August 21, 2009


August 21, 2009 7:47 A.M.

I read a post from one of my favorite bloggers the other day, the Anti-Supermom, about her frustrations with her "check engine" light on her van, among other things.

I was inspired by that post to begin one of my own, thanks Anti-Supermom! Haven't thought of a good name for it yet, but it will be a weekly post with advice on cars. I have over 10 years of experience as a professional auto mechanic, and I hope by posting the mechanic's point of view, it will be an entertaining and enlightening part of my blog. Any suggestions or comments about a name would be greatly appreciated.

As I got the idea from the Anti-Supermom, my first piece of advice will be about her problem, the dreaded "check engine" light.

I posted a comment on her blog about the fact that I have seen this issue on countless occasions, and believe me when I say that was not an exaggeration. Many of them involve a simple problem, a loose or misplaced fuel cap, but that is only one cause of that particular "trouble code" as they are called to be "set" in the engine's computer.

Due largely to emission laws and gas mileage concerns, cars today are built with a closed system for containing gasoline fumes from escaping into the atmosphere from the fuel system, which of course includes the fuel tank. The way the integrity of this closed system is monitored is the main reason so many people see the check engine light, and why it seems to stay on for a long time after the problem is corrected.

Now in order to keep this short and sweet I will just say that the engine vacuum is used to check that the system is secure and leak free, and if it is not, then the light comes on. Mechanics can check the engine computer for the trouble code, and correct the problem easily if it is just the fuel cap, but often the fuel cap is not the problem, or is not the only leak in the system. The tubing that carries the vacuum and the fumes from the fuel tank to the engine is most often made of plastic, and is easily punctured or cut by road debris or damaged from underbody collisions with the pavement, as in "bottoming out" over a large speed bump. Even so, some of these punctures may be very small, pinhole sized, and very hard to find.

The "evaporative emission" system is not a constantly monitored system, like most of the engine's sensors and control mechanisms, and is only tested under certain engine conditions. This is the main reason the check engine light seems to stay on for an indefinite period after the problem is corrected. If the light remains lit, and the fuel cap is verified to be secure, then there is likely to be another leak somewhere else in the system, or a malfunction in the valves that are part of the monitoring and operating system itself. It is always a good idea to have a mechanic check the trouble codes, and to run tests on the system just to make sure that there are no other leaks.

In general the check engine light DOES indicate a problem with the engine, so any time the light comes on and stays on, or goes on and off regularly, even if the engine seems to be running normally it saves money in the long run to get it diagnosed early. Some failures don't cause any perceptible engine performance issues, but if left unrepaired, can cause costly repairs later on as other systems are affected. So please, if you do see the check engine light, make sure you find out what it is, to avoid more problems down the road.

Well that's my advice on that issue. I welcome any comments or questions on car stuff, I love to tell people what other mechanics seem to like to keep secret. This stuff isn't rocket science, and lots of things can be diagnosed and repaired by anyone that can pick up a wrench. I guess it's a matter of money with most mechanics, they figure the more people know, the less money they'll make. Not me!

Until next time!