Keep Your Diesel Purring

Posted: August 1, 2014

Valve adjustments are vital to engine health.

By: Deane Hislop

Diesel engines are legendary for their reliability, durability and fuel economy. There’s a catch to sustaining those attributes, however, and that’s proper maintenance of key components. A deferred valve adjustment may negate all of a diesel engine’s virtues. Valve adjustment is critically important, yet it’s among the most frequently overlooked or forgotten tasks, because to adjust valves, you have to delve into the heart of your engine.

Intake and exhaust valves are found within an engine’s cylinder head and under the valve cover. Valves, which are shaped vaguely like mushrooms, are opened and closed through a combination of action from the camshaft and the valve springs. The former opens a valve by lifting a pushrod and rocker arm that depresses the valves in a set order. Springs on the valve stems close them again.

When the engine’s running, the valves move at eye-blurring speed in a delicate and precise manner. However, if the balance and precision of just one of the moving parts associated with the valve lifters, pushrods, springs, rocker arms and the valves themselves are affected, chaos quickly ensues. The opening and closing of the valves allows fresh air to enter the cylinder and exhaust gases to exit once combustion is complete. If the clearance between the valve stem and the rocker isn’t within the engine manufacturer’s tolerance, the valve will either open too far or not far enough; it may also stay open too long or close too soon. Any of these scenarios can cause decreased power, hard starting, poor fuel economy and excess smoke.

Valve adjustment should occur at the engine manufacturer’s specified intervals, so consult the owner’s manual for your engine. Adjustment requires a clear understanding of the procedure. While experience helps, it’s not necessary, if you follow the service manual instructions and have the ability to use feeler gauges, a screwdriver and a wrench.

While each engine is different, a basic walk-through of the job is feasible. Following the sequence specified in your engine manual, use the starter and a breaker bar or a crank to rotate the engine — always while it’s turned off — until a specified valve is closed and its associated piston has traveled to the top of the cylinder.

The correct clearance between a valve and the rocker arm or cam varies by engine and sometimes between intake and exhaust valves. In general, it’s usually in the range of .010 to .012 inch, but check your motor maintenance manual for the proper clearance before you start. If the clearance isn’t correct, take your wrench and loosen the locking nut on the rocker arm, then use a screwdriver to obtain the correct amount of space by turning the adjustment screw. When set properly, the feeler gauge should move smoothly into the opening between the valve stem and the rocker, but there shouldn’t be any play there. When the clearance is correct, use your screwdriver to hold the set-screw steady, and tighten the locking nut. Recheck the clearance and proceed to the next valve, rotating the engine accordingly.

Remember, a valve must be fully closed and in its seat before it can be adjusted.

Valve adjustment usually takes no more than a few hours, and when you’re done, remember to replace the gasket on the valve cover. The value of keeping the little workhorses in proper adjustment is incalculable. If valve adjustment is carried out at the manufacturer’s specified intervals, you’ll enjoy economical and reliable engine operation for many more hours.

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