See more of the story

Q: I have a nice '93 GMC conversion van that has been stored winters since new. There is a clunking sound when moving forward or backward while turning left or right, and it's getting worse. It has been in two shops with no results. One shop removed brake backing plates and lubricated parts, the other said they couldn't tell where the noise was coming from. I feel this needs to be repaired before something happens.

A: Without identifying which model van — full-size or mid-size — or which end of the vehicle the clunking originates from, troubleshooting is much more difficult. But here are several possibilities.

If the noise is coming from the front, the most common source is a broken sway bar link. These tend to rust and when broken often generate a solid clunk. Other potential front-end culprits are worn ball joints, control arm bushings, worn steering tie rods or drag link, a failed shock absorber, broken coil spring or loose steering box or rack. If the vehicle is equipped with a tilt steering wheel, the noise could also originate in the steering column itself due to a burr or wear in the upper bearing assembly. This noise is typically heard as the steering wheel returns to center.

If the noise originates in the rear, possible culprits include a rear axle/bearing assembly, differential or spider gear problem, leaf spring bushing, failed shock or loose exhaust.

If the clunk originates in the brake system, it could be the brake pads moving back and forth in the calipers. If the issue is with the rear drum brakes, wear on the backing plate can cause the brake shoes to "catch" or clunk when applied.

Q: I've been changing my own oil for nearly 50 years and I have some questions. Is there really a necessary difference between 5-20 and 5-30? Why do manufacturers have such odd capacities? My Escape needs 5.3 quarts, my Tacoma uses 5.5 and my son's Contour uses 5.8. Can't they just keep it in full-quart or half-quart amounts for ease of changing? If a car uses a blend can you top the odd amount with a regular oil, blend or full synthetic? Finally, when I first started to change oil I was told to put a bit of oil on the filter gasket to help it seat. Is this correct?

A: One of the easiest ways to improve fuel economy is to fill the engine with lighter weight oil. The less viscous the oil, the less power needed to pump it through the engine under pressure. I remember when car makers switched from 10W-30 to 5W-30, fuel economy improved by 1/10th of 1 mpg. Today's lubricants are so far superior in performance that we now see 0W-20 oil recommended for many new cars.

Regarding oil capacities, I suspect there are two main factors. First, adequate volume to continually lubricate and cool the engine's mechanical components while maintaining continuous flow during acceleration, braking and cornering. Secondly, design constraints for oil pan size, shape and location. And finally, some engines were designed to metric sizes, which are different than SAE measurements. The issue of partial quarts isn't much of a factor since today's oils come in resealable plastic bottles.

As long as the oil meets the car maker's specs, there is no harm in topping up with petroleum, semi-synthetic or full synthetic.

Not only is it a good idea to "wet" the rubber seal on an oil filter to help it seal, partially filling (if possible) the filter before installation helps the engine build oil pressure faster at first start-up.

Q: Is it better for mileage or the car to cycle heating/cooling on and off or just leave them on?

A: Just leave the HVAC system on. Cabin heat is generated by waste heat from the engine in the coolant, so there's no significant cost or impact on the vehicle. The A/C compressor cycles on and off automatically to minimize cost, wear and tear.