Mercedes-Benz Mislabeled Parts

If you own an R107 (convertible) and need either a new fuel tank filler neck grommet (seal) or a fuel filler door vacuum lock, they have been improperly labeled by Mercedes.

107 Fuel Tank Filler Neck Grommet

If you order this part from any of the resellers or directly from Mercedes-Benz, for your R107, you will most likely receive part number 123 997 35 81. This is incorrect, as it is for a W123 and does not fit the R107. The correct part number is 107 997 24 81. This however does not end the confusion. This part number, although correct, is labeled in the Mercedes system to only fit the C107 (coupe) which is incorrect. It fits both the R107 and C107 cars. 

Yes, this seal is important so water does not enter the trunk during rain or washing. 

The photo shows an old R107/C107 seal and a new W123 seal. As you can see, very different design.

107 Fuel Filler Door Lock

Similar to the grommet, if you purchase the R107 fuel filler door vacuum lock for an R107, it will turn up an incorrect part. The correct part is 107 800 12 75 which once again shows it will only fit the C107 (coupe).

NOTE:
As with any purchase, make sure to check your individual chassis and part. Mercedes, like many manufacturers changed parts etc., throughout production.

If you are unfamiliar with the Mercedes part numbering system, the first three digits of the 10 digit number usually indicate the body/chassis production design, i.e., 123 000 00 00 is for a W123 (240, 300...) and 107 000 00 00 would indicate R107/C107 (350, 450 560...)

While Mercedes does supersede part numbers on occasion, it will still generally be superseded with in the same similar time period of production. For example a W116 might have a part number supersede to an R107 and visa versa. 

Mercedes W123 Brake Peddle Repair

With the introduction of automotive plastics in the 1970's, time is now and will continue to take its toll on these parts. 

A part that will more than likely fail if you own a W123, will be the plastic brake peddle bushing/retainer. Thankfully, Mercedes still has the parts to get the job done.

If you notice side-to-side movement in our brake peddle, it's time make the repair. By leaving this part unattended to can cause brake booster failure.

STEP 1
Remove lower dash covers

STEP 2
Unplug wiring as needed and lower cruise control module. You can let the cruise control unit hang to the side

STEP 3
Remove brake master cylinder and booster

STEP 4
Unplug brake light switch. This is a two part plug, but can be unplugged as one.

STEP 5
Remove brake peddle holder assembly. To do this, wiggle assembly back and forth while pulling toward back of car. It will take a bit of work to release the assembly from the dash brace and rubber buffer. Don't lose the rubber buffer, and check to make sure it is in good shape. You will want to make sure you have this rubber part as it will cause vibration and other noises if you do not.

STEP 6
Remove return spring from peddle and remove brake peddle from assembly.

STEP 7
Gently tap bushing into brake peddle with rubber or plastic mallet until fully seated.

STEP 8
Reinstall brake peddle to assembly.

STEP 9
Do not order the bushing until you remove the entire unit first. It appears some of the cars needed shims. If all is not very tight it will cause excess movement in the peddle and cause the new bushing to fail. 

STEP 10
Return assembly as it was removed.

STEP 11
Return booster, master cylinder and fluid reservoir

STEP 12
Refill system with approved brake fluid and bleed braking system. Make sure there is no air in the lines. You can make this process much easier and a one man operation if you purchase Speed Bleeder brake bleeders. Excellent product and excellent customer service.

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.

W123 Mercedes 240D Idle Test - Video

A well running W123 diesel should be very smooth. Over the years I have been told "they all rattle and vibrate, they always have, even new." 

I have had a hard time believing Mercedes-Benz would or could ever sell a car that vibrated as much as I have been told is "normal." Mercedes was up against high-end luxury cars in the United States and had to meet a fairly high standard of luxury and comfort regardless of whether or not it was a diesel.

With that in mind, I set out on a journey to see just what was necessary to make my 1981 240D run like Mercedes designed it to. With many checks and a number of maintenance items replaced, this car is once again running excellently and I am pretty convinced it is running close to factory if not at factory spec. 

It should be noted that the process of "tuning" should begin with a compression test. If that fails, no need to go further until that is rectified. 

The following test is usually performed on Rolls-Royce, but I thought it would be a great demonstration for a diesel, and the results truly surprised me. Prior to the work I performed, this glass would have bounced right off the valve cover.