Question: My 1980 Datsun pickup wears out brakes every 17,000 miles, and, along with this, black dust collects on the front wheels. No other car of mine, even a Datsun 510, got less than 40,000 miles to a brake job. The dealer sees nothing wrong with this kind of mileage. I have put three sets of brakes on, and I want to know if you think this is acceptable.--D.T.L.
Answer: I would consider 17,000 miles between brake jobs premature wear-out in normal usage. Other Datsun owners have raised similar concerns, and the company is currently working on a new brake pad for extended wear.
Datsun is developing and may already have available at dealerships a semi-metallic brake pad. These pads have a higher metallic content in the friction material and should last longer. They are marketed as "heavy duty" pads.
If the dealer does not have the new brake pads, you might consider buying a semi-metallic pad from an independent auto-parts supplier. They won't be original equipment, but there are plenty of top-quality manufacturers to chose from. In addition, your driving style and the type of service you subject your truck to may account for the rapid brake wear.
If you use your truck to haul cargo, you have to expect greater wear on the brakes. A lot of city-street driving will result in greater wear than long-distance highway driving. If you venture regularly into the hills or mountains, that's another factor that increases wear.
Finally, the problem may also be related to a proportioning valve in the brake system. The valve distributes hydraulic pressure between the front and rear brake. If it malfunctions, it causes rapid wear on the front or the rear brakes.
Q: My 1973 Ford Courier is backfiring really badly. The engine runs smoothly. The explosions are worse when going down hill. What could be the matter?--F.S.P.
A: The problem is most likely in the fuel mixture. A slight, almost undetectable, backfire is normal in a situation where you are going downhill without any accelerator pressure. But a severe backfire can cause major engine damage, such as burning out your valves.
One possible cause of the problem is a defective "decel" valve, which is a switch on the carburetor that cuts off fuel when you lift your foot from the accelerator. The valve is operated by a solenoid, a magnetic switch.
If the valve is malfunctioning, the engine is continuing to get fuel in a situation where the ignition timing is not advanced very far. The exhaust valve may be opening before the fuel is burned completely in the cylinder. That sends out a loud explosion that you hear as a backfire.
Another possible cause is incorrect valve timing. Usually, valves don't lose their proper timing unless you have a serious mechanical problem. And backfiring caused by a valve timing problem would not be limited to downhill driving conditions.
Q: About three weeks ago, I bought a new set of tires and had my car's front end aligned. Now when I go 50 m.p.h., the floor seems to rattle. I am a senior citizen and can't afford a big repair bill. My car is a 1976 Plymouth with about 137,000 miles on it.--A.T.
A: You should have had all four wheels spin-balanced when you bought the new tires. A high-speed shake is most often caused by an out-of-balance condition.
On a high-mileage car such as yours, you should also have a qualified front-end mechanic examine the car for worn steering and suspension parts.
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.