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A burnt valve has lost its seal because the sealing face has been channelled, guttered or eroded by hot gas. It is almost always an exhaust valve. The exhaust valve is cooled by sitting on its seat, which conducts heat into the cylinder head and the coolant around it. If the valve does not sit, or sits on a speck of carbon, that path is lost and the face overheats. Gas then leaks through the gap, the leak torches a channel, and the channel grows. Inlet valves rarely fail this way. On a direct-injection petrol engine they coke up instead, which is a different fault and is described on carbon deposits and cleaning.
The complaint is easy to file under coils, injectors or a head gasket. The test that moves it into this page is a leak-down hiss at the tailpipe on one cylinder, after timing has been confirmed. Common petrol engine problems and common diesel engine problems both link here. Diesels do burn exhaust valves, but a diesel that shakes at idle is far more often an injector or a compression loss at the rings. Do the return test and the compression test before the head comes off for a valve.
How the face burns
AE’s service bulletin on valve-face burning describes a local burnout, usually in one place on the head of the valve, forming a deep channel. Radial cracks can follow, and pieces can leave the head if it is left running. The bulletin’s causes for both austenitic heads and hard-faced valves are:
Not enough tappet clearance. The valve seats lightly. Carbon builds on the face. Both destroy the heat path from the face, through the seat, to the coolant. Face temperature rises until the material cannot resist hot corrosion. The gas leak then torches the gutter. On an engine with adjustable clearance this is a maintenance fault. On a hydraulic lifter the same loss of seat time can come from a collapsed lifter, a worn cam, or a stem and guide that do not let the valve close.
Carbon packed between the face and the seat. The bulletin names over-rich combustion and excessive oil consumption as routes to that deposit. The heat path is lost in the same way. If a piece of the deposit breaks away, the leak starts at that point and accelerates.
Pre-ignition. A wrong air-fuel ratio, wrong ignition timing, or a hot spot at the edge of the valve head. The bulletin notes that on some engines the piston fails before the valve shows the damage. A lean mixture raises combustion and exhaust temperature. So does ignition that is late, because the charge is still burning as the valve opens. Both are reasons to look at fuelling and at ignition when more than one exhaust valve is burnt. One burnt valve with the others intact is more often seating, clearance or a local hot spot than a mixture that the whole engine ran.
Distortion and alignment. The bulletin also lists guide-to-seat misalignment, a weak valve bridge, overload, and a valve head distorted by poor cooling. A cooling-system fault belongs on engine overheating and coolant faults as well as here. Replacing the valve and leaving the engine to overheat again repeats the gutter.
LPG and other gaseous fuels come up in owner reports because a lean or poorly calibrated gas system, and a seat that was not specified for a dry fuel, can overheat the exhaust valve. The bulletin’s mechanism still applies: temperature, seating and the heat path. It is not true that every gas conversion burns valves. Calibration, valve material and seat material decide it. A gas system that is right at idle and lean under load is the one to measure, under load, before the head is reassembled and run the same way.
What it feels like
A valve that leaks a little misfires on that cylinder, more so when cold or at idle, and may tick. A valve that has guttered loses compression and the misfire is constant. The spark plug on a petrol engine may look unusually clean if the leak has been blowing the electrode, or the misfire count sits on one cylinder no matter which coil is fitted. Moving the coil and the injector, and having the misfire stay in the hole, is the point at which compression testing starts. Until that swap is done, a burnt valve is only a guess.
Engine Australia’s SB041 separates the low-compression causes. A wet test that does not rise points away from the rings and towards the valves, the valve gear or the timing. Leak-down air at the tailpipe is an exhaust valve that is not sealing, or timing that is holding it open. Leak-down air at the intake is the inlet valve. Air from the next cylinder’s plug hole, or bubbles in the radiator, is a head gasket or a crack, not a burnt valve. Timing has to be confirmed. A belt or chain that has jumped leaves air at the intake or the exhaust of a valve that is not burnt at all.
Petrol and diesel
On a petrol engine the valve is one of the last misfire causes, after plugs, coils and fuel. Haynes puts compression loss, including damaged valves and the head gasket, at the expensive end of a misfire diagnosis for that reason. Direct-injection carbon on the inlet valve is the lookalike that forums mix up with a burnt valve. Carbon upsets airflow and can misfire. It does not put a gutter through the exhaust face, and a leak-down test does not hiss at the tailpipe because the back of an inlet valve is dirty. Inspection through the port, or a borescope, finds the carbon. The leak-down test finds the burnt exhaust valve.
On a diesel the same leak-down result means the same thing, but the everyday rough idle is an injector, a glow circuit on a cold start, or low compression from the piston. Quantity corrections that follow an injector when it is swapped, within the coding rules on the injector page, stay with the injector. A correction that stays in the cylinder, with leak-down air at the tailpipe, is the valve. Exhaust temperature on a diesel that is over-fuelled, or that has been working with a restricted particulate filter, is also a reason a valve runs hot. Fix the reason the exhaust was hot.
The repair has to include the seat
A new valve on a burnt seat will not seal and will not cool. The seat is recut or replaced, the guide is checked, and the clearance is set to the specification for that engine. A hydraulic lifter that has collapsed, or a cam lobe that has worn, is the reason the valve was not seating, and it is still there after the valve is changed. On engines where the clearance is adjusted by shim or screw, the clearance is measured hot or cold exactly as the procedure says. Too little clearance is how this failure starts. Too much clearance is noisy and hammers the seat, which is the bulletin’s related wear path, not the gutter itself.
If several exhaust valves are burnt, the mixture, the ignition timing and the cooling system are the question. One valve, with a gutter in one place and carbon on the face, matches the bulletin’s loss of the heat path at that seat.
Sources
AE / Garage Gurus, service engineering bulletin on valve face burning (SB2135.1). Guttering, loss of the heat path through the seat, insufficient clearance, carbon on the face, and pre-ignition.
Engine Australia, service bulletin SB041. Wet compression against leak-down, and air at the tailpipe or the intake as a valve-sealing finding. Timing has to be ruled out.
Haynes, fault code P0300. Compression loss, including valves and the head gasket, as the end of a misfire diagnosis rather than the start.
Professional Motor Mechanic, GDi carbon. Inlet-valve deposits on direct injection, which misfire without burning the exhaust valve.
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Revision 1: Burnt valves
A burnt valve has lost its seal because the sealing face has been channelled, guttered or eroded by hot gas. It is almost always an exhaust valve. The exhaust valve is cooled by sitting on its seat, which conducts heat into the cylinder head and the coolant around it. If the valve does not sit, or sits on a speck of carbon, that path is lost and the face overheats. Gas then leaks through the gap, the leak torches a channel, and the channel grows. Inlet valves rarely fail this way. On a direct-injection petrol engine they coke up instead, which is a different fault and is described on carbon deposits and cleaning.
The complaint is easy to file under coils, injectors or a head gasket. The test that moves it into this page is a leak-down hiss at the tailpipe on one cylinder, after timing has been confirmed. Common petrol engine problems and common diesel engine problems both link here. Diesels do burn exhaust valves, but a diesel that shakes at idle is far more often an injector or a compression loss at the rings. Do the return test and the compression test before the head comes off for a valve.
How the face burns
AE’s service bulletin on valve-face burning describes a local burnout, usually in one place on the head of the valve, forming a deep channel. Radial cracks can follow, and pieces can leave the head if it is left running. The bulletin’s causes for both austenitic heads and hard-faced valves are:
- Not enough tappet clearance. The valve seats lightly. Carbon builds on the face. Both destroy the heat path from the face, through the seat, to the coolant. Face temperature rises until the material cannot resist hot corrosion. The gas leak then torches the gutter. On an engine with adjustable clearance this is a maintenance fault. On a hydraulic lifter the same loss of seat time can come from a collapsed lifter, a worn cam, or a stem and guide that do not let the valve close.
- Carbon packed between the face and the seat. The bulletin names over-rich combustion and excessive oil consumption as routes to that deposit. The heat path is lost in the same way. If a piece of the deposit breaks away, the leak starts at that point and accelerates.
- Pre-ignition. A wrong air-fuel ratio, wrong ignition timing, or a hot spot at the edge of the valve head. The bulletin notes that on some engines the piston fails before the valve shows the damage. A lean mixture raises combustion and exhaust temperature. So does ignition that is late, because the charge is still burning as the valve opens. Both are reasons to look at fuelling and at ignition when more than one exhaust valve is burnt. One burnt valve with the others intact is more often seating, clearance or a local hot spot than a mixture that the whole engine ran.
- Distortion and alignment. The bulletin also lists guide-to-seat misalignment, a weak valve bridge, overload, and a valve head distorted by poor cooling. A cooling-system fault belongs on engine overheating and coolant faults as well as here. Replacing the valve and leaving the engine to overheat again repeats the gutter.
LPG and other gaseous fuels come up in owner reports because a lean or poorly calibrated gas system, and a seat that was not specified for a dry fuel, can overheat the exhaust valve. The bulletin’s mechanism still applies: temperature, seating and the heat path. It is not true that every gas conversion burns valves. Calibration, valve material and seat material decide it. A gas system that is right at idle and lean under load is the one to measure, under load, before the head is reassembled and run the same way.
What it feels like
A valve that leaks a little misfires on that cylinder, more so when cold or at idle, and may tick. A valve that has guttered loses compression and the misfire is constant. The spark plug on a petrol engine may look unusually clean if the leak has been blowing the electrode, or the misfire count sits on one cylinder no matter which coil is fitted. Moving the coil and the injector, and having the misfire stay in the hole, is the point at which compression testing starts. Until that swap is done, a burnt valve is only a guess.
Engine Australia’s SB041 separates the low-compression causes. A wet test that does not rise points away from the rings and towards the valves, the valve gear or the timing. Leak-down air at the tailpipe is an exhaust valve that is not sealing, or timing that is holding it open. Leak-down air at the intake is the inlet valve. Air from the next cylinder’s plug hole, or bubbles in the radiator, is a head gasket or a crack, not a burnt valve. Timing has to be confirmed. A belt or chain that has jumped leaves air at the intake or the exhaust of a valve that is not burnt at all.
Petrol and diesel
On a petrol engine the valve is one of the last misfire causes, after plugs, coils and fuel. Haynes puts compression loss, including damaged valves and the head gasket, at the expensive end of a misfire diagnosis for that reason. Direct-injection carbon on the inlet valve is the lookalike that forums mix up with a burnt valve. Carbon upsets airflow and can misfire. It does not put a gutter through the exhaust face, and a leak-down test does not hiss at the tailpipe because the back of an inlet valve is dirty. Inspection through the port, or a borescope, finds the carbon. The leak-down test finds the burnt exhaust valve.
On a diesel the same leak-down result means the same thing, but the everyday rough idle is an injector, a glow circuit on a cold start, or low compression from the piston. Quantity corrections that follow an injector when it is swapped, within the coding rules on the injector page, stay with the injector. A correction that stays in the cylinder, with leak-down air at the tailpipe, is the valve. Exhaust temperature on a diesel that is over-fuelled, or that has been working with a restricted particulate filter, is also a reason a valve runs hot. Fix the reason the exhaust was hot.
The repair has to include the seat
A new valve on a burnt seat will not seal and will not cool. The seat is recut or replaced, the guide is checked, and the clearance is set to the specification for that engine. A hydraulic lifter that has collapsed, or a cam lobe that has worn, is the reason the valve was not seating, and it is still there after the valve is changed. On engines where the clearance is adjusted by shim or screw, the clearance is measured hot or cold exactly as the procedure says. Too little clearance is how this failure starts. Too much clearance is noisy and hammers the seat, which is the bulletin’s related wear path, not the gutter itself.
If several exhaust valves are burnt, the mixture, the ignition timing and the cooling system are the question. One valve, with a gutter in one place and carbon on the face, matches the bulletin’s loss of the heat path at that seat.
Sources
- AE / Garage Gurus, service engineering bulletin on valve face burning (SB2135.1). Guttering, loss of the heat path through the seat, insufficient clearance, carbon on the face, and pre-ignition.
- Engine Australia, service bulletin SB041. Wet compression against leak-down, and air at the tailpipe or the intake as a valve-sealing finding. Timing has to be ruled out.
- Haynes, fault code P0300. Compression loss, including valves and the head gasket, as the end of a misfire diagnosis rather than the start.
- Professional Motor Mechanic, GDi carbon. Inlet-valve deposits on direct injection, which misfire without burning the exhaust valve.
Revision 1: Burnt valves
A burnt valve has lost its seal because the sealing face has been channelled, guttered or eroded by hot gas. It is almost always an exhaust valve. The exhaust valve is cooled by sitting on its seat, which conducts heat into the cylinder head and the coolant around it. If the valve does not sit, or sits on a speck of carbon, that path is lost and the face overheats. Gas then leaks through the gap, the leak torches a channel, and the channel grows. Inlet valves rarely fail this way. On a direct-injection petrol engine they coke up instead, which is a different fault and is described on carbon deposits and cleaning.
The complaint is easy to file under coils, injectors or a head gasket. The test that moves it into this page is a leak-down hiss at the tailpipe on one cylinder, after timing has been confirmed. Common petrol engine problems and common diesel engine problems both link here. Diesels do burn exhaust valves, but a diesel that shakes at idle is far more often an injector or a compression loss at the rings. Do the return test and the compression test before the head comes off for a valve.
How the face burns
AE’s service bulletin on valve-face burning describes a local burnout, usually in one place on the head of the valve, forming a deep channel. Radial cracks can follow, and pieces can leave the head if it is left running. The bulletin’s causes for both austenitic heads and hard-faced valves are:
- Not enough tappet clearance. The valve seats lightly. Carbon builds on the face. Both destroy the heat path from the face, through the seat, to the coolant. Face temperature rises until the material cannot resist hot corrosion. The gas leak then torches the gutter. On an engine with adjustable clearance this is a maintenance fault. On a hydraulic lifter the same loss of seat time can come from a collapsed lifter, a worn cam, or a stem and guide that do not let the valve close.
- Carbon packed between the face and the seat. The bulletin names over-rich combustion and excessive oil consumption as routes to that deposit. The heat path is lost in the same way. If a piece of the deposit breaks away, the leak starts at that point and accelerates.
- Pre-ignition. A wrong air-fuel ratio, wrong ignition timing, or a hot spot at the edge of the valve head. The bulletin notes that on some engines the piston fails before the valve shows the damage. A lean mixture raises combustion and exhaust temperature. So does ignition that is late, because the charge is still burning as the valve opens. Both are reasons to look at fuelling and at ignition when more than one exhaust valve is burnt. One burnt valve with the others intact is more often seating, clearance or a local hot spot than a mixture that the whole engine ran.
- Distortion and alignment. The bulletin also lists guide-to-seat misalignment, a weak valve bridge, overload, and a valve head distorted by poor cooling. A cooling-system fault belongs on engine overheating and coolant faults as well as here. Replacing the valve and leaving the engine to overheat again repeats the gutter.
LPG and other gaseous fuels come up in owner reports because a lean or poorly calibrated gas system, and a seat that was not specified for a dry fuel, can overheat the exhaust valve. The bulletin’s mechanism still applies: temperature, seating and the heat path. It is not true that every gas conversion burns valves. Calibration, valve material and seat material decide it. A gas system that is right at idle and lean under load is the one to measure, under load, before the head is reassembled and run the same way.
What it feels like
A valve that leaks a little misfires on that cylinder, more so when cold or at idle, and may tick. A valve that has guttered loses compression and the misfire is constant. The spark plug on a petrol engine may look unusually clean if the leak has been blowing the electrode, or the misfire count sits on one cylinder no matter which coil is fitted. Moving the coil and the injector, and having the misfire stay in the hole, is the point at which compression testing starts. Until that swap is done, a burnt valve is only a guess.
Engine Australia’s SB041 separates the low-compression causes. A wet test that does not rise points away from the rings and towards the valves, the valve gear or the timing. Leak-down air at the tailpipe is an exhaust valve that is not sealing, or timing that is holding it open. Leak-down air at the intake is the inlet valve. Air from the next cylinder’s plug hole, or bubbles in the radiator, is a head gasket or a crack, not a burnt valve. Timing has to be confirmed. A belt or chain that has jumped leaves air at the intake or the exhaust of a valve that is not burnt at all.
Petrol and diesel
On a petrol engine the valve is one of the last misfire causes, after plugs, coils and fuel. Haynes puts compression loss, including damaged valves and the head gasket, at the expensive end of a misfire diagnosis for that reason. Direct-injection carbon on the inlet valve is the lookalike that forums mix up with a burnt valve. Carbon upsets airflow and can misfire. It does not put a gutter through the exhaust face, and a leak-down test does not hiss at the tailpipe because the back of an inlet valve is dirty. Inspection through the port, or a borescope, finds the carbon. The leak-down test finds the burnt exhaust valve.
On a diesel the same leak-down result means the same thing, but the everyday rough idle is an injector, a glow circuit on a cold start, or low compression from the piston. Quantity corrections that follow an injector when it is swapped, within the coding rules on the injector page, stay with the injector. A correction that stays in the cylinder, with leak-down air at the tailpipe, is the valve. Exhaust temperature on a diesel that is over-fuelled, or that has been working with a restricted particulate filter, is also a reason a valve runs hot. Fix the reason the exhaust was hot.
The repair has to include the seat
A new valve on a burnt seat will not seal and will not cool. The seat is recut or replaced, the guide is checked, and the clearance is set to the specification for that engine. A hydraulic lifter that has collapsed, or a cam lobe that has worn, is the reason the valve was not seating, and it is still there after the valve is changed. On engines where the clearance is adjusted by shim or screw, the clearance is measured hot or cold exactly as the procedure says. Too little clearance is how this failure starts. Too much clearance is noisy and hammers the seat, which is the bulletin’s related wear path, not the gutter itself.
If several exhaust valves are burnt, the mixture, the ignition timing and the cooling system are the question. One valve, with a gutter in one place and carbon on the face, matches the bulletin’s loss of the heat path at that seat.
Sources
- AE / Garage Gurus, service engineering bulletin on valve face burning (SB2135.1). Guttering, loss of the heat path through the seat, insufficient clearance, carbon on the face, and pre-ignition.
- Engine Australia, service bulletin SB041. Wet compression against leak-down, and air at the tailpipe or the intake as a valve-sealing finding. Timing has to be ruled out.
- Haynes, fault code P0300. Compression loss, including valves and the head gasket, as the end of a misfire diagnosis rather than the start.
- Professional Motor Mechanic, GDi carbon. Inlet-valve deposits on direct injection, which misfire without burning the exhaust valve.