W123 Diesel • Rough Idle

If you have come this far, you either own a W123 diesel or are interested in acquiring one. In either case, there is no need to settle for a rough running diesel Mercedes.

If you have a W123 diesel, 4 or 5 cylinder, there is no doubt they vibrate and are certainly louder than modern diesels, but that is no reason not to make sure your Mercedes is not running as smoothly as possible. 

When I acquired my 1981 240D, and not being familiar with just how these diesels were to perform, I was more forgiving than I am now. Regardless, the following are a few very important items to look at and know about.

Let's work from back to front to find out why it might be running rough.

• Is the fuel tank filter clean or new? If you don't know it's time to find out and most likely replace it.

• Are the rubber connecting lines from the fuel tank filter and along the way up to the front of the car new? If not they are most likely brittle and can easily be drawing air in to the system. Even if they are not leaking they can be allowing air in.

• The two fuel filters, primary and secondary, in the engine bay also need to be changed at specified intervals. 

• Check the plastic fuel lines that are probably no longer clear but dark brown. These too can become brittle over time and begin to allow air into the system.

• The primer plunger must also be checked. If you still have the all metal unit it is most likely leaking and should be changed for the the newer Bosch unit. If you have the newer plunger, depress it 6 or so times and see if it leaks around the base of the plunger. If it does, you are drawing air in to the system from there. 

• Another important factor are the injectors. How long has it been since they were rebuilt or replaced? Again, if you don't know it is time to pull those and have them tested, and if need be rebuilt. Also make sure they are not leaking. If they are, air will be introduced into the system at that point as well. 

• Check the air filter!

• Make sure all the hard injector lines are tight at the pump and injectors. Do not over tighten!

Image: 1981 Mercedes-Benz 240D

Once all these items have been checked and or replaced, you should notice a major difference in performance and smoothness of operation. I also recommend running a lubricant/cetane booster in every tank. With the growing distribution of low sulfur and ultra low sulfur  (LSD/ULSD) diesels, the older pumps are not sufficiently lubricated and require additives to reduce excessive wear. 

I would recommend Red Line 85 Plus Diesel Fuel Additive. This also keeps the engine running much smoother and appears to provide a touch more pep. Every little bit helps. If you are not running additives in every tank to help keep water and sludge from clogging up the system, your entire system might be in need of a good cleaning as well. A very good cleaner I use every 6 months is Liqui Moly Diesel Purge.

It should be noted, that there are many other factors that can cause rough idle such as poor compression, worn timing chain and injection pump timing.