Check fuel cap message

The first thing is to have the diagnostic fault codes read and recorded. Almost certainly there is going to be one related to a "gross evap. Leak", meaning the vapor recovery system has a large leak. A loose gas cap is the most common cause for this code, but definitely not the only one. The entire system must be inspected visually for loose or rusted hose clamps, dry-rotted hoses, and things like that. When no other cause can be found, your mechanic will use a smoke machine to inject a white, non-toxic smoke, at 2 psi, then he will look for where it is sneaking out.

The fact the warning light is not on all the time suggests the leak is after a valve that cycles on and off at different times. That makes using the smoke machine a little more involved. The conditions that lead to the leak occurring must be present for the smoke to find the leak so it can be identified.

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.

This sounds like you have a fuel tank pressure sensor that is not working right Here is the location so you can replace it. Check out the diagrams (Below). Please let us know what happens.

Hi moejoe1238,

It probably is some gas vapor leaking. The seals might look good but within the cap there is a pressure valve which might be faulty resulting in leakage.

You might have to replace the cap to rectify the problem. You also need to check other evap system components.