How do you tell when your egr valve is going bad.

You will need a vacuum guage.

Unplug the vacuum hose to the egr and connect the vacuum guage to the egr where you disconnected then apply vacuum with the vacuum guage. The engine should stumble or die, if it does your egr works. If not the passage to the egr might be clog more common then the egr being bad.

Now you should check the hose that you disconnected to see if it has vacuum. Hook up the vacuum guage to the hose that goes to the egr (one that you disconnected to check your egr) you will need a reverse piece, raise the rpm you should see the guage move up I think (8-21 hg inch of vacuum). If not the soleniod might be bad or the vacuum hoses is routed wrong or clog.

If you gone this far I would check your back pressure transducer disconnect 4 hoses, 1 on one side, two on the other and one on the bottom. Blow through the bottom if you cant that mean it good and if you can that means there is a hole in the diapham and it needs to be replace.

If this is because you fail smog on emission tail pipe for nox I would check egr,o2 sensor, cat, and timing.