Can't say until we know the cause. At a minimum, fuel vapors will leak into the atmosphere and cause pollution. As far as the engine is concerned, the Engine Computer opens a "purge valve" at specific times to draw stored fumes from the charcoal canister. Doing that changes the fuel / air mixture, but since the computer knows to expect that, it modifies fuel metering calculations to take that into account. Where the problem comes in is if that leak results in fresh air being drawn in, the fuel calculations will be wrong. There won't be enough of an error for you to notice a running problem, and that won't hurt the engine, but there is a less-known problem that could result.
The Engine Computer is constantly performing hundreds of tests on components, circuits, and operating conditions. It compares numerous things to each other to know when there's a problem, then it sets a diagnostic fault code to tell the mechanic where to start the diagnosis. The problem is though, once a fault code is set, the computer knows it can't rely on that circuit's information to compare to other things, so it suspends some of those tests. As a simple example, the computer knows that when the engine has been off for at least six hours, the engine coolant temperature sensor had better be reporting the same temperature as the intake air temperature sensor. If a fault code is set for one of them, the tests for the second one might not run, and a problem won't be detected.
When some of those tests are suspended, a new problem may not be detected. That new problem could cause a running problem, like a hesitation or stumble, and while the cause could be minor, if ignored, it could lead to an expensive repair.
The second problem is once the original problem is corrected and the fault code is erased, the rest of the tests resume, and THAT is when the new problem is detected. This is a common cause of taking the car to the mechanic, he provides an estimate for repairs based on the fault codes he is aware of, the repairs are made, then Check Engine light comes right back on during the test drive or within a few hours of you taking the car, and you incorrectly assume the car was not diagnosed or fixed properly. This is most likely to happen when you wait a long time to get the first problem fixed. That provides plenty of time for a second problem to occur.
Thursday, June 18th, 2015 AT 3:40 PM