QUESTION: I have a Mercury Sable wagon with an automatic transmission. The engine is the 3.0L V6, with 113,000 miles.Consistently while accelerating between 35 and 45 mph, the engine shudders and bucks as though starting off in third gear with a manual transmission. Stepping on the accelerator through this window only postpones the shuddering until the accelerator is released at the higher speed. At higher speeds, the auto runs fine.New plugs and wires and fuel filter did not help, nor did good injector cleaner in a few tanks of gasoline. The Mercury dealer's computer found a bad valve in 6-cylinder causing about 15 percent loss of power, and they think this is the problem. I have doubt about the valve causing this intermittent shuddering. Other mechanics are baffled. I don't know what else to do. This has been going on consistently for almost a year. What do you think is the cause?ANSWER: The first thing I'd do is to be certain the transmission is in the appropriate gear for the vehicle speed and engine load applied. If manually selecting a lower gear causes an engine RPM increase yet the shuddering still exists, we can concentrate our efforts on the engine.If the engine truly runs smooth at certain times and rough at others, the cause isn't likely to be mechanical. A slightly bad valve in cylinder 6 is certainly something to keep an eye on, but this sounds more like an electrical demon.Had you not already renewed the spark plugs and plug wires I'd would have bet money on a cracked spark plug or plug wire boot insulation fault. Could one of the spark plugs have been damaged during installation? Is the distributor cap free of cracks and carbon tracking? Does the ignition secondary system pass an available voltage test? Could a dirty fuel injector be the cause?Identifying which cylinder is misfiring should be our next step. It sounds like the Sable is a pre-1996 vehicle, as you didn't mention the check engine light illuminating or flashing. Post '96 OBD-II equipped vehicles are generally able to identify a cylinder miss and in many cases, in which cylinder it has occurred. Can the misfire be replicated with the vehicle stationary and perhaps half throttle applied against the brake? If so, each cylinder can be momentarily disabled, with the offender showing little or no difference in engine speed and shake. If a specific cylinder is identified, a very close look at all components supporting it should reveal the cause. Should cylinder 6 be the offender, a valve job might be the solution.You mentioned using fuel injector cleaner. A test of each injector's flow rate also can be performed, using a fuel pressure gauge and a triggering device. If all injectors flow evenly, they can be discounted. Other things to check include a careful look at the engine control system's RPM signal and fuel injector commands, among other circuit signals, using an oscilloscope or graphing multimeter. If erratic, these could certainly cause a sputtering, misfiring engine.

QUESTION: My 2005 Toyota Tacoma with less than 20,000 miles has brakes that continuously squeal upon application. The local Toyota garage claims they disassembled each brake, in turn, and found no root cause for the noise, which is now getting even worse. This inspection was done within the three-year warranty period at about 15,000 miles. Can you shed light upon this situation?ANSWER: Brake squeal can originate from a disc or drum brake, but is far more common with disc brakes. The causes of this harmless but annoying sound are vibration of the disc pad and possibly the caliper within its mounts.Noise can occur because of brake pad composition or condition, the pad's relationship with the brake rotor surface, the pad vibrating in its nest, or dry/unlubricated brake caliper slide pins.If the disc pad employs springs or shims to dampen movement, it's important these parts are present and in proper condition. It should be noted worn-out disc brakes can also be noisy, because of metal-to-metal contact between the disc pad backing and rotor (the friction material is worn away), or a metal wear sensing tab is rubbing against the rotor.Disc brake pads come in many formulations. Some offer exceptional brake performance, shed lots of dust and wear out quickly. At the other end of the spectrum a pad might be designed for very long life, adequate performance and less dusting but be prone to noise because of hard material composition.Brake pads also can become glazed because of overheating from heavy use. The shiny and hardened friction surface skips and bounces across the brake rotor surface.When renewing disc brake pads, you should be aware of and select the pad composition — perhaps of three to four choices — best suited to your needs.There are various ways vibrating disc pads can be quelled. Shims behind the pad can help reduce vibration, as well as a cured layer of rubbery adhesive. A very tiny application of high-temperature grease at the edges of the pad backing where they contact the brake caliper and the contact points between the brake rotor and spindle can reduce noise. It's really important grease does not get onto the disc pad's friction surface. Chamfering (tapering) the leading and trailing edges of the pad's friction material might also help keep things quiet.It sounds like you are about halfway through your original set of disc pads. Your best chance of quieting the squeal would be to renew the pads with some of a slightly softer composition, inspect the brake rotor surface — correcting via machining if not proper, and apply the adhesive pad backing and grease mentioned above. Block sanding the original pad's friction surface (removing glazed material) and chamfering the edges could also work if you'd prefer not to change them.If you're still under the 3-year/36,000-mile comprehensive warranty, I'd ask for the fix.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose. E-mail under-the-hoodearthlink.net. He cannot make personal replies.