1999 Ford Expedition 5.4L The service engine light come on and at the same time it starting idling weak and shaky like it is about to die at a stop. When I open the hood when it is on it sounds similar to a high pressure air leak.

Kberryman,

Sounds like you are right on top of this one. Scan the computer for codes. Most parts stores will do this for free. My guess is that you have a lean code (maybe a P0171). Before you do that (since it sounds like you have a vacuum leak) try to pinpoint the location. The PCV valve should be in the top of the passenger side valve cover about mid way. Run your hand around the hose all the way back to the source (especially around elbows). If you hear the hissing stop or change drastically you have nailed it. This isn't the only vacuum leak you can have but it is the most common and is fairly cheap and easy to fix.
Post back here and let us know what you find. We'll be glad to help.

Kberryman,

The check engine light is on because that's it's job. It's tellin you that there is at least one stored error code in the PCM. To find out what that code (or codes are) the PCM has to be scanned. Auto Zone does scans like this as a free service. Go to any Auto Zone store, have a scan done, then post any codes that are present here so we can see them. The high pressure air leak that you mentioned is a vacuum leak. If you don't know to check for vacuum leaks, then let us know.

check the pcv vacuum line behind the intake manifold--they sometimes melt and break open, causing a leak.