Q I have a 2002 Chevy Blazer Xtreme with the 4.3-liter engine and automatic transmission. I use the truck to tow my Austin Healey on a trailer to vintage-car racing events.

For towing, I added an aftermarket transmission-oil cooler, and it seems to work well. Is there any need to cover the cooler in winter to prevent the transmission from running too cool? I have been towing in third gear because I understand towing in overdrive will overheat and stress the transmission. Is that always true, or is towing on a flat freeway OK?

I changed the transmission fluid and filter when installing the cooler and used Dexron VI fluid, which replaces Dexron V. How is that different? Is it synthetic?

A The installation instructions for most aftermarket transmission coolers suggest "plumbing" the cooler "in series" with the cooling chamber in the radiator by connecting the cooler into the fluid return line from the radiator to the transmission. Plumbed in this way, transmission fluid is pumped from the transmission, through the cooling chamber in the radiator, through the aftermarket cooler, then back into the transmission. This is the most effective way to keep transmission fluid temperature under control -- meaning in the range of 200 to 230 degrees -- in extreme conditions.

A quick look at the flow of transmission fluid illustrates the benefit of the extra cooler. Hot transmission fluid, perhaps above 250 degrees, flows from the transmission through the radiator where it is cooled by 190-230 degree engine coolant -- not a great deal of cooling effect from only a 20- to 60-degree difference in temperature. But as the fluid flows through the aftermarket cooler, it's cooled by ambient airflow with a temperature differential of 200 degrees or more. The greater the temperature differential, the faster heat is dissipated from the fluid.

But what about winter? If ambient air temperature is extremely cold, the fluid-to-air cooler may overcool the fluid. There are three ways to deal with this. A number of newer transmission coolers feature a self-regulating characteristic. When the fluid is thick or viscous because of cold temperatures, it will flow only through the larger-diameter upper tubes and plates in the cooler, limiting the amount of additional cooling effect. As the fluid warms up and becomes less viscous, it flows through the smaller tubes and plates for additional cooling.

Or you can plumb the cooler so that the fluid flows through the extra cooler before it flows through the cooling chamber of the radiator. In this scenario, the fluid can actually be warmed to normal temperature as it flows through the radiator chamber, a useful benefit in sub-freezing temperatures.

Or you can cover the extra cooler to limit airflow and cooling effect.

The logic behind towing in third gear rather than overdrive is a question of available engine torque and fewer shifts. If the vehicle delivers enough power to operate in overdrive on flat roads without frequently shifting in and out of overdrive, leave it in overdrive for best mileage. In hilly terrain or if the transmission wants to frequently downshift out of overdrive, shift down to drive until you're on flat ground again. There's absolutely no harm in driving in Drive, and the torque converter will still "lock up" to maximize fuel mileage and minimize heat buildup in the transmission.

As for the new Dexron VI transmission fluid, I found a technical article from Kendall Oils that describes the benefits nicely.

"It is formulated to provide improved oxidation stability, shear stability, friction durability and foam resistance compared with earlier generation Dexron fluids to help ensure long service life, better performance in extreme conditions and consistent shift performance for the life of the fluid. It is particularly recommended for use in new GM six-speed automatic transmissions, and is fully backward-compatible with older GM automatic transmissions."

Q I have a 2005 Chevy TrailBlazer that I love to drive, but it seems to have a very intermittent loss of power that lasts only for a split second. The engine never stalls or dies, but the headlights blink and you can hear a hesitation in the blowing sound of the climate controls when you have them on. Any idea about what's causing this? Maybe the voltage regulator?

A By "intermittent loss of power" I assume you mean electrical power, not engine power. I don't think the problem is the voltage regulator, but I do suspect there's an intermittent ground connection somewhere. I checked my Alldata database, and it pulled up TSB 04-08-45-010 dated September 2004 that identifies the possibility of a poor ground connection at "ground splice pack G201" on the right side at the front of the console area. If your vehicle is still covered under warranty, the dealer would be the place to go for repair.

The repair involves removing the center console and carpet panel, then locating, disassembling, cleaning and properly reassembling the ground connection.