7 Symptoms of a Bad or Burnt Intake and Exhaust Valves

Valves are part of the engine that helps with the combustion process. There are two types of valves. The valves that allow the air and/or fuel to enter the combustion chamber are called intake valves. The valves that are allowing the exhaust to leave the combustion chamber are called exhaust valves.

Both valves play a very important role and if one of them malfunctions, the engine will not work as it should. Luckily, there are signs of bad or burnt intake and exhaust valves that you can look out for.

Symptoms of a bad or burnt intake and exhaust valves include a decrease in power and acceleration, trouble starting the vehicle, poor fuel mileage, burning smell, rough idle, valve noise, and backfiring.

Key Takeaway

7 Symptoms of a Bad or Burnt Intake and Exhaust Valves

  • Decrease in power and acceleration
  • Trouble starting the vehicle
  • Poor fuel mileage
  • Burning smell
  • Rough idle
  • Valve noise
  • Backfiring

7 Symptoms of a Bad or Burnt Intake and Exhaust Valves

7 Symptoms of a Bad or Burnt Intake and Exhaust Valves

Here are the 7 most common signs of bad or burnt car valves:

1. Decrease in power and acceleration

7 Symptoms of a Bad or Burnt Intake and Exhaust Valves

One of the most common symptoms of a bad or burnt intake and exhaust valves is a decrease in power and acceleration. Usually, this is caused by burnt valves. Basically, when combustion gases escape between the valve and valve seals, this will cause the exhaust gas to erode the exhaust valves causing premature failure.

When valves are not sealing all the way, the cylinder compression will start to decrease therefore causing low engine power and acceleration. You can run a compression test to determine which cylinder is causing the performance issue. The compression test can also diagnose and determine if there are any worn piston rings, worn cylinder walls, and bad head gaskets.

Conduct the compression test on all cylinders and compare the readings. If the compression is low, make sure there is no blue smoke coming out from the exhaust which is an indication of worn piston rings and cylinder wall. If the blue smoke from the exhaust lasts only 10 minutes, that means that the valve seals are bad. When white smoke appears from the exhaust after a compression test, that is a sign of a blown head gasket.

However, if there is light white smoke coming out from the exhaust and smells like gasoline after the compression test, that is a sign that the valves are not sealing all the way and not all of the fuel is not burning in the combustion process. The unburned fuel will eventually cause the catalytic converter to fail. You should replace the valves to avoid premature parts wear.

Is your engine oil due for a change?

2. Trouble starting the vehicle

7 Symptoms of a Bad or Burnt Intake and Exhaust Valves

A bad or burnt exhaust and intake valves will often result in trouble starting the vehicle because of the improper ratio of air and fuel in the combustion chamber. The valves regulate the air-fuel mixture entering and exiting the engine. If they’re not functioning properly, the engine may fail to start at all or have difficulty igniting.

Combustion occurs in the engine of a car, where gasoline is mixed with air that has been drawn in from outside the vehicle and into the combustion chamber by the intake valve. When these two components are combined, an electrical spark causes them to ignite and form a powerful burning explosion. The exhaust gases from the combustion process are then released through the exhaust valves and the combustion chamber is ready for another cycle.

However, when the intake or exhaust gases are bad or burnt, it can lead to a lot of air being sucked into the combustion chamber, or not enough air. Both of these conditions will result in trouble starting the vehicle. In order for a combustion cycle to occur, a pre-determined amount of air and fuel is needed, no more no less.

3. Poor fuel mileage

7 Symptoms of a Bad or Burnt Intake and Exhaust Valves

The intake and exhaust valves of an engine play a crucial role in allowing the car to draw in air and expel exhaust gases, thus creating fuel efficiency. However, if either valve is damaged or dirty, it can restrict the flow of air, resulting in less efficient combustion of fuel.

Bad intake and exhaust valves can reduce fuel economy significantly because more fuel will be required for the same amount of power, meaning you’ll spend more money on gas than normal.

Basically, when the intake valve is bad or burnt, it can allow more air to enter the combustion chamber than needed. The vehicle’s computer will respond to this by spraying more fuel into the combustion chamber than needed, resulting in poor fuel mileage.

4. Burning smell

7 Symptoms of a Bad or Burnt Intake and Exhaust Valves

A bad or burnt intake valve can cause less air to get sucked into the combustion chamber which will result in improper combustion because the extra fuel won’t get ignited and you will feel a burning smell. For a perfect combustion cycle to occur, there should be a 50/50 air-to-fuel ratio in the combustion chamber.

If the intake valve is blocked, there will be more fuel than air in the combustion chamber. Not all of the fuel will be ignited and most of it will exit through the exhaust valve and into the exhaust system. This will result in a burning smell.

5. Rough idle

7 Symptoms of a Bad or Burnt Intake and Exhaust Valves

A rough idling engine is usually indicative of an underlying issue with the car. Rough idling occurs when the engine shakes, has low rpm and is struggling to remain running at a steady state.

A rough idle is often caused when the intake or the exhaust valves are bad or burnt and are unable to hold compression. This allows for more air to get inside the combustion chamber but also allows the air and fuel mixture to escape through the exhaust valve before getting properly burnt.

Keep in mind that rough idle can be caused by a number of things, but it really comes down to two main reasons: not enough air, a lot of air, not enough fuel, or a lot of fuel in the combustion chamber.

6. Valve noise

Bad valves in engines are more than just an inconvenience – they can be a source of very loud, noticeable noise. This happens because of the worn-down material on the valves and their inability to form a proper seal when closed.

When the valve is opened, air escapes from around the edge rather than remaining trapped inside. This leaking air causes pressure to build up, and as it rushes to escape it produces a loud whistling sound.

7. Backfiring

Backfiring in cars is when the fuel and air are not fully combusting inside the engine, causing some of the unburnt fuel to escape through the exhaust system. This can cause a loud bang or noise when this happens. Backfiring is often caused by an incorrect air-fuel mixture inside the combustion chamber.

The intake valves regulate how much air is present in the combustion chamber and if there is not enough air, the fuel will escape through the exhaust system unburnt, resulting in backfiring.

What Would Cause a Burnt Valve?

Some of the most common causes of burnt valve include clogging and contamination due to lack of frequent cleaning, corrosion due to harsh environments or chemical exposure, and warping and fatigue caused by rapid material changes in temperature or pressure.

1. Clogged and contamination due to lack of frequent cleaning

Clogged valves commonly occur when the heads of an engine haven’t been cleaned adequately or if they have been sitting idle for long periods of time without starting. This results in burnt valves.

Dust and dirt can collect on the valves and, if left untreated, build up until the valves become blocked. Of course, there are other factors which can contribute to clogged valves in engines such as water damage due to leaks or corrosion as well as carbon deposits caused by burning too rich a fuel/air mixture.

2. Corrosion due to harsh environments or chemical exposure

7 Symptoms of a Bad or Burnt Intake and Exhaust Valves

Valves are essential parts of the engine that are designed to facilitate air and fuel intake as well as exhaust release. Unfortunately, if left unchecked or ignored, these pieces can corrode over time, causing all sorts of problems. The corrosion itself happens because metal surfaces in the engine become exposed to oxygen, moisture and other environmental contaminants.

If these metal surfaces happen to have contaminants on them like acids from gasoline and oil residue, even more corrosion can occur. While avoiding this issue may seem difficult, there are some steps drivers can take to avoid experiencing the negative effects of valve corrosion inside their engines.

Regularly changing oil and air filters, avoiding low quality fuel sources, and taking care of any known issues could all be beneficial in helping preserve the condition of your valves over time.

3. Warping and fatigue caused by rapid changes in temperature or pressure

Increased engine temperature can be incredibly detrimental to your vehicle’s performance, and one possible result is burnt valves. This is because increased engine temperature causes thermal stress on your valves, which can cause them to warp, weaken and even crack.

As the pressure in your cylinders increases beyond the ability of your valves to seal it off, combustion gases are allowed to escape and cause severe damage. Ultimately, burnt valves lead to poor engine performance and a drop in fuel efficiency.

Can You Drive With a Burnt Valve?

If you are able to start your vehicle with a burnt valve, then you can technically drive it. However, keep in mind that driving with a burnt valve can result in serious damage. If you drive with a burnt valve, the engine’s combustion chamber will not regulate the compression of fuel and air in the correct manner, leading to poor performance and soot build-up on the cylinders due to improper combustion.

The purpose of intake valves is to allow clean, fresh air into the engine from outside the vehicle. Intake valves act as a gatekeeper into the engine, carefully regulating the amount and type of air that enters.

Without these valves, a car’s engine would not be able to run at its peak efficiency, as the combustion process relies on properly metered and oxygenated air for optimal performance.

Exhaust valves help to control the flow of hot gases exiting an internal combustion engine. These valves are designed to open and close in a careful sequence following the combustion cycle, allowing more or fewer exhaust gases depending on the engine’s need at that moment. Without proper functioning exhaust valves, engines fail to ignite properly and do not reach desired RPMs.

A burnt exhaust valve can cause the engine to overheat by not allowing the hot exhaust gasses to escape from the combustion chamber resulting in increased temperature. This is why you shouldn’t drive a car with burnt valves.

Igor Iwanowski

I am a certified Automotive Service Excellence (ASE) mechanic since 2018. I specialize in Brake systems, dashboard warning lights, EGRs, general engine problems, EVAP and Emissions issues.

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