a lot of times carbon builds up where the egr valve bolts to the mounting area get a can of wd 40 and a wire brush and clean it good use a new gasket and give it a try there is a plunger on the valve and the carbon prevents it from moving up and down

You should be able to blow through it one direction and not the other. These can be rinsed out and cleaned with gasoline. After you rinse it with gasoline, if you can not blow through only one direction then it needs to be replaced.

you do not want to clean an EGR valve with gasoline or solvents as they will damage the diaphragm.

the way to test the actual egr valve (efi or carb)is to 1.pull it off 2. visually inspect the valve and the exaust gas entrance. theres always a little bit of carbon buildup. 3. clean out any chunks of carbon. 4. you can use solvents as long as they're not really hot like laquer thinner. I use brake non-chlorinated brake cleaner. if you think about it really hot exaust gases (even though inert) hold pressure inside this valve untill it opens. 5. there will be a nipple that you're vaccum hose was attached to. simply using your lips suck on the nipple and you should be able to tell from feel if the valve is not seated all the way or if the diaphram is void.... now if you are doing this on an EFI engine then you also need to make sure that IF there is a sensor on top or on the side that there isn't a bad seal. this could cause a slight vaccum leak. you will also want to make sure none of you vaccum lines are cracked, lose, or misrouted. vacuum port caps can also, with age and heat become loose and create a bad seal. EFI will have a vacuum "modulator" or pump that applys the correct amount of vacuum at certin times (when the engine is hot and at cruising speeds) the modulator may have a bad seal or bad electrical connection. OR another way is to go buy a chilton and read how to check the egr system. EGR systems are sometimes really hard to diagnose. OR The easiest way ever might be to go get an emmissions test if the valve isn't working than you'll fail and they will usaully tell you what is defective. (thats how most places make their money, by fixing a problem most technicans have experience in idetifying after a failed test. So, they will tell you the reason then you leave and fix it yourself. This is all based off my experiences in ohio, massaschusetts and anchorage alaska. results may vary depending on location.