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7 Symptoms Of A Bad Valve

At the heart of all our vehicles, beating the drum day in and day out is the engine. A precision engineered feat of genius that starts up and runs everyday. 

But, sometimes things go bad inside of the engine. Unlike brakes, or tires that are easy to see when you have a problem. Internal engine components are not so easy to see. If you are unfamiliar with what your engine should sound like, abnormal sounds and noises are masked by normal engine noises. 

Let’s take a look at the most common symptoms of a bad valve  and how much an engine valve repair will cost if your vehicle starts to show one of these symptoms. 

Common Symptoms Of A Bad Valve

Here are the most common symptoms of a faulty cylinder head valve. 

Loss of power

The valves inside of your engine allow air/fuel to enter the engine when required and seal the combustion chamber to allow for the combustion process to occur. 

If you have a valve that is not sealing correctly it will not allow the cylinder to seal correctly. This causes many issues but a lack of power can be very common.

If you have a burnt exhaust valve, when the cylinder is moving up it will force the air/fuel mixture out of the cylinder. This will cause your engine to not have enough fuel resulting in a loss of power. 

Imagine if you had a campfire and you removed half of the wood. The fire would start to reduce in temperature and height. This is very similar to what is happening if you have a burnt exhaust valve.

Indicator light (CEL) illuminated

If you drive a modern vehicle, the indicator light turning on in the dash is very common when you have a bad valve. If you have a faulty engine valve it will cause the engine to misfire. 

The Powertrain Control Module (PCM) monitors the engines condition at all times. Once it notices that your engine is not running correctly, it will turn the indicator light on in your dash. 

If you notice this light come on, read the codes with a code reader. Or, make a visit to your local auto parts store. They will generally scan a vehicle for customers at no charge. 

engine smoking out the tailpipe which is a symptom of a bad valve

Blue smoke from the tailpipe

This symptom is found when starting your vehicle after it sits for a period of time. A blue smoke coming from your tailpipe is caused by a faulty valve or valve stem seal

A valve seal is a seal that prevents oil in your valve cover from leaking into the combustion chamber. Over time if your valve becomes worn or the valve seal fails this oil can make its way into the combustion chamber. 

When the vehicle sits for a while this oil will build up inside of the engine. This is what causes the blue smoke when you first start the engine. 

If you see blue smoke on your car when starting, get it repaired as soon as you can. Burning too much oil can cause your engine to run low on oil, if low enough it can also cause engine failure. 

Rough idle

A bad valve can cause your engine to shake or vibrate at idle. It is normal for an engine to run smoothly at all engine speeds. 

If you raise your hood and the engine is shaking all over the place, you may have an issue with a bad engine valve. 

Engine backfire

This is the most alarming of the symptoms of a bad valve. Engine backfiring can happen in the intake or exhaust depending on what valve is damaged. 

A faulty intake valve will make the most concerning noise. A backfire occurs when the valves are not sealing correctly. If the intake valve is not sealing correctly, compression pressures and gases will be released back through your intake. 

This release of those pressures is what causes the loud pop, pop noise that is common to engine backfiring. This symptom is rare but if it occurs it is very noticeable.

Close up of a cylinder head showing carbon buildup from a bad valve.

Engine misfire

When the valves do not seal correctly, an engine misfire can occur. This symptom is generally a cause of other symptoms of a bad valve that you may notice. 

A check engine light that is illuminated or a rough idle are only a few of the symptoms that can be caused by an engine misfire because of a faulty valve. 

Engine smoking

A more uncommon symptom of a bad engine valve is smoking while driving. This is generally caused by a valve oil sealing issue, very similar to blue smoke on start up. 

If you have a valve sealing issue that you do not get fixed, it can eventually cause the vehicle to smoke at all times. This causes valves to carbon up and can cause more valve sealing issues. 

Close up of cylinder head that had a bad valve that broke.

How Do You Test A Bad Engine Valve?

While there are many problems on your car that you can test quickly and easily. A bad engine valve requires a few special tools. If you have these tools you can do the testing on your car to determine if you have a bad valve. Here is the process below of how to test a bad engine valve.

Cylinder leak down test

  1. Borrow or purchase a cylinder leak down tester. 
  2. Remove the spark plugs from the engine and rotate the engine to Top Dead Center(TDC). It is important that you are at TDC with both valves closed when testing. If you are not, it can lead to incorrect results. 
  3. Attach the spark plug adapter that works for your engine. 
  4. Hook up the compressed air and set the tester to zero using the steps provided in the operators manual for the cylinder leak down tester. 
  5. Complete leak down test. Make sure that you write down the results from the test.

The cylinder leak down test is the best way to test a bad engine valve because it checks valve sealing. Because a misdiagnosis at this step can cost you thousands of dollars in unneeded work. If you are not comfortable with this test, ask someone experienced with this test to confirm your test results. 

Having the best auto repair manual by your side will help you interpret these results. 

A closeup of a engine valve being removed from a cylinder head

How Much Does It Cost To Replace A Bad Engine Valve?

On average, expect to spend from $1000 to over $5000 to replace a bad engine valve. Parts will range in cost from $100-$500, with labour costing you anywhere from $800 to thousands of dollars.

With some mechanical knowledge and simple tools you can remove the cylinder head yourself, send it to a professional to replace the valve, and save yourself some money. 

To get a better idea of the costs you will need consider the following things:

  • Make and model: Depending on the vehicle that you have it will change the cost of doing this repair. Luxury vehicles generally have higher parts cost that an economy vehicle. 
  • Engine: This is the biggest factor in labour costs. A four cylinder engine will cost much less than a V-8 engine for example. 
  • Labour costs: Labour costs add up quickly when you can pay up to $160 per hour to get your vehicle worked on. These costs change from country to country and even city to city. But remember, do not shop on labour cost alone, you want a technician that is skilled in these types of repairs. 
  • Aftermarket vs OEM parts: Using aftermarket parts can save you money on the repair costs. These parts are built to meet or exceed OEM specifications. Depending on your engine, there may be no aftermarket parts available. Shop around to get the best deal.

Frequently Asked Questions (FAQs)

Can I drive with a bad engine valve?

It is possible to drive with a bad engine valve, however it is not suggested. If you continue to drive with a faulty valve you can cause more damage to your engine. By making regular inspections you can prolong the life of your engine and your engine’s valvetrain.

Can I fix a bad valve myself?

Yes, with the right tools you can replace a valve yourself. There are some special tools that you will need to complete the job. These tools are not common in the home garage. If you are looking to save some money on the repair we suggest removing the cylinder head and sending it to a machine shop.

This way you save some money on the labour and the special tools needed to repair the valve are not needed because the engine machine shop completes this repair.

Are bad valves preventable?

No, it is not easy to prevent damage to engine valves. The best way to prevent damage is to make sure your engine is running properly. If your cooling system, fuel system and other engine components are working correctly it reduces the chance of a bad engine valve.

How often should I replace my engine valves?

Replacing engine valves is not a repair that you need to complete. Checking valve clearances or valve lash is recommended maintenance but not replacement. 

John Morris
John Morrishttps://autoknowit.com
John Morris is the technical editor for AutoKnowIt.com. His years of experience in automotive repair as well as an automotive professor have prepared him to ensure that even the most technical information is accurate and concise at all times.

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