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Mercedes-Benz C-Class and C-Class AMG: Why is My Engine Knocking?

What could be causing your engine to knock? Aren't all Mercedes supposed to be smooth and quiet? Unwanted sounds are a pest, especially scary and evil engine related ones. Let's take a closer look at your C-Class, and get to the bottom of this issue.

By Jerry Perez - June 9, 2015

This article applies to the Mercedes-Benz C-Class and C-Class AMG (2007-2014).

Mercedes-Benz engines are regarded as some of the most highly advanced and reliable in the world. Even though most would say that an engine is only as reliable as the maintenance it's given. Sometime failures happen for no good reason at all. These failures can cause unwanted sounds such as knocking. To add to the issue, finding the source of the knocking can be quite tricky. We've compiled a guide with the most common reasons for engine knocking in the Mercedes-Benz C-Class.

Step 1 – Pumping the wrong gas?

It could make the engine knock.

Most people would think that requiring a high octane premium gasoline is just a measure of telling if it's a fancy car or not, but that isn't true. Just like using the right viscosity engine oil, using the right octane fuel is crucial for optimum performance. Mercedes engines are engineered to operate at a minimum of 92 octane fuel, and failing to do so could clog, damage and even throw the valves off-timing. Use the right octane fuel as noted on your owner's manual for optimum performance.

Figure 1. Choosing fuel might be confusing, but always go with what your manual says.

If that doesn't solve the knocking sound, then attempt the following Step.

Step 2 – Is the sound occurring during start up?

It could be the upper main bearing.

At the very moment when you start your car, lots of different components get put through certain actions that result in ignition. A couple of those parts are called main bearings. Main bearings exist in different sizes and create a certain pressure when the engine starts. It is known that on certain C-Class vehicles Mercedes has equipped them with the wrong size bearings, resulting in not making the right amount of pressure at start up. Keep in mind, this knocking sound would only happen when you start the car.

Figure 2. Mercedes-Benz engine bearings.

If the engine bearing isn't the root of the issue, continue with the next Step.

Step 3 – Is the oil pressure low?

Could the oil pump be failing?

Sometimes during a cold engine start up, the oil pump doesn't get enough oil in the engine to properly lubricate it. This is a good reason why you should always let your car idle for a couple of minutes before driving it. If the engine knock is happening during cold starts only, it could be a troubled oil pump, oil filter or even just low oil. Correct this immediately, as this could lead to severe damage.

This may be a bit tricky, as Mercedes-Benz does not equip their cars with an oil temperature or oil pressure gauge. However, aftermarket OBD-II (On-board Diagnostic port) scan tools can often read both temperature and oil pressure. Good oil pressure for most engines is between 30-50 psi once warmed, and oil temperatures should be between 180-240 F.

Figure 3. A Mercedes-Benz oil pump.

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