The Fifteen Dollar Diesel Tune-up

With over 150,000 miles on the 30 year old OM617A turbo diesel, there has to be some build up in the fuel system. Many Mercedes diesel owners have spent countless days tweaking the performance of their engines and many talk about the benefits of a diesel flush. It looked easy and inexpensive (two of my favorite things) so thought I would give it a try. The entire process took less than an hour, gave me more power, smoother running engine, less valve noise and a more responsive engine. 

Some folks advocate removing the injectors and cleaning or replacing them rather than the Diesel Purge. Here is my thinking: The purge will not fix a clogged injector, nor will it clean as well as removing them and cleaning them. But even if you do clean or replace the injectors, there is still carbon build up in the prechambers and fuel delivery system. This will clear the ENTIRE fuel system and unless you are doing a rebuild, a diesel purge is a great option. 

The product I used is called Liqui Moly Diesel Purge available from just about auto shop or you can order it from Amazon for about $15 and easy to find for even less. If you are comfortable under the hood and understand fuel systems, skip the rest of this blog and follow the instructions in the video to your right. Warning it is a little bit of an ad, but does provide good information. 

If your under the hood skills are limited to checking and topping off the oil, I've got step by step instructions that will work on just about any diesel engine. I'm using the OM617A that is in my 300GD for an example. 

A little background first. The diesel fuel is pumped from your tank, through a filter (maybe 2), through a throttle that modulates the amount of fuel, then eventually to the injectors that feed it into each cylinder. Now the injectors can't use all the fuel so some is fed back into the fuel tank. So the entire system is kind of like a loop with the engine sipping off what it needs. 

OM617 Fuel Lines

Here is a quick summary of what we will be doing:

1. Disconnect the fuel line going into the filter - this is where the Diesel Purge needs to go into the system.
2. Disconnect the fuel return line going back to the tank - this will need to be fed back into the Diesel Purge bottle. 
3. Take the line you disconnected in step one (the one that came from the tank and fed into the filter) and plug it into the line going back to the fuel tank. This will keep the lines from getting air and the pump from spilling fuel all over the place while you purge the system. 

Before you begin you will need:

1. A can or two of Diesel Purge
2. A 10" tube of rubber fuel line hose
3. A clear plastic bottle - large enough to hold all the product.

NOTE: During the process you don't what to run the engine while there is no fuel going into the system or you will need to prime the fuel system to get the air out. 

I also suggest warming up the engine before starting this process. Things just seem to clean better when they are warm, right? 

When disconnecting the fuel lines you will have to loosen the clamps with a screw driver or clamp remover. No need to tighten them during the purge but remember to tighten them back up when you put everything back together. 

Lets start by removing the line coming back from the injectors. In the OM617 the return line comes from the injector closest to the front of the engine and runs to the top of the filter. From there it feeds back to the tank. Remove the hose that feeds the tube going to the tank so that you have about a foot or so of hose hanging loose. I like to point the hose up to reduce the amount of fuel that will leak out.  

Return Fuel Line Removed

Then disconnect the line going into the primary filter. This line is coming from the fuel tank and supplies the diesel fuel to the system. If this primary filter is not present you should install one. In cold climates, this filter will become clogged first when it gets cold with the waxy diesel and is easy to change. 

Primary Fuel Filter Line

Then connect it to the return line going back to the tank. You will still have the return line you disconnected before sticking up in the air. This is important because when you start the engine a fuel pump will pump fuel from the tank. By hooking it back to the return, it will simply be pumped back into the tank and not spill all over your garage.

You can feed those tubes directly into the can of Diesel Purge but the metal bottle makes it hard to see how much purge is left. By using a clear bottle or glass jar, you can see how much purge is left and cur off the engine prior to in running out. This keeps air out of the system.  

If you use a plastic water bottle, make sure is is DRY and clean. Then cut a hole in the top and feed the return line coming from the injectors into the bottle. Then take the 10" extra fuel line you purchased and connect to the small filter to supply the engine. Make sure the second line going to the filter stays submerged and rests in the bottom of the bottle. The return does not need to be inserted to far into the bottle. Wedge the bottle in a safe place so it stands up. 

Once everything is hooked up, turn on the engine and let it idle for a while. Once you are sure it is working, increase the throttle from under the hood and run the purge through the system at a verity of RPMs. If you are using a clear bottle, you may notice it getting a little dirty. That is the carbon build being removed up BEFORE the injectors, just think of what is being cleaned from the injectors and the pre-chamber!

Purge System in Place

As the bottle becomes empty, shut off the engine remove the return tube and use a funnel fill the bottle with more purge. If you have not done this before, use two full bottles of product. The entire process takes about 10 minutes per bottle. 

Remember to stop the process before the product runs out. I play it safe and shut off the engine with about a 1/4 inch of purge left in the bottle. 

When you are done, reconnect all the fuel lines as they were connected before and test you engine. It is a good idea to change at least the primary fuel filter now as it will have collected some carbon residue. Go for a test drive now and you will notice less smoke, smoother running engine, more power better cold starts and a quieter idling engine. All of this for $15 and about an hour of shop work.  

As you can see this process is pretty easy. I'm planning on doing this about every 12 or 18 months just to keep the engine in good shape.