Things that Cause Knocking Noise from Vehicle When Accelerating - Service Questions serving Sacramento, CA

April 23, 2019

Knock, knock! Who's there? Your car, and it's got bad news. If you hear a knocking or pinging sound coming from your Subaru vehicle's engine when you accelerate, it could be a sign that something is wrong. There are many things that could make this noise, from unevenly burned fuel (due to an improper air-fuel ratio) to unlubricated pistons. Below are four of the most common causes of an engine knocking noise. No matter what the cause, it's best to bring your Subaru to an authorized service center to have it inspected.

A close-up of a gas pump with three different octane levels listed: Regular (86), Plus (88), and Premium (91).

4. You've Used the Wrong Fuel

Your Subaru will have a recommended octane rating. If you fill it up with gas with an octane rating that's below the manufacturer's specs, you may hear a knocking or pinging noise coming from the engine. If you've filled up with lower-octane gas, you can try adding an octane booster to stop the knocking. Then, be sure to use the right gas next time.

A close-up view of a spark forming in the gap of a spark plug.

3. The Combustion Chamber Has Carbon Buildup

Even though gasoline has added detergents, it's still possible for carbon to build up on various components of your vehicle's engine, including within the combustion chamber. This can also be a relatively easy fix. There are fuel injector cleaners that can help break down carbon deposits.

A close-up view of an illuminated red Check Engine light on a dash.

2. Spark Plug/Timing Problems

If your vehicle's spark plugs have problems, they can create a knocking noise. If the spark plug gap is too narrow or too wide, or if you've replaced old spark plugs with incorrect new ones, this might be the source of the problem. Spark plugs can cause the fuel-air mixture to ignite at the wrong time, causing knocking.

1. Problems With the Knock Sensor

The knock sensor in your Subaru helps prevent knocking or pinging. It does this by sensing vibrations that a knock sends out, then letting the computer know so that it can adjust the timing in order to stop the knock. However, if the knock sensor has a problem, it can actually cause knocking. That's because it may think it senses a vibration where one doesn't exist. It will send the wrong signal to the computer, which could actually cause the engine to knock.

Because it's hard to know exactly what's causing the knocking, it's best to bring your Subaru to an authorized service center.

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