there is no check valve only a diaphragm pump on the side of the engine. if this is what you are talking about it is a diaphragm sandwiched between two aluminum blocks and then a gasket between that and the engine.
how to change a valve cover gasket on a 97 mercury grand marquis ?
The PCV valve is located on the back side valve cover. It should be a plug in type and easy to change.
check your idle air control valve
The 2000 Mercury is right at the model change for PCV valves. Some had the valves in a hose under the throttle body. The newer style has no PCV valve at all.
ask a car dealer
Valve springs
You'll need to install a tee into the line by the fuel filter to check it, there is no shrader valve.
Vacuum check valves are usually located close to the vacuum source.
Remove the bolts around the valve covers then the hoses. If your not familiar with working on cars you can take a picture of it before you start removing parts that way you will now where everything goes . Also check out youtube
On a 2000 Merc. it's usually an emissions problem. You can try a water dryer for the fuel and change the PCV valve. If the light remains on, then more than likely it's the EGR valve. Have it tested for free at autozone or Advance auto.
There is no shrader valve on these vehicles, the only way to check fuel pressure is to tee into the fuel line by the fuel filter.
The 2002 Mercury Mountaineer EGR valve can be found in the engine compartment, on the firewall. The EGR valve will be labeled as such.