One of the most alarming sounds you can hear from your GMC Sierra is the engine knocking. Unfortunately, an engine knock can be a sign of significant engine problems.
The first thing you should do is check your Sierra’s oil. If it’s low, fill it up. If it appears frothy, there may be coolant getting into it.
This guide is designed to help you assess what is causing the knocking sound in your GMC Sierra. When applicable, we’ve added an audio clip of the typical sound made by a failing part.
Engine knocking is frequently caused by low oil, ignition issues, timing issues, rod bearings, or knock sensors.
Engine Knock Diagnosis: GMC Sierra
Before you do any looking around, is the check engine light on?
If it is, it’s time to get the diagnostic codes from the ECM and use those to help you find the problem. If you don’t have an OBDII scanner, the local parts store will usually scan for codes for you as a courtesy.
If there’s no check engine light, going off of the sound type can help you deduce why your GMC Sierra is knocking.
GMC Sierra Engine Knocking Causes
Here are a few of the most common issues that would cause your Sierra’s engine to knock:
Low Oil
If your GMC Sierra is low on oil, it’ll cause a harsh knock as the engine is starved for oil. There are three major reasons why oiling issues will cause your Sierra to knock.
The knocking sound isn’t as deep as throwing a rod, but it’s very noticeable.
- No oil at all
- Lack of oil pressure (oil pump, worn engine, or sludge buildup from infrequent oil changes)
- Water or coolant in the oil.
Bad Spark Plugs
A prevalent reason that your Sierra’s engine will knock is bad spark plugs. Spark plugs are vital to proper engine combustion. There must be a solid spark to achieve clean combustion. When the spark plugs aren’t burning right, your truck will spark knock.
Over time, spark plugs can fail for a few different reasons:
- Wrong Plug– If you aren’t using the spark plug recommended by GMC, it would be a good idea to go ahead and swap them for the correct plug. The wrong plug can burn too hot or cold, which has a direct effect on the combustion and cause engine knocking.
- Worn-Out Plug– Spark plugs can wear out over time. You need to replace the spark plugs at the recommended intervals. If you have high mileage or the plugs look worn, go ahead and change them all.
- Improper Gap– Almost all modern vehicles take pre-gapped spark plugs. But you should still check the gap when you install them. Now and then, you’ll be surprised by one. Check your gap if you’ve recently had your Sierra’s plugs replaced and you’re now hearing a knock.
- Spark Timing– All of the internal parts of your GMC Sierra’s engine, as well as the sensors and computer, coordinate the engine timing so that the spark plug fires at precisely the right moment. If something is off with this timing, the fuel will fire at the wrong time, and there can be a knock.
Spark plugs are the most common part of the ignition system that can cause your Sierra to knock, and definitely where you should start looking. But there are other parts, such as the coil packs and plug wires (if equipped), that you should be looking at.
The coil pack sends a spark to your truck’s plugs. If they send a weak spark, the air/fuel mixture will not be able to properly fire, no matter how good the spark plugs are.
See also:
Timing Chain Jumped Symptoms and Diagnosis
GMC Sierra: Bad Timing Belt or Chain Symptoms
Rod Knock/Worn Bearings
A rod knock is perhaps the easiest knock type to identify. It’s deep and loud.
Rod bearings exist in between the crankshaft and piston connecting rods. When they wear out, the space between the rod bearings and connecting rods grows. As the crank turns in a circle, the connecting rods bang into the bearing and make a deep knocking sound.
If your Sierra has a sound coming from it like the sound above, the engine will need a rebuild or replacement.
Other Causes
Here are other knocking causes, but they aren’t as common as the ones listed above.
Carbon Deposits
Carbon can form at the top of your Sierra’s combustion chamber. This effectively increases the compression ratio. Too much compression can lead to detonation.
Modern fuels are required to keep carbon cleaning detergents mixed in with them. Even so, deposits still build, leading to a knocking sound.
If this is the case, you will need to bring your Sierra into a shop to have it cleaned. You could also try one of those cleaners in a bottle, although we recommend getting it to a professional.
Wrong Octane/Bad Gas
Bad fuel can cause your Sierra’s engine to knock. The wrong fuel can also cause a knocking sound.
Using fuel with the proper octane rating is essential. If you need to be using premium fuel, use it.
Air/Fuel Mixture Too Lean
Sensor issues can cause the air/fuel mixture to lean out. If the mixture gets incredibly lean, it can cause your Sierra’s engine to knock. Bad fuel injectors, oxygen sensors, MAF sensor, and more can cause a lean air/fuel mixture.
Knock Sensor
Modern engines use a knock sensor to correct knocking automatically. If the knock sensor goes bad, then the motor can knock. A bad knock sensor will often throw a trouble code, such as P0325 (knock sensor “1”).
Exhaust Leak
An exhaust leak at or around the exhaust can make a noise, often mistaken for a knocking sound.
Conclusion
Above, we identified the most common causes of knocking in the GMC Sierra. Most of them are serious engine problems, so if you can’t figure out what’s causing them, we recommend bringing your truck to an experienced mechanic.