7 Symptoms of a Bent Engine Valve: How to Test and Fix the Problem

Chances are you probably don’t spend a lot of time lost in deep contemplation about the state of your car’s engine valves. That is right up until the car starts shaking, you notice a massive drop in fuel economy, and/or the engine starts backfiring.

These are just some of the more common symptoms of bent engine valves. Unfortunately, it’s not a problem that is cheap or easy to fix. Especially if you let it persist for too long.

So, becoming aware of the symptoms of bent engine valves, and what causes them is a critical first step toward minimizing your repair costs.

What Are Engine Valves?

In an internal combustion engine, the engine valves play a critical role in allowing or restricting the flow of fluids like oil and gas to and from the cylinder’s combustion chambers. They are purely mechanical device that works with other components like the rocker arms in order to open and close at the correct time and in the correct sequence.  

Sometimes referred to as “Check Valves” engine valves also play a role in the air injection process as part of the emission control system. They also play a role in the engine’s exhaust gas recirculation system, which is part of the emission control system, as well as being involved in helping to keep the engine block within safe operating temperatures. 

This much interaction with so many critical engine components and systems places engine valves under a lot of stress. This is even more likely to be an issue if you’ve had overheating issues or low engine oil which are two of the more common causes of bent engine valves.

7 Common Symptoms of Bent Engine Valves You Can’t Ignore

Bent valve symptoms can manifest over time but are often pronounced. You often notice them after another serious mechanical problem such as the engine overheating, a timing belt failure or the car letting out a major backfire.

Here are seven symptoms to watch out for that may indicate that your engine valves are bent, and what they may mean for your car’s health:

1. Bent Valves Can Cause Your Engine To Start Backfiring

The Engine Starts Backfiring

Backfiring is one of the first signs that one or more engine values it bent. The deformation in the valve essentially changes how the exhaust gasses leave the cylinder. Unfortunately, the engine relies on the exhaust valves opening and closing at precise times to ensure a consistent level of pressure is always present within the cylinder.

So the bent valve ends up being in the wrong position, and the exhaust valve is essentially open while the air-fuel mixture is still igniting. This causes the miniature explosion to spill out of the cylinder, with a loud popping noise. Sometimes a severe backfire can make it all the way through the exhaust chamber to spill flames out of the tailpipe.

2. Engine Shows Signs of Running Rich

When the car’s ECU detects a problem in the fuel/air mixture caused by a bent valve it can sometimes attempt to overcompensate by increasing the amount of fuel that’s delivered to each of the cylinders.

This causes all the cylinders to run slightly rich, which then allows unburnt fuel to leak into the engine and the exhaust system. As the engine continues to run rich you might notice other symptoms of a bent engine valve manifesting as poor fuel consumption, fouled spark plugs, a rough idle, or even occasional misfires.

These are often symptoms of other problems, which might lead you to investigate other sources, such as a stuck-open fuel injector. When all along it’s a bent engine valve that’s causing the rich-running engine condition.

3. Low Engine Compression

Low Engine Compression

A bent valve essentially prevents the full compression needed to maintain vigorous internal combustion and the efficient redistribution of exhaust gasses. The resulting low compression environment in the affected cylinder(s) drastically hampers engine performance. Especially if you have multiple bent valves.

If you suspect that the engine being down on power is directly related to low compression due to a bent valve, you can confirm your suspicion with a simple compression test. You can buy test kits at an auto parts store or online for less than $50. Otherwise, a mechanic can perform a compression test as part of the diagnostic process.

4. Engine Feels Down on Power

The Engine Feels Down on Power

Low compression and poor engine performance caused by a bent valve will often make the car feel down on power. This is most prevalent when under load or when you need to accelerate hard. You might even notice the engine backfire as you step on the accelerator pedal right before the other cylinders with normal operating valves attempt to compensate to meet your acceleration demand.

5. Strong Engine Vibrations

Strong Engine Vibrations

Since engine valves play such a critical role in how the engine performs, it’s no surprise that things like small misfires, timing issues, and compression problems can lead to strong engine vibrations. If it’s a single valve that’s bent, the problem might feel more like a rough idle.

If more than one valve is bent, the vibrations can feel very aggressive. Depending on how other engine systems are affected, the strong engine vibrations will likely cause the ECU to turn the check engine light on.

6. Low Engine Oil

Low Engine Oil

Many times, a bent engine valve is caused by low oil or a fault in the oil system, like an oil leak or a failing oil pump. At the same time, a bent valve also allows fuel and sometimes oil to escape from the cylinders. While it’s only a minute amount with each piston stroke, the oil level gets lower and lower.

You’ll likely also notice a blue tinge to the smoke and possible symptoms of a partially clogged catalytic converter. This is due to the oil burning or smoldering in the exhaust system after it manages to pass out of the cylinders.

If you’ve been seeing other signs of a bent engine valve, and you see low oil on the dipstick, then it might be a bent engine valve or a problem with the oil system that caused the bent engine valve in the first place.

7. Check Engine Light

Check Engine Light

The performance issues caused by a bent engine valve typically cause the check engine light to come on. When this happens, you can hook the ECU up to a code reader.

There isn’t a specific code for a bent valve. So, what you’re looking for is some type of rich condition code, such as P0172 or P0175.

This might also happen in conjunction with other codes for fuel injector issues, misfires on specific cylinders that start with P030, or even a code P0420 for a catalytic converter problem.

What Causes Engine Valves to Bend? Uncovering the 6 Most Common Culprits

Bent engine valves occur when the valves themselves manage to make contact with the pistons or mechanical faults that cause them to get stuck in the valve guides. Though this is just the end product of a chain event due to one of the following underlying causes.

1. Low Engine Oil

Low Engine Oil

Low engine oil increases friction and heat within the engine, which can cause bent engine valves. All the moving parts inside an internal combustion engine need to be constantly lubricated to run smoothly with minimal friction.

A fault in the oil system such as a fuel leak or a gradual reduction of oil from something like a blown head gasket can starve the engine of the oil it needs to run smoothly. This rapidly increases friction, which causes severe wear on a lot of components, including the engine valves.

It’s also possible for low oil and severe friction to cause one or more engine valves to get stuck in the valve guide. The forces exerted on the stuck valve can cause it to become bent over time.

2. Frequently Revving at High RPMs

Frequently Revving at High RPMs

Frequently running the engine at revs can stress engine components, and drive up the heat, which increases the risk of causing bent valves. Essentially every engine has a maximum number of RPM it can operate within before components approach their tolerance level.

If you have a bad habit of constantly pushing the engine revs to near the max you are increasing the risk of bent valves. Not to mention a lot of other potential engine faults.

3. A Broken Timing Belt

A Broken Timing Belt

A broken timing belt is one of the more common causes of bent valves. This is especially true for cars with an interference engine where there is minimal spacing tolerance between the pistons and the engine valves.

When the timing belt breaks all timing is lost and the pistons can easily crash into the valves with extreme force. This often causes bent valves as well as piston damage and even damage to the cylinder head.

4. Overheating

Overheating

Severe overheating or a car that is allowed to frequently run very hot can also cause bent valves by warping the metal. This is usually linked to some other fault in the engine or cooling system such as severe friction from a failing oil pump, a fault in the cooling system, or even something like a stuck EGR valve.

The longer you let your car run hot, or if you push your car to the point of severely overheating you increase the risk of causing bent valves. Not to mention damaging other heat-sensitive engine components such as the head gasket, the cylinder head, or the spark plugs.

5. Incorrect Valve Lash

Incorrect Valve Lash

Anything that the valve lash clearance isn’t properly set you risk contact between the pistons and the valves. This is even more likely to be an issue in an interference engine where the tolerances are often very tight.

When contact occurs it can damage the valve, bend the valve and impact other components inside the engine. This also increases the risk of the engine overheating, which can cause multiple valves to bend due to one small fault.

6. Poor Maintenance Habits

Poor Maintenance Habits

It’s an unfortunate truth that poor maintenance habits are one of the most common underlying causes of bent valves. The most common amongst them are car owners who go far too long between regularly schedule oil changes, as well as failing to check the motor oil, or ignoring mechanical faults that cause the engine to run hot.

How to Confirm if Your Engine Has Bent Valves

How to Confirm the Valves Are Bent

The surest way to confirm bent engine valves is to remove the cylinder head and inspect each valve. Though this is labor-intensive, and not the sort of thing that the average home mechanic can do on their own.

If your car has been showing symptoms of bent engine valves, such as backfiring, low engine oil, and other signs of running rich, a leak-down compression test can help you dial in the problem.

A simple compression tester will allow you to quickly assess just how much compression each cylinder is producing. If it reveals that one or more cylinders are showing low, you can manually turn the crankshaft until the cylinder is at the compression stroke. At this point, you hear where the air rushing out of the engine.

If the rushing air is only coming from the intake, then you might be dealing with a bent intake valve. If the rushing air is coming from the exhaust points then you might have a bent exhaust valve.

While this isn’t 100% confirmation of bent valves, it certainly indicates that there is a serious valve problem that needs to be addressed. If the compression leak isn’t being caused by one or more bent valves, the next likely issue is carbon buildup on the valves resulting in a very poor seal.

3 Ways to Test for Bent Engine Valves Without Removing the Cylinder Head

A leak-down compression test where you test the compression and airflow of each intake and exhaust valve will help you assess if the valves are sealing as they should.

This isn’t a 100% confirmation, but if you find one or more valves that aren’t sealing in the leak-down compression test, and you’re seeing symptoms of bent valves, it’s a good indicator that it’s time to remove the cylinder head for final confirmation of the problem.

Perform a Leak-Down Compression Test

  • Step One: Turn the vehicle off, set the parking brake and have an assistant sit in the driver’s seat.
  • Step Two: Remove a spark plug, preferably from a cylinder that threw a code earlier in your troubleshooting.
  • Step Three: Connect the pressure gauge to the spark plug well.
  • Step Four: Have your assistant attempt to turn over the car through 4 to 6 revolutions. This should give you a compression reading. If it’s low, then you have a compression loss issue, likely with one of the valves.
  • Step Five: With the engine off, rotate it to the point that the piston is at top dead center of the cylinder you are testing.
  • Step Six: Insert the leak-down tester into an open spark plug well. Connect the tester to an air compressor and set the tester’s regulator to 100 PSI.
  • Step Seven: Read the second gauge on the leak down tester to confirm the difference in pressure.
  • Step Eight: Repeat this test with each cylinder to assess which ones, or if all of them have bent valves.

Visually Inspecting for Bent Valves

Visually Inspecting for Bent Valves

If you want visual confirmation without removing the head you can do so with a small, flexible borescope. You’ll need to bend the miniature camera back on itself and the use the following steps to inspect your suspected bent valves without removing the cylinder head.

Inspect Suspected Bent Valves with a Borescope

  • Step One: Turn the vehicle off, set the parking brake and remove negative (Black) terminal first.
  • Step Two: Remove the spark plug from every cylinder you suspect has a bent valve associated with it.
  • Step Three: Slowly lower the borescope down the spark plug well, monitoring the progress as it descends.
  • Step Four: Look for marks, burns and deformations on the head of each valve for signs of collision with the pistons.

If you see any visual faults or deformation and the valves also failed the leak-down compression test, you will have solid confirmation of bent valves that need to be replaced.

How Much Do Bent Valves Repair Cost?

How Much Do Bent Valves Repair Cost?

The average cost to repair bent engine valves can vary wildly from as little as $150 for a single valve to as much as $2,000 to replace all the valves in the car’s engine. It all comes down to the number of valves that were bent.

Though this is just the cost for the bent valve repair/replacement. Most of the time, another mechanical fault caused the bent engine valve(s) in the first place. If something like a failing oil pump, damaged timing belt, or major coolant system fault isn’t also repaired, you’ll likely suffer bent valves again in no time.

Frequently Asked Questions

Can you drive a car with a bent valve?

You might be able to drive a car with one or two bent valves for a while, but it’s not a good idea. The risk of damage to engine components, misfires, and exhaust system damage increases the more you drive a car with bent engine valves. Not to mention the increased risk of overheating damage to other engine components like the cylinder head and/or the head gasket.

Will an engine start with bent valves?

If you only have one or perhaps two bent valves you might get lucky, and the car will start and run very rough. Though most of the time multiple bent valves prevent the car from starting effectively. The lack of compression in one or more cylinders makes it hard for the engine to cycle properly to maintain continuous internal combustion.  

Conclusion

The symptoms of bent engine valves often show up after a serious event such as a hot-running car overheating or a failure in the timing belt or the oil pump. When the pistons contact the valves the extreme force exerted can warp or deform them, which prevents them from sealing properly as the engine cycles.

If you’ve been noticing symptoms of one or more bent engine valves such as recurring backfiring, surprisingly low oil, hard engine vibrations, and a check engine light, it’s best to park the car to accurately access the problem. Usually, a leak-down compression test will reveal a valve problem serious enough to warrant removing the cylinder head.

If you do find that you have bent engine valves, the price will vary based on the number of valves that are damaged, as well as any other repairs that need to be made to the timing belt, oil system, or other engine components. If it’s just a single bent engine valve, you might get away with a mechanic’s bill as low as $150 to $200. Though $900 to as much as $2,000 is more realistic. Especially if you have multiple bent engine valves that were caused by a major mechanical fault.

Jason Farrell

Written By

Jason Farrell

Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.

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