Question: I just had my car in for its 90,000-mile service and was told I needed to replace the water pump. I trust my mechanic's judgment but would like to know more about what the water pump does and why mine needed replacement. The bill came to an additional $178 beyond the normal service.Answer: A water pump, attached to the front of the engine, circulates coolant through the engine and the cabin heater. Water pumps are typically belt-driven either by the timing (camshaft drive) belt, a serpentine belt (a wide belt that snakes around many pulleys) or an individual V-belt. Transverse (sideways-mounted) engines used on most front-wheel-drive vehicles typically tuck the water pump up very close to the engine, under a plastic cover. It's driven by the timing belt to save space. Longitudinal (in-line) engines are more likely to have a visible water pump, driven by a serpentine or V-belt.There are basically two things that go wrong with water pumps: leakage or bearing failure. Since the pump is driven by a shaft and pulley, a seal is needed to keep the coolant in, relative to the rotating shaft. Seal failure typically results in a small to moderate rate of coolant leakage to the ground, often with little visible symptoms (dripping, spray or steam) under the hood. A puddle of orange, red or green fluid under the front of the engine and a declining level in the radiator overflow container might be caused by a leaking pump or pump-to-radiator hose. It's important to notice these symptoms promptly, to prevent engine overheating and serious damage.The water pump's shaft and drive pulley are supported by a bearing located just in front of the seal. Excessive belt tension, normal long-term wear or seal failure can cause the bearing to become loose and noisy. Bearing looseness can be checked on a longitudinal engine by wiggling the fan blades (engine off, of course). Unfortunately, bearing looseness is not detectable on a timing belt-driven pump. A deep growling/grating noise also might occur when the bearing goes bad, and should be investigated right away. A severely failed bearing on a longitudinal engine can result in the fan assembly wobbling badly enough to carve fatal grooves in the radiator.A timing-belt-driven water pump that seizes up due to bearing failure can throw or break the timing belt, crashing the engine's valves into the pistons, and that's a very expensive repair. For this reason, many shops recommend renewing the water pump along with the timing belt as a precaution.The moderate cost of your water pump replacement leads me to believe it might have been replaced as a preventive measure in conjunction with a 90,000-mile timing belt renewal. When the covers and belt are removed, it's far easier to renew the pump, compared with going in for the pump alone. Periodic coolant renewal, perhaps every three years or 36,000 miles, will help ensure the new pump lives a long and happy life.

QUESTION: The oil-can light has begun flashing on my '89 Golf at times after I've been driving for a half-hour or more. What could be causing this? Is it serious?ANSWER: I'm glad you checked on this, as it could be a serious problem in the making.A low oil level is the most likely cause, but you've checked that. The next most likely cause could be worn engine bearings, which allow the oil pump's volume to squirt more out of places it would normally be retained. It's also possible your oil has been contaminated with fuel and is thinner than it should be. Finally, the oil pressure sending unit — the warning lamp's boss — could be giving a false indication.Worn engine bearings might provide some indication. Usually they make a knocking sound at start-up, but the problem also can be impossible to verify without engine disassembly and measurement. Changing the oil and filter might make enough difference to turn off the light, although if the previous oil change was on schedule, the cause of contamination should be verified and corrected.The oil pressure sending unit is a bit awkward to test, so the best way to verify its operation is to remove it and temporarily substitute a mechanical gauge in its place. The oil sending unit on your Golf is amazingly easy to access. It's located on the driver's side end of the cylinder head. This will set the record straight on what's really happening inside the engine. Be sure to run the engine through several cooling fan cycles (idling for perhaps 15 to 30 minutes) or drive the car at freeway speed to replicate the hot engine condition when the lamp would glow.