I just returned from a trip to the mountains and was surprised to find that my GMC pickup occasionally blows a cloud of smoke after I use a lower gear on steep inclines. It also produces a much smaller but noticeable puff of smoke upon starting, after it’s already warm. How serious is this? It has 134,000 miles and other than this, runs perfectly.

-Mike Chavez Prunedale, Calif.



Mike, it sounds as though your pickup’s engine is suffering from worn/faulty valve guide seals and/or worn valve guides. The upper (tip) end of the each valve is located in an oil bath environment and the opposite (head) end is subject to reduced pressure (vacuum). Intake valves are the greatest oil-consuming offender, although the flow of exhaust gases past the valve neck can draw oil down an exhaust valve guide/stem as well. Besides causing occasional smoking, faulty valve guides and seals can allow carbon buildup on the neck of the valve, reducing airflow and engine performance.

Depending on the engine type, a variety of rubber seals might be used to regulate oil passage into the valve stem/valve guide region and, if dried, cracked or faulty, oil consumption can dramatically increase. A worn valve guide allows rocking of the valve stem and stresses the seal, accelerating leakage. Smoking during or just after a long period of deceleration is common as intake manifold vacuum is at its greatest and there is little or no fuel present in the combustion chambers to dilute the oil.

It’s possible on many engines to renew the valve guide seals without removing the cylinder heads. Before doing so I’d run a compression test to determine if the engine is worthy of a midlife repair of this moderate magnitude, and consider renewing the timing chain and gears at the same time. If you’ve changed the oil regularly and have never overheated the engine, another 100,000 miles of service might be possible before engine renewal or rebuild.

Renewing and/or upgrading the seals involves disengaging/removing each valve’s keeper and spring, while being sure not to allow the valve to fall into the cylinder. This can be accomplished by bringing the related piston to the top of its travel and pressurizing the combustion chamber with air. A length of limp rope also might be inserted into the spark plug hole, filling the combustion chamber. Depending on the vintage of your cylinder heads, it might be possible to upgrade or supplement the original seal design to help compensate for moderately worn valve guides.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif. E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190.