Why valves burn out

Timing valves are located precisely in the combustion chamber and are designed for high-temperature loads. However, if the normal operation of the internal combustion engine is disturbed, even the heat-resistant material from which they are made is destroyed over time. How quickly the valves burn out depends on the nature of the malfunction. Characteristic signs that the valve in the cylinder has burned out are uneven operation and difficult starting of the internal combustion engine, as well as loss of power. However, these same symptoms can occur with other problems. This article will help you figure out what “burned out the valve” means, why this happened and learn about methods for diagnosing the timing without removing the head.

Symptoms of a burnt valve

How to understand that burned valves? The easiest way to install this is by visual inspection, but for this you would have to remove the cylinder head, which is quite laborious and expensive. Therefore, to begin with, it is worth being guided by indirect signs. Knowing what happens when the valve burns out, and how this affects the operation of the internal combustion engine, it is possible to determine the breakdown without disassembling the motor.

How to tell if a valve is burned out see the table for typical symptoms and underlying causes.

Symptom	Causes	Why is this happening
Detonation ("knocking fingers")	The octane number does not correspond to the recommended by the manufacturer. ignition set incorrectly	If gasoline is low-octane or ignites at the wrong time, then with a strong compression of the mixture, instead of its smooth combustion, an explosion occurs. Combustion chamber parts are subjected to shock loads, valves overheat and may crack
Increased fuel consumption	Incorrect operation of the timing	The operation mode of the timing belt with a damaged valve is disrupted, the power drops, and with it the efficiency of the engine, which can lead to increased consumption
Deterioration of traction and dynamics	The drop in the total power of the internal combustion engine	A burnt valve does not allow reaching working compression in the cylinder, as a result, the necessary force is not created to move the piston
Difficult launch	Reducing the speed of the piston	The piston is not able to create the necessary force to rotate the crankshaft
Shaking and uneven idling, change in the sound of the engine	Cylinder misfire	Normally, flashes in the internal combustion engine cylinders occur at even intervals (half a turn of the crankshaft for a 4-cylinder internal combustion engine) and with the same force, so the motor rotates evenly. If the valve burns out, the cylinder cannot do its job and the internal combustion engine is subjected to load fluctuations, causing tripping and strong vibrations.
Silencer shots	Ignition of the VTS in the exhaust manifold	In a leaky cylinder, the air-fuel mixture does not burn completely. As a result, the remaining fuel enters the hot exhaust tract and ignites.
Pops in the inlet	the air-fuel mixture returns to the manifold and receiver	If the inlet valve burns out and poisons, then during compression, part of the mixture returns to the inlet receiver, where it burns out when a spark is applied

The valve burned out and can no longer provide tightness

By the symptoms listed above, you can find out that the valves in the internal combustion engine have burned out. A combination of several signs indicates this with a higher probability. The seat to which the valve must fit snugly when closing can also burn out, although this is a less common failure.

If the symptoms indicate the presence of cracks in the valve or that the valve seats are burnt out, what is the cause of the breakdown can only be reliably established with the help of a complete diagnosis and troubleshooting. To carry out the repair. Whatever it was, you will have to remove the cylinder head, and then change the failed parts.

The cost of fixing the problem

You can personally replace the valve on a domestic car at minimal cost, spending about 1000 rubles on the valve itself, a new cylinder head gasket, lapping paste, and antifreeze for topping up. But usually everything does not end with one burnout: milling or replacing a cylinder head deformed due to overheating, as well as turning valve seats, may be required. A pinched valve entails the development of a camshaft cam.

At the service station, they are reluctant to change one valve, and the full maintenance and repair of the cylinder head starts from 5-10 thousand rubles for a VAZ - up to tens of thousands for modern foreign cars.

After replacing burnt valves and repairing the cylinder head, it is important to eliminate the root cause of burnout. If this is not done, then soon the part will fail again!

Why do engine valves burn out?

What causes a valve in an internal combustion engine to burn out? the underlying reason is violation of the temperature regime in the combustion chamber. As a result, the part is subjected to overheating, the metal begins to melt, or vice versa, it becomes more brittle, crumbles and cracks. Even a small valve defect gradually progresses, due to which it becomes unusable over time.

There are 6 basic reasons why valves on a car burn out:

1. Poor mix. A lean combustible-air mixture burns more slowly than normal (stoichiometric), part of it burns out already at the exit from the combustion chamber, so the heat load on the exhaust tract increases. The reasons why the exhaust valve burns out usually lie precisely in the lean mixture or in the next problem.
2. Incorrect ignition timing. The higher the octane number of the fuel, the more evenly and more slowly it burns, therefore, with an increase in the octane, an increase in the ignition timing is also required. With late ignition, the mixture burns out already in the exhaust tract, overheating the valves. With early gasoline ignites prematurely, shock loads and overheating appear.
3. soot deposition. At the moment of closing, the valve fits snugly against the seat, which is involved in the removal of excess heat. With the formation of soot on their surface, heat transfer deteriorates significantly. Cooling only through the neck is not as effective. In addition, the layer prevents the valves from fully closing, resulting in a breakthrough of the burning mixture into the intake or exhaust manifold, exacerbating overheating.
4. Incorrect valve clearances. On a cold engine, there is a gap between the valve lifter and the camshaft eccentric, which is a margin for metal expansion. It can be periodically adjusted manually by means of washers or cups of the required thickness, or automatically by hydraulic compensators. In case of incorrect adjustment or wear of the hydraulic compensator, the part occupies an incorrect position. When the valve is pinched, it cannot close completely, the burning mixture breaks into the gap between it and the seat, causing them to overheat. If the inlet valve is burnt out, the reasons for this most often lie precisely in the clamping or in deposits on its surface that prevent locking.
5. Cooling system problems. If the circulation of coolant in the cylinder head is disrupted or the antifreeze simply cannot cope with heat removal, as a result, the head parts overheat, and the valves and their seats may burn out.
6. Incorrect dosage of fuel. On diesel engines, valve burnout occurs due to the same excessive thermal loads caused by incorrect fuel dosage. The reason for them may be incorrect operation of the injection pump or fuel injectors.

Exhaust valve burnt out

Carbon deposits on valves and seats lead to burnout

From the foregoing, we can conclude which valves burn out more often - exhaust valves. Firstly, they are smaller in size, and therefore heat up faster. Secondly, it is through them that hot exhaust gases are removed. The intake valves are constantly cooled by the air-fuel mixture or clean air (on direct injection engines) and therefore experience less thermal stress.

What causes valves on a gasoline engine to burn out?

The answer to the question “why did the exhaust valve burn out on a gasoline engine?” can be found in the previous section in points 1-5 (mixture, ignition, carbon deposits, gaps and cooling). At the same time, the fourth reason is most relevant to DVSm, in which manual adjustment of the thermal gap is provided. Do valves with hydraulic lifters burn out? This also happens, but most often for reasons beyond the control of automatic compensators - they themselves rarely fail.

The most common reason why a valve burns out in a VAZ ICE with an 8-valve timing is precisely untimely or unqualified clearance adjustment. On older engines installed in the VAZ 2108 and VAZ 2111, the problem manifests itself more often due to the shorter adjustment interval. On the ICE of the 1186 series, installed in Kalina, Grant and Datsun, where the interval is increased due to the refinement of the ShPG, it is slightly less pronounced. Nevertheless, valve pinching is one of the main reasons why the intake valve burns out. And this applies not only to VAZs.

The fact is that due to the subsidence of the seats and the gradual self-grinding of the valves, freely rotating around their axis, they gradually rise up. As a result, the gap between the pusher and the camshaft eccentric cam is reduced, the adjustment is lost.

A lean mixture, which causes overheating of the exhaust port, is the main cause of burnout on gasoline engines with hydraulics. But incorrect ignition and cylinder head overheating are equally common on all engines, regardless of the valve adjustment mechanism.

Why do valves burn after installing HBO?

The main reason why gas valves burn out is incorrect setting of the internal combustion engine for HBO. Gaseous fuel differs from gasoline in octane number: propane-butane usually has an octane rating of 100 units, and methane has 110 units. If a ignition adjusted for petrol 92 or 95 - the mixture will be burn out already in the exhaust tract.

When installing HBO (especially methane), be sure to install the UOZ variator in order to correct the moment of sparking when driving on gas! Or install the dual-mode firmware "gas-gasoline". On cars that originally come with HBO (like the Lada Vesta CNG), such firmware is installed from the factory; for other models, similar software is created by chip tuning specialists.

The second common reason why valves burn out from gas is lean mixture operation. A lean mixture ignites worse, burns longer and burns out already in the exhaust channel, thereby exposing the valve and its seat to overheating.

Any HBO needs tuning. On 1st to 3rd generation systems, it is important correctly adjust the gearbox, and on the 4th and newer - set injection corrections relative to petrol in the gas ECU. If you incorrectly adjust the system or deliberately “strangle” it for the sake of economy, this is fraught with burnout.

Gas consumption on a modern engine cannot be 1:1 to gasoline. Their calorific value is comparable (within 40–45 kJ/g), but the density of propane-butane is lower by 15–25% (500–600 g/l versus 700–800 g/l). Therefore, gas consumption on a normally enriched mixture should be more than gasoline!

As with gasoline, common causes of valve burnout in an internal combustion engine with LPG can be incorrect clearance adjustment, coking with soot, and cooling problems. Therefore, when troubleshooting a motor with a burnt valve, you should make sure that these problems do not exist.

On motors with manual adjustment of valves operating on gas, when adjusting the gaps, it is worth making an amendment of +0,05 mm. For example, for an 8-valve ICE VAZ, normal intake clearances are 0,15–0,25 mm, and exhaust clearances are 0,3–0,4 mm, but on gas they should be shifted to 0,2–0,3 mm for intake and 0,35–0,45 mm for release.

Why do diesel valves burn out?

The reasons why diesel valves burn out are different from gasoline ICEs. They do not have spark ignition, and a lean mixture is a sign of normal operation, since air must always be supplied in excess for complete combustion of diesel fuel. Typical reasons why valves burn out on a car with a diesel engine are:

• too early injection of fuel into the cylinders;
• re-enrichment of the mixture due to excessive pressure of the injection pump or overflow nozzles;
• incorrect adjustment of thermal gaps or breakdown of hydraulic lifters;
• overheating of the cylinder head due to a violation of the circulation of antifreeze or a deterioration in its properties.

Most often, the valve on a diesel engine burns out precisely because of the above reasons. On older ICEs with a mechanical injection pump, early injection may occur due to a breakdown of the timer (advance machine) of the pump that controls the moment of fuel supply. In modern ICEs with a Common Rail system, the cause of valve burnout can be sensors that incorrectly determine the moment for injection, and worn nozzles that pour fuel in excess of the norm.

The reasons why the valves in the internal combustion engine of a car on diesel fuel burn out may be problems with the air filter and intercooler (on a turbodiesel). A clogged filter restricts the air flow, due to which there is a relatively large amount of fuel with a constant supply volume. An intercooler that overheats (for example, due to pollution) acts similarly. It cannot cool the air normally, as a result, although it develops the necessary pressure in the intake from expansion when heated, the amount of oxygen in it ultimately turns out to be insufficient, since the air is deficient in mass relative to the norm. Both factors cause an over-enrichment of the mixture, which on a diesel engine can lead to valve burnout.

How to identify a burnt valve without removing the cylinder head

Inspection of valves using an endoscope connected to a smartphone

There are two basic ways to determine a burnt valve with high accuracy without disassembling the motor:

• compression measurement;
• visual inspection with an endoscope.

in order to understand that the valves have burned out, you can carry out these operations yourself or contact a car repair shop. A budget endoscope, like a compressometer, will cost 500–1000 rubles. Approximately the same amount will be taken for diagnostics and the master at the service station. Inspection with an endoscope connected to a smartphone, tablet or laptop allows you to clearly see the damaged valve, and the "compressometer" will show the pressure drop in the cylinder.

Before checking a burnt valve, you need to make sure that there are no gap problems. They must be correctly set, because also a whole pinched valve that cannot close completely behaves the same as a burnt one.

To measure compression, especially on motors with an electronic throttle, you need an assistant, because at the time of testing the damper must be fully open. also the assistant will start the starter.

How to find a broken cylinder

You can determine a cylinder with a burnt valve by measuring compression or removing wires / coils from candles with a running engine. How to check a burnt valve on a gasoline engine by sound:

Identifying a Cylinder with a Burnt Valve

1. Start the engine, let it warm up and open the hood.
2. Remove the wire or coil from the candle of the 1st cylinder.
3. Listen to whether the sound of the motor has changed, whether the vibrations have increased.
4. Return the wire or coil to its place, again listen to the changes in work.
5. Repeat steps 2-4 for the rest of the cylinders.

If the cylinder holds pressure properly, then when it is turned off, the internal combustion engine starts to work worse, triple and shake, and when connected, the work returns to normal. But if the valve is burnt out, the cylinder is not fully involved in the work, so the sound and vibration of the motor after disconnecting and connecting the candle do not change.

For diesel, only the option with a compression gauge is available due to the lack of spark plugs. In a cylinder with a defective valve, the pressure will be approximately 3 (or more) atm less than in the rest.

How to determine what the problem is

Since it is possible to recognize a burnt valve with an endoscope for sure, it is better to choose this option if possible. For inspection you need:

Burnt valve in the picture from the endoscope

1. Connect the "endoscope" to a laptop or smartphone and display the picture on the screen.
2. Put a mirror attachment on the camera (optional if the “endoscope” is with a controlled head).
3. Unscrew the candle and place the “endoscope” into the cylinder through the hole.
4. Inspect valves for defects.
5. Repeat steps 3-4 for each cylinder.

Checking with a compression gauge is based on understanding what happens to pressure when a valve burns out. For a warmed-up gasoline internal combustion engine, normal compression is 10–15 bar or atmospheres (1–1,5 MPa), depending on the compression ratio. The pressure in the diesel cylinder is 20–30 bar or atm. (2–3 MPa), therefore, to check it, you need a device with a pressure gauge that has a wider measurement range.

How to determine that a valve has burned out using a pressure gauge is indicated in the instructions below. If the tip of the compression gauge is not equipped with a thread, but with a rubber cone, an assistant will be required.

The procedure for checking burnt valves with a compression gauge:

1. Unscrew the spark plugs (on a gasoline engine), glow plugs or injectors (on a diesel engine) from the cylinder head. in order not to confuse them during assembly, number the spark plug wires or coils.
2. Shut off the fuel supply, for example, by turning off the fuel pump (you can remove the fuse) or by disconnecting the line from the injection pump.
3. Screw the “compressometer” into the hole of the 1st cylinder or press it tightly with a cone to the hole.
4. Have an assistant turn the engine over with the starter for 5 seconds while pressing the gas pedal to the floor to properly fill the cylinder with air.
5. Record the pressure gauge readings, compare them with the normal ones for your internal combustion engine.
6. Zero the “compressometer” by depressurizing it.
7. Repeat steps 3-6 for each of the remaining cylinders.

Gasoline "compressometer" with thread and cone nozzles

Diesel "compressometer" with a measurement scale up to 70 bar

After making compression measurements, compare the readings of the device for each of the cylinders. Normal values ​​for different internal combustion engines are indicated above, the spread over the cylinders should be within 1 bar or atm. (0,1 MPa). A sign of burnout is a significant (3 atm or more) pressure drop.

A burnt valve is not always the culprit for low pressure. Poor compression can be caused by stuck, worn or broken rings, excessive cylinder wall wear, or piston damage. You can understand that a burned-out valve behaves this way by injecting about 10 ml of engine oil into the cylinder and re-measuring compression. If it has increased - a problem with the rings or cylinder wear, if it has not changed - the valve does not hold pressure due to burnout.

Oil also will not help increase compression if it is not there due to a piston that has burned out or burst from detonation - the symptoms will be the same as when the valve burns out. You can check the integrity of the piston indiscriminately with an endoscope or by feeling it with a long thin rod through the candle well.

Can you drive with burnt valves?

For those who, by symptoms, have determined that their car has problems with valves, and is interested in: is it possible to drive if the valve is burnt out? - the answer is immediately: it is highly undesirable, this can lead to additional costs. If the valve really burned out, the consequences can be disastrous for the motor:

• pieces of a collapsing valve damage the piston and cylinder head, peel off the cylinder walls, break the rings;
• when the intake valve burns out, the air-fuel mixture that breaks into the intake receiver can flare up there and break it (especially true for plastic receivers);
• a burning mixture, breaking through a leaky valve, leads to overheating of the manifold, exhaust pipe, gasket, leading to burnout of exhaust parts;
• a mixture that cannot burn normally in the cylinder burns out in the exhaust, damaging the catalyst, oxygen sensor;
• due to continued local overheating, the cylinder head may lead, which will require its milling during repair or even replacement.

How to avoid burnt valves

• Control the quality of mixture formation by periodically inspecting the candles for carbon deposits. If it is white, the mixture is poor and needs to be adjusted.
• Observe the intervals for replacing spark plugs prescribed in the regulations of your car.
• when driving on gas, reduce the interval for measuring valve clearances. Check them every 10 thousand km (at each oil change) and, if necessary, adjust.
• Refuel with the manufacturer's recommended octane rating.
• when driving on gas, use the UOZ variator or the dual-mode firmware of the gas-gasoline ECU.
• Change the oil in time, using products with tolerances recommended by the car manufacturer.
• Change antifreeze every 3 years or after 40-50 thousand km, in order to prevent deterioration of its properties, monitor its level in the tank and the temperature when driving.
• When a “Check Engine” notification appears on the instrument panel, diagnose the engine using OBD-2 for quick troubleshooting.

By adhering to these recommendations, you will extend the life of the motor, since it is easier and cheaper to prevent burnout of internal combustion engine valves than to replace them. In the case of a VAZ, there is a chance to buy a “live” head inexpensively at a disassembly, but even a used part for foreign cars can hit your wallet.