Question: We have a 1982 GMC 6.2-liter Diesel Suburban and have had trouble with the transmission, a four-speed automatic overdrive. It has been repaired four times, and the last time we almost started a fire while pulling our 300-pound trailer. All the transmission fluid gushed out. Only then did GM tell us about an extra oil cooler for $200. Is it worth it?--P.M.
Answer: If you plan to continue towing your trailer, it would be a wise investment. Your GM transmission, like so many others, is not meeting the service demands it was designed for. A cooler might get you out of the repair rut you seem stuck in.
In towing heavy loads, your transmission gets very hot. At a certain point, the transmission fluid foams up, then sprays out the dipstick hole.
Automatic transmission fluids take a real beating when they overheat. At the normal temperature of 175 degrees, the fluid should last 100,000 miles. At 195 degrees, its longevity drops to 50,000 miles. At 315 degrees, it will break down after only 750 miles.
Another part of the problem is that automatic transmission fluids don't have much lubricating property left in them. Oil from sperm whales was the predominant lubricant in the fluids prior to 1973.
But a ban on killing the rare and endangered sperm whales cut off that lubricant. Most petroleum and synthetic substitutes attack rubber parts in the transmissions. It appears, however, that the oil from the jojoba plant is gaining increasing acceptance in the automobile lubricating market, according to Wynn Oil Co.
Jojoba oil is chemically similar to oil from the sperm whale. It is derived from the seed of the jojoba plant, which is grown in the California and Arizona deserts. But it is still very expensive. General Motors dealers market an additive for automatic transmissions that may also help.
Q: We have a 1977 Honda Accord, which has a strange sound coming from the transmission. It is a deep, shuddering thud that comes most often when the car is in reverse. There may be a brief loss of power, then it's as if nothing happened. Several Honda men had no explanation, other than to tear down the whole transmission. Do you have any advice?--O.M.
A: I would not suggest you tear down the transmission for an intermittent noise as long as it is providing good service. But you certainly should have the transmission and engine thoroughly inspected on a number of safety points.
A loose motor mount will often cause a loud hammer sound when changing transmission settings if the transmission is an automatic or when engaging the clutch if it is a manual.
You should also check the gear-shift cables, which may need adjusting. The noise could result from the transmission shifting lever not moving to the correct position.
Q: With the big reduction in lead content of leaded fuel, are there any additives available yet that help compensate for the lead reduction?--A.V.
A: Unfortunately, the major oil companies have not yet reacted to the large potential market among older vehicles that could benefit from a lead-substitute additive.
Several smaller oil companies are reportedly developing an additive that would help protect engine valves and valve seats in older cars.
Ralph Vartabedian cannot answer mail personally but will respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, You section, The Times, Times Mirror Square, Los Angeles 90053.