Your car’s engine contains a collection of valves working together to let air into the cylinders and exhaust gases out. When one of the valves runs too hot, it can become burnt, resulting in inadequate sealing of the cylinder and a loss of compression.
If the engine suffers from a burnt valve, you’ll likely notice one or more troublesome symptoms that you’ll want to address right away.
What is a Burnt Valve?
An internal combustion engine has one or more camshafts that open and close a series of valves. The valves are located in the engine’s cylinder head(s) and held in position by guides. Each of the cylinders typically has two to four valves, depending on the engine’s design.
There are two types of valves: intake and exhaust. Intake valves allow air (and fuel, in the case of port injection) into the engine’s cylinders, whereas exhaust valves allow exhaust gases to exit.
Simply put, a burnt valve is a valve that has been damaged by excessive temperatures. The extreme heat will often cause the edges of the valve to burn away. In some cases, the valve may experience thermal shock, causing a portion of the valve to crack or break off. In any of these scenarios, the cylinder with the burnt valve will experience a loss of compression.
The exhaust valves are constantly subjected to extremely hot exhaust gases. Also, unlike the intake valves, the exhaust valves don’t benefit from cool air entering the engine. For these reasons, the exhaust valves run hotter and are more likely to burn than the intake valves.
What Causes a Burnt Valve?
Any issue that creates excessive heat in a valve or cylinder head can cause a valve to burn. Some of the most common causes of a burnt valve include:
Valve Not Seating Properly or Sticking Open
When a valve sticks open or does not seat properly in the cylinder head, hot gases can sneak past the valve, causing it to burn. Common causes of improper valve seating include incorrect lash adjustment, valve and seat wear, improper machine work, and valve seat erosion. Weak valve springs, carbon deposit build-up, and improper valve stem-to-guide clearance can also cause the valve to stick open, preventing it from seating properly.
High Operating Temperatures
Extreme operating temperatures can lead to burnt valves. For example, cooling system problems, such as a low coolant level or defective water pump, can cause the engine to run hotter than normal, resulting in burnt valves. Localized high-temperature concerns (e.g., blocked coolant passages or a corroded head gasket) can have the same effect.
Engine performance problems
Burnt valves can also result from any type of engine performance problem that causes an increase in combustion temperatures. For instance, burnt valves can stem from detonation, a lean air-fuel mixture, or pre-ignition.
Burnt Exhaust Valve Symptoms
Do you think you might be dealing with a burnt valve? If your vehicle is exhibiting one or more of the following symptoms, you might be right.
Note: Because other problems can present the same symptoms as a burnt valve, you’ll want to perform a thorough diagnosis of the vehicle before performing any repairs.
Rough Running and Misfiring
A gasoline internal combustion engine needs three primary ingredients to run well: a precise air-fuel mixture, proper spark, and adequate compression. If a cylinder has a burnt valve, that cylinder won’t seal properly, leading to a loss of compression. The engine will begin to misfire and run rough as a result.
Illuminated Check Engine Light
Your car’s engine computer, which is often referred to as the powertrain control module, looks for problems, such as a misfire caused by an exhaust valve leak, that could lead to an increase in vehicle emissions. If the module detects such a problem, it turns on the check engine light and stores a corresponding diagnostic trouble code (DTC) in its memory.
How Much Does it Cost to Fix a Burnt Valve?
Replacing a burnt valve (or the entire cylinder head) is a costly and labor-intensive repair. If you choose to have a professional do the job, you can usually expect to pay somewhere between $1,000 and $2,500 to get the job done. Of course, the exact cost will depend on various factors, such as the year, make, and model of your vehicle.
Burnt Valve FAQ
How to Check for Burnt Valves
A professional will usually check for a burnt valve by performing a compression test, a leak down test, or both. A leak down test is usually the most accurate method. The test involves bringing the cylinder in question up to top dead center on the compression stroke (to seal the cylinder) and then adding compressed air to check for leaks. The video below explains how to perform this test:
If the cylinder head is removed—and you want to determine whether a suspect valve is burnt and leaking—you can perform a liquid leak down test. The video below explains how it’s done:
How to Fix a Burnt Valve
A burnt valve cannot be fixed—it must be replaced. In some cases, a machine shop may be able to replace the valve and recondition the cylinder head for you. In other instances, the cylinder head may need to be replaced. You’ll also want to address whatever caused the valve to burn in the first place to avoid repeat failure.
Can You Drive with a Burnt Valve?
You should not continue to drive with a burnt valve—additional damage to the vehicle may result. For example, a portion of the valve could break off, damaging other parts of the engine. Or a misfire (caused by the burnt valve) could harm the catalytic converter.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.