Q My car’s right brake light won’t work when the headlights are on, although it’s fine when they’re off. I’ve tried changing the bulb. Any ideas?

JON REMOLONA   
   Folsom, California  

A Electrical systems can show a perverse creativity in the difficulties they present. Taillight bulbs that double as brake lights have two filaments. If a filament burns out, one of the functions goes out with it. Installing a monofilament bulb in a dual-function socket can have the same effect. If you have the right bulb with two good filaments, the likely culprit is either a bad ground connection in the bulb socket or a poor connection that, when there’s greater demand on the electrical system, prevents adequate voltage from reaching the bulb.

Q My 1986 Toyota truck, with 20,000 miles on a rebuilt engine, has two problems. First, the engine knocks on acceleration when it has regular (87 octane) gas. Switching to 89 octane cures the knock, but it’s supposed to run on regular. Second, it needs a quart of oil (I use 10W40) every 500 to 600 miles. How can I correct them?

THUONG V. LUU  
San Jose, California  

A Given the esoteric electronic things that can go wrong with modern engines, it’s almost like greeting old friends to encounter knocking and excess oil consumption. Knocking happens when gasoline burns in the cylinder before it should. These days, it’s usually caused by a timing problem, so check the timing and inspect the distributor for wear, as it may have been reused. You might also try a different brand of gas. As for oil, 10W40 is another blast from the past; it’s heavi-er than is proper for most engines of the last 20-odd years. Try 10W30. Still, 600 miles to the quart is pretty grim. It’s possible that rings, valve seals, or valve guides have not seated properly, letting oil into the combustion chambers.

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