1989/11/01 Caterpillar

Reference: Guidelines for Reusable Parts: Valve and Valve Springs, SEBF8002-02; Analyzing Engine Valve Failures, SEBD0668; Valve and Valve Spring Specifications, SEBF8034-04.

Dealers have reported an increase in exhaust valve failures following an overhaul using either Caterpillar's Remanufactured cylinder head assemblies or dealer's rebuilt head assemblies.

Engine valves that are reused in rebuilt engines should be carefully examined. The reference publications are useful in examining the valves that are to be reused. After rebuild, exhaust temperatures should be monitored to ensure that the engine is operating within exhaust temperature limits.

Follow the references closely. Surface nicks, corrosion or erosion in the area under the valve head and on the top, or flat, part of the valve head can become stress risers that can develop into a crack and eventual valve failure. Caterpillar's Remanufacturing and reuse criteria will be reviewed. Dealer's reuse criteria should be made more conservative.

Engine valve material is designed to withstand 650°C (1200°F) on a prime power rating and 705°C (1300°F) on an intermittent rating. All vehicle ratings are intermittent because they have a loading cycle, haul and carry cycle, unloading and return cycle. High power demand is on the loading and haul cycle. The unloading and return cycle requires much less power. The unloading and return cycle allows the exhaust temperatures to stabilize at 565 to 620°C (1050 to 1150°F). When the complete work cycle is short, exhaust temperatures are less likely to stabilize.

There are several factors that can contribute to abnormally high exhaust temperatures. Some of these factors are:

... High exhaust restriction caused by faulty muffler. Low exhaust gas flow caused by leaks in the exhaust elbow or manifolds. Manifold gaskets and sealing joints should not leak.

... An inefficient turbocharger that does not provide correct boost. Closely follow correct overhaul and maintenance procedures.

... High fuel flow. Over a period of time, fuel nozzle orifices may erode and allow excessive fuel flow. Rack settings higher than specification will also result in high fuel flow and excessive exhaust temperature.

... High intake restrictions caused by plugged air cleaners will raise exhaust temperatures to unacceptable levels.

A pyrometer with a warning buzzer that activates at 690°C (1275°F) can be used to alert the operator to high exhaust temperature and allow engine load to be reduced.

With correct exhaust flow to the turbocharger, boost pressure and air flow should be within the correct range. With fuel flow within specification, correct air fuel ratios will be maintained. These two factors will ensure reasonable exhaust temperatures. Any engine operating with exhaust temperatures exceeding their limits will have short valve life and the possibility of catastrophic exhaust valve failures.