Valve Face Burning & Guttering
Valve face burning and guttering are serious problems which can lead to cracks in or breakages from the cylinder head. Hence, it is important to be able to identify the types and causes of valve face problems.
Valve face wear can be serious, but mainly occurs in older engines since modern fuel does not provide the lubrication they require. Valve face burning or guttering – where a deep channel is formed in the valve head- can happen to any engine, so it’s worth being aware of the causes.
WHERE’S THE WEAR?
Older engines rely on exhaust gases to lubricate the valve faces. However, modern low-leaded and LPG fuels don’t provide enough lubrication resulting in wear, especially in an engine with slack tappets, weak springs, or valve to port seat misalignment.
In turbocharged petrol engines, wet fuel is a sufficient lubricant; however turbocharged diesel engines can suffer from face wear. Since turbocharging raises air pressure in the inlet port above normal atmospheric air pressure, lubricating oil isn’t taken into the air stream. This causes the valve face and seat to run dry enough for wear to occur.
A BROADER ISSUE
Guttering on the inlet valve is known as broad-face burning. Material corrodes over the valve face, resulting in an escape of gas affecting performance.
This is most likely to affect non-hard-faced valves and can be caused by:
LACK OF TAPPET CLEARANCE
Lightly sitting valves allow carbon to build up on the seating face, which destroys the thermal heat path to the coolant. With nowhere for the heat to escape to, the temperature of the valve head and face rise until hot corrosion or burning takes place, creating the gutter.
HIGH CARBON DEPOSIT BUILD UP
Over-rich combustion and excessive oil consumption can also create carbon deposits. This again destroys the thermal path to the coolant, resulting in valve burn out. If deposits break away it can cause a localised gas leakage which accelerates the failure.
TORCHING DURING EROSION
Nickel-alloy is used for the valve faces for its high hot-corrosion resistance. Yet the alloy is weak in the erosion phase, which can lead to torching. This is a rarer occurrence, but can be identified by the deep gutter caused by the local gas leak.
An incorrect air/fuel ratio, inaccurate timing, or incandescent hot spots on the valve head periphery can all produce high gas loads, leading to localised high temperatures. This results in guttering, although the piston can sometimes fail before guttering occurs.
PREVENTION IS BETTER THAN CURE
Manufacturers attempt to prevent this kind of exhaust valve face wear with amended valve geometry and more wear-resistant materials for the cast iron head.
For effective prevention in all cases, it’s important to fit and adjust carefully and accurately. Installers are advised to use parts from reputable manufacturers, which incorporate the latest proven materials, design innovations and manufacturing techniques.