QUESTION: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.ANSWER: 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.

QUESTION:A couple of weeks back you told how to check your air conditioning system to be sure it was working properly. Mine unfortunately didn't pass the temperature test you mentioned. My question is, what can I expect if I take it in for service? I keep receiving mailers from my repair shop offering an "air conditioning service" special for $99. Will this correct the problem or could I be in for a larger bill?ANSWER: Let's start with a look at some of the reasons an A/C system might fail to function properly, then we can tackle the service options.The most common condition and owner complaint is the system blows warm air. This is often the result of insufficient or no refrigerant - because of leakage or an electrical fault disallowing compressor operation. A system that blows barely cool air or intermittently cool air might be suffering from moderately insufficient refrigerant, condenser cooling fan problems or a mix-up of heated and chilled air in the air-distribution system. It's also possible the refrigerant control device (either a thermostatic expansion valve or metered orifice) might be contaminated with debris or be faulty.A worst-case scenario is compressor failure, which involves not only replacement of this expensive part, but careful flushing of debris from the system's many passages.Refrigerant leakage is common, as the tiny refrigerant molecules can sneak past worn or dry seals, and through rubber hoses, and the system is under continuous pressure, even when inactive. In severe cases, corrosion-caused perforations or damage to the aluminum condenser or evaporator can lead to rapid leakage and a hefty repair bill. Determining the cause of leakage and correcting it is essential to protect the environment from escaping refrigerant and to provide a lasting repair. R-134a, the more environmentally friendly refrigerant used since the early `90s, has a greater tendency for leakage than the older and now very expensive R-12 refrigerant used in the past.What's involved in an A/C service? This is a real wild card. At the very least, this service should include a system performance (temperature and air distribution) check; visual inspection of hoses, fittings and the compressor drive belt; and an accurate estimate for additional work needed. Additional inspections might include refrigerant identification (important on older, often compromised R-12 systems), a refrigerant leak check and light diagnosis of possible electrical faults.On R-134a vehicles, because of the modest price of refrigerant, it's possible a shop might add a pound of refrigerant to top off a moderately low system with no obvious leaks. This is a poor practice, though, as it's virtually impossible to determine system fullness, and an overcharge condition can cause problems. The right way to add refrigerant is to evacuate the system fully and recharge with the specified quantity. Leaking R-12 systems require repair and retrofitting to R-134a, because of the astronomical price of the older, no longer manufactured refrigerant.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. He cannot make personal replies.