Valve failure

Valve Failure

Valves will wear out, many valves expire long before they should because of burning or breakage.

Exhaust valves are most likely to burn because they run hotter than the intakes. The intake valves are cooled by the incoming air and fuel, and consequently operate at about 800 degrees F. Exhaust valves receive no such cooling and are blasted by the hot combustion gases exiting through the exhaust port. Exhaust valves run at 1200 to 1350 degrees F. on average, which makes them much more vulnerable to burning and erosion. The higher operating temperature requires a tougher alloy, so exhaust valves are usually made of stainless steel or have stainless steel heads typically 21-2N or 21-4N alloy with a high chromium and nickel content.

The intake and exhaust valves rely on contact with the valve seat and guide for cooling. About 75% of the combustion heat that is conducted away from the valve passes through the seat, so proper seat contact is essential to prevent burning. The remaining 25% of the heat is dissipated up through the valve stem and out through the guides. Sodium filled hollow valve stems can be used to draw even more heat up through the stems to aid cooling.

Anything that interferes with valve cooling or creation of extra heat in the valve or cylinder head can lead to premature valve failure. A buildup of carbon deposits on the valve face and seat have an insulating effect that slows cooling and makes the valve run hot. In addition poor contact between the valve and seat if the seat is too narrow, nonconcentric or off-square. If deposits are developed in one spot or flake off in another, the posibility of a leak will that create hot spots on the valve and result in creation of grooves that are eroded or burned into the valve.

Weak valve springs or insufficient valve lash can also prevent good valve-to-seat contact and allow excessive heat to build up in the valves. Also a loose vavle seat or a worn valve guide can prohibit the heat transfer to the head and contribute to burning.

Ignoring the installed valve height when doing a valve job can lead to burning. When valves and seats are ground or cut, the valves sit deeper in the head than before. This allows the valve stem to stick up higher which changes the rocker arm geometry and may lead to a loss of valvelash when the engine gets hot. If the proper geometry can’t be restored by grinding the tips of the valve stems no more than about 0.010" maximum or you will grind through the case hardened layerof the valve stem tip.

The valve seats can be replaced to correct the installed valve spring height but it can be an expensive repair. Another option is to install slightly oversize valves that ride higher on the seat to compensate for seat machining, in addition to giving you an increase in performance.

Valve recession can cause the identical problem, because as the valve seat wears away causing the valve to recede into the head, thus reducing the valve lash. Leading to little or no lash left and the valve makes poor contact with the valve seat, overheats and burns.

Hot running engines can lead to valve sticking and burning if the operating temperature gets too high. Due to the fact that excessive heat makes the valve stems swell which may cause them to gall or stick in their guides if there isn’t enough clearance. If the valve sticks open, it can burn or be destroyed if it hits the piston.

Valves can also run hot because of elevated combustion temperatures. Factors such as retarded ignition timing, lean fuel mixtures, detonation from too high compression and low octane fuel or preignition developed from hot spots caused by deposits in the combustion chamber or even too hot a spark plug can all result in elevated combustion tempertures.Also exhaust restrictions such as a crushed exhaust pipe can make the valves run hot.

Valve Breakage is the other main type of valve failure and can happen to either intake or exhaust valve. Valves break in one of two places, where the head is joined to the stem, or where the keeper groove is machined into the end of the stem. Either way, breakage is bad news because the pieces fall into the combustion chamber and damage the piston and the combustion chamber of the cylinder head.

The causes of valve head breakage include fatigue failure from valve flexing due to off-square seating that causes the stem to flex every time it seats, pounding from excessive valvelash, stretching from excessive heat and RPM, and thermal shock sudden temperature changes as when abruptly shutting off a highly loaded engine.

Breakage of the valve stem tip can result from excessive sideloading if the installed stem height isn’t right and causes rocker arm misalignment. It can also be caused by excessive valvelash that prevents the valvetrain from cushioning the valve as it closes. Another condition that may cause the tip of the stem to snap off is wear or scratches in the keeper groove or valve spring coil bind due to using a camshaft with excessive lift.

Concerning two-piece valves, the junction between the stem and head is where the valve often cracks and separates, not because the valve is defective but because this area is highly stressed due to the joining of two dissimilar alloys.

Beside burning and breakage, there are a number of other valve problems that can occur. Bent valves are the result of contact between valve and piston. The causes are a broken cam chain, weak or broken valve springs, overrevving the engine, a valve sticking because of insufficient guide clearance or lubrication or overheating and insufficient valve-to-piston clearance from excessive valve lift, valve reliefs not cut deeply enough into the pistons, the wrong pistons, not enough deck height, or the cylinder head has been milled (shaved) too much.

Worn valve stems are a normal condition for high mileage valves but scuffing or scoring may be a clue to insufficient guide clearance, overheating, lack of lubrication or dirty oil. Using the wrong type of valve guide seals can sometimes be a factor. The valve seal controls the amount of oil that lubricates the guides. Positive seals provide the most oil control because they remain on the guides and act like a squeegee to scrape oil off the stems. Positive seals are used on Suzuki engines because there is more oil flow in the head that needs to be controlled. Umbrella and O-ring deflector seals, on the other hand, ride up and down with the valve and allow more oil into the guides. Substituting a positive seal for an umbrella or O-ring seal, therefore, may starve the guide for oil and create a scuffing problem on some applications, you should always use the same type of seal that was originally used on the engine.

Damaged or mushroomed valve stem tips are the results of worn or damaged rocker arm adjustment screws. Incorrect installed valve stem height can be the underlying cause here by creating too much back and forth scrubbing across the tip of the stem every time the valve opens and closes. Abnormal valve stem wear can also occur if the tips of the valves are ground excessively in an attempt to correct valve stem height. Grinding through the hardened surface layer leaves the soft metal underneath in direct contact with the rocker arm adjustment screw.

In conclusion valve failure analysis is this: replacing a bad valve with a new one will not fix anything if there’s an underlying problem that hasn’t been corrected. You should follow accepted valve and seat refinishing techniques leaving adequate valve margin, proper seat positioning and width, in addition to checking and correcting installed stem height and installed spring height also maintaining adequate stem to quide clearance and valve lash, and ruling out other engine problems such as overheating, detonation, air/fuel mixture or timing problems will prevent valve failures.