just how much bad information gets on the web. What's worse is how gullible we often are to buy into it without question or further research. 

On occasion I scour the web to see what bad information I can find on ways people suggest fixing particular automotive problems. After just a few minutes of reading or watching a Youtube video, it becomes evident most people never read the workshop manual. For 99% of any problem that will arise on your car, there is a corresponding fix from the manufacturer. No guessing required what so ever.

Unfortunately so many great older cars are being butchered from bad advice, laziness, or the assumption the manufacturer had no clue what they were doing from the start. Indeed there are manufacturers who built very unreliable products that should have never been put on a car, but as it pertains to Mercedes, there are few components that were engineered that did not perform near flawlessly.

With that in mind, if your diesel is still running rough with all the previously mentioned items checked or replaced, there is one last component to getting your W123 diesel to run like it did from day one. The Fuel Pressure Relief Valve.

STEP 1
Locate Fuel Pressure Relief Valve highlighted in green.

STEP 2
Remove valve. Fuel will run out so have a rag under the valve and/or a drip pan under the car. Note from photo wish bolt to loosen. 

Once the valve has been removed, it will need to be separated and checked. To separate, either hold with two wrenches or carefully place in a vice and loosen the top "bolt" cap. WARNING: Only loosen! Separate valve by hand and over a pan or rag. A spring and ball bearing are inside. 

Once separated, the is what you will see.

Mercedes-Benz Fuel Pressure Relief Valve from a 1981 240D

STEP 3
Make sure everything is thoroughly cleaned after taking this apart. It should already be very clean since fuel is constantly flowing through this valve. It can however attract sludge depending on how the car has been maintained. 

Once clean, measure the spring using a micrometer. When I measured the spring that was removed from my valve, it measured 19.76mm. According to the Mercedes-Benz worship manual it can measure up to 27mm max. Based on the max of 27mm, I gently stretched the spring to 25mm exactly, and reinstalled.

This simple procedure made the most drastic difference of anything I could have done. While the car was running much better with all the other adjustments and replacement parts, this "put the icing on the cake, as the video in my last post demonstrated.

Mercedes must have known this could or would become an issue since it is specifically mentioned in the manual. Regardless, it's worth the check to see if your spring has compressed over the many years in service. I'm sure it has, and could use a stretch.

I would not recommend starting off with 27mm however. Good luck.