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In last week's discussion I told you a little knowledge of your car's OBD
II system could save you a $70 service call (I've actually heard reports of up to $200!). Even if you don't own the Craftsman 1200 piece tool set there's no need to pay a professional tech to tighten your gas cap. Want to know more? Read on!
If you're in a hurry, here's the short version. Your vehicle's on-board computer can tell if you didn't eat your Wheaties before tightening your gas cap. A loose gas cap allows fuel vapors (hydrocarbons) to evaporate into the atmosphere. That's considered an emissions problem, so your check engine light turns on. So, just tighten your gas cap and the light will turn off? It's not quite that simple. In fact, that Check Engine light has enough quirks that I'll make that the topic of next weeks email.
If you have a couple minutes, here's the whole story. Your car doesn't *exactly* know your gas cap is loose. It just knows that there's a leak in the fuel system somewhere that's letting fuel vapors escape into the atmosphere. More often than not, the fuel cap is the culprit. Either it's loose or the seal is leaking. How does the car know this? It uses a fairly complex test, applying a vacuum to the fuel system and monitoring how long it takes the vacuum to "leak down". On new vehicles, it can detect a hole as small 0.020 of an inch!
So how do you know for sure? The best way is to use a scantool to read the code from your car. The AutoTap OBDII scantool for PC or Palm can read the codes (and much more). One of the most common codes that results from a loose gas cap is P0440. If you read this code, check your gas cap. Is it loose? If so, tighten it up, use AutoTap to clear the code and drive on. If the gas cap wasn't the culprit the Check Engine light will come on again within a couple "drive cycles" (another good topic, lets make that the subject of another email) and you have more diagnostic work to do. Odds are good though that the gas cap was the problem and you can drive with the peace of mind that your car isn't a breakdown waiting to happen.
What if you don't yet own an AutoTap scantool? It's a little like flying at night with no instruments; you don't have much to go on. If you've recently refueled, double check your gas cap. If it's loose, tighten it. If that was the problem you'll have to complete a drive cycle or two before the light will turn itself off. Your dealer will be happy to scan your car for codes, but has a one hour minimum on labor so bring your Visa card. Some parts stores will loan you a code-reader tool. You can use that to read the code, then look it up in the store's manuals, or jump on www.autotap.com and plug the code into our online DTC Lookup Tool.
Next week I'll talk about the secrets of the Check Engine light - what can make it turn on and off, and what you need to know about it.
Happy Repairs,
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