They say you can’t tune up a diesel engine. They mean that you do not tune a diesel engine the same way you tune up gasoline engines. There are no spark plugs or ignition coils to replace; we often think there is nothing you can do to tune a diesel engine.

This guide will show you how we turned a neglected and loud engine into a quiet, smooth-running engine.


Here are the problems this diesel engine had:

  • Poor Throttle Response
  • Black Smoke
  • Hard time starting in cold temp.
  • Engine sounds loud

The most important tip for owning a diesel engine is to perform regular oil and filter changes. When you buy a diesel car or have one in the shop that you need to work on, you often have no records of past maintenance history.

That was the case with a Mercedes R320 CDI we recently worked on. This past week I was working on a  diesel engine that had been neglected for over 164 days.

neglected diesel engine

Looking at the oil, was extremely dark, and it felt more like sand than liquid. It looked like the oil hadn’t changed for a long time.

change engine oil helps tune diesel engine

Our challenge was to make this great car run smoother and quieter. Because it is a diesel, it doesn’t have to sound like a tractor, but this R-Class did. The first thing we did to this car was an oil change. You can see how dirty this oil was.

Ideally, I wouldn’t add any additives to the oil. Since this car had a lot of black smoke and some built up in the engine, we decided to run a couple of oil and fuel additives.

The first one that we used was Stiction Eliminator. It is designed to restore engine power and compression. It also protects injectors and rings. It is a synthetic blend of additives. Our goal is to let it do its jobs or remove stiction/sludge for about 1000 miles.

tune diesel engine additive

We mixed half a quart of the additive with 6 quarts of engine oil.

diesel stiction eliminator oil additive chart

Because this car hasn’t had an oil change for a long time, it will get an oil change a lot sooner than recommended so that we can remove as much sludge and contamination from the engine.

diesel tune up box

Next on the list are the fuel injectors. Yes, you can remove and rebuild them, but we are not at that point yet. We are looking for a cheap way to tune up this diesel engine. The primary concern with diesel engines is that the injector tips can get dirty, reducing fuel delivery to the cylinders and, therefore, performance.

After spending several hours researching fuel injector cleaners, we tried Hot Shot’s Fuel Additive. It is designed to clean internal diesel injector deposits (IDID), including waxy and polymeric deposits.

diesel fuel additive tune up diesel engine

This diesel fuel additive also includes a cetane boost and fuel stabilizer. You would mix about 4 oz with 10 gallons of fuel.

diesel tupe up additive chart

The bottle comes in 32 oz, which means it will take a month or two for you to use the whole bottle, depending on your driving. Add it to the fuel tank right before filling up the tank.

diesel engine tune up procedures

With these two simple steps, this diesel engine is running much better. But we are not going to stop here. We changed the engine air filters to give this car some fresh air.

change engine air filter to tune diesel

Next on the list is to change the fuel filter.

Lastly, we are planning on changing one of the motor mounts.

The driver-side motor mount has completely failed. Causing vibrations in the seats and steering wheel when the car is driving. You may be tempted to think that is normal because you are driving a diesel, but that’s not true.

Check the motor mounts. If your diesel engine is not running smoothly, keep tuning it until it gets there. Once you have changed the oil, oil filter, air filter, and fuel filter and may be used a couple of additives, and your diesel still runs rough, it may be the time to consider rebuilding the fuel injectors. Another common problem that can cause poor engine performance on diesel engines is the fuel pump.

We hope you find the How to tune a neglected Diesel Engine guide helpful. Check these troubleshooting and repair guides for more help on your vehicle.

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