Thoughts on possible bad gas - the car kind?
February 18, 2008 7:08 AM   Subscribe

Bad Gas, the car kind, I think - advice?

My 18 month old to me Newsed car, 17,000 miles, started knocking the other day. I got cheap gas a few days ago, and it's been wet, humid, and rainy. I thought I might have gotten bad gas. So this morning, with less than a quarter tank left, but before the low gas warning went on, I topped off at my regular station. Driving home from that, a warning light came on. The owners manual says this particular light is a diagnostic that could indicate a loose gas cap or bad gas, and if it stays on through a few normal driving cycles, to take it in for service under it's own power. It now occurrs to me that my instincts may have been wrong and I should have burned off as much as the alleged bad gas as possible before refuleing.

Any suggestions or alternate diagnosies before I call tomorrow for a garage appointment? What could I be looking at and what might the garage do?
posted by rainbaby to Travel & Transportation (11 answers total)
It could be any number of things involving the fuel or ignition systems, but many of them should be covered under the emissions warranty, if not the manufacterers warranty.
posted by TedW at 7:16 AM on February 18, 2008

my engine light was on about a month ago. I had filled up the day before and my gas cap was on tightly, so i decided i would go through two fillups and then call for service. The first fill up after it came on (still had about 1/4 of a tank of the "bad" gas) it stayed on the entire time. The next time I got gas, it immediately went off. If you notice loss of power or problems shifting, call immediately. If it is still knocking, take it in. If no problems other than the light, go through one more tank before you take it in.
posted by domino at 7:53 AM on February 18, 2008

There is a product called ''Heet'' made for getting water out of your gas. Should be available at wal-mart or an auto parts store, and won't hurt anything, even if water isn't the problem. Might be worth a shot.
posted by entropic at 7:57 AM on February 18, 2008

Try some Dry gas- unless it's a diesel.
posted by jenkinsEar at 8:21 AM on February 18, 2008

I recommend Heet too.
posted by bprater at 8:47 AM on February 18, 2008

I'm thirding Heet. I had the exact same problem with my car after filling up at a seedy gas station (a Mazda Protege ) and it stopped the problems. It only costs $2.

If you're paranoid you can stop by any Advance Auto Parts and use their diagnostic equipment for free. There will be a plug somewhere on the drivers side that you will plug the device into and it will tell you what is wrong. If there is water in your gas it will say something like 'fuel injector #x not functioning'. If Heet doesn't fix your problem you might want to see a mechanic. Ignoring check engine lights is dangerous business.
posted by Alison at 9:17 AM on February 18, 2008

So-called "dry gas" (which I used to use faithfully) is pretty much obsolete since the introduction of Ethanol at the gas pump. I don't know how it is in the rest of the country, but when you buy gas in Massachusetts you are pretty much buying gas with 10% "dry gas". I believe "HEET" is just a brand-name for "dry gas".

In any case I don't think you problem has anything to do with adding "dry gas" to your gas tank (we used to do this to prevent gas line freeze-ups, which were fairly common - in that event your car just doesn't run, until you melt the ice in the fuel line).

Your problem, especially the fact that it was humid and warm, sounds vaguely familiar from my nightmare days of owning a Volkswagen "new" beetle (the worst vehicle I have ever owned). I would definitely try higher-octane gas but I think there may be something wrong with the emissions sensor. My beetle had so many problems, it's hard to remember them all.
posted by thomas144 at 9:38 AM on February 18, 2008

I had this exact same problem over New Years in my truck while travelling cross-country.
Since the manual said that it was an emissions problem and wouldn't cause harm, I just kept on my trip. I didn't want to try to deal with some middle of nowhere mechanic.
I filled up about 5 times, always making sure it was from one of the Top Tier Gasoline retailers, and the light went off and has stayed off since.
I didn't use any Heet type product.
posted by whoda at 10:53 AM on February 18, 2008

(I'm not a mechanic, but I do like cars. My experiences are all with Mercedes, so YMMV.)

What kind of car do you drive? By "cheap" gas, did you mean a lower octane than what the car was originally designed to use? If that's the case (or, for that matter, if you unwittingly received some bad gas), it's possible that the ECU has retarded the timing in order to prevent premature detonation ("pinging"). Depending on the car, I can imagine that might trigger the "check engine" ("service engine soon," "multifunction indicator lamp," or "MIL") light.

In most cases, the engine computer will go through cycles in which certain diagnostic trouble codes (which are set by error conditions) are reset after a period of time or number of restarts. This is especially true of the loose gas cap (vacuum loss), a condition which could allow hydrocarbons to escape through the filler neck. The emissions system monitors whether or not it can maintain a vacuum; if it can't, the "check engine" light illuminates and a trouble code is set.

That said, you'll really need to have the code(s) retrieved from the engine computer for a reliable diagnosis. Most of the time, they'll point directly to the problem at hand. Keep in mind that trouble with an O2 sensor or a mass airflow sensor (MAF) — common trouble spots, especially on some BMW, Mercedes, and Volkswagen cars — could resemble the symptoms of bad gas. Either of those issues would have to do with fuel mixture, and could cause the car (once again, speaking generally) to run too rich or too lean.

As long as the light is on, it's best to treat the car as though it's just being broken in. Stick with light throttle, and try to avoid placing any kind of heavy load on the engine. If the light doesn't go out, head for your dealership or trusted service center. In addition to detecting a loose gas cap, the "check engine" light can also indicate serious emissions problems. Some of those can cause damage if left unchecked. If the light starts blinking, then it's indicating a critical problem that needs immediate attention.

Good luck, and happy motoring!
posted by scoria at 11:03 AM on February 18, 2008

thomas144 writes "I believe 'HEET' is just a brand-name for 'dry gas'."

Heet is Methyl Hydrate, the alcohol in pump gas is usually ethanol.
posted by Mitheral at 12:20 PM on February 18, 2008

Response by poster: Very much thanks, people. It was not the check engine light, but another type of light, a diagnostic thingum. . . I did a gas treatment product and the knocking and the warning light has gone away. If it comes back, I'll take it in.
posted by rainbaby at 6:09 PM on February 18, 2008

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