Your Car: Car Exhaust and Upgrading Gas
Q: There are three grades of gasoline available at many filling stations. Are they checked for octane accuracy? Is the sound of pinging a good way to determine whether an engine needs higher octane fuel?
MICHAEL GAUTIER
Marinwood, California
A: Most states—including California, Idaho, Montana, Nevada, Oregon, Utah, and Wyoming (but not Alaska)—inspect gas sold at the pump for octane level, either as a regular program or on consumer complaint. But the average motorist can't determine the accuracy of advertised octane. An engine might ping because it was designed to use higher grade fuel or because it has mechanical problems such as incorrect timing, running too hot, or carbon deposits in the cylinders. If the car pings when using fuel of the octane recommended in its owner's manual, going up a grade in gas may be a short–term solution. If the car used to run well on a particular grade but no longer does, you might try the same grade from another station before you do anything more expensive.
Q: I've just had the oil in my 1991 Cadillac changed, and now when I start the engine black exhaust comes out of the tailpipe. What do you think can be causing this?
ALBERTO TEVES
Lakeport, California
A: Car exhaust comes in three colors plus invisible. Black indicates unburned or poorly burned fuel (gasoline, not oil). This could be related to changing oil if, in the process, something was knocked loose; the vacuum hose of the fuel pressure regulator seems the likeliest suspect. But black exhaust may have no relation to your oil change—a bad sensor, computer error, or other gremlins could be the cause. If the car were burning oil excessively, the exhaust would be blue. And if coolant were leaking into the combustion chambers, billows of white would follow you wherever you drove, which wouldn't be far. A lesser plume of white is usual on cold starts as water in the exhaust system is vaporized, but this ends when the car warms up—and then you can't see the exhaust at all.
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Please send car problems to Your Car, VIA, 150 Van Ness Avenue, San Francisco, CA 94102. Send email to viamail@csaa.com. Questions will be answered only in the magazine.
This article was first published in May 2006. Some facts may have aged gracelessly. Please call ahead to verify information.
