Cadillac Repair: egr valve, oxides of nitrogen, vacuum leak

Question
A Guy that does my car a back street machanic took my egr valve lose because my car was acting up> I forget now. what the problem was .anyway he took that lose and said it was bad what is that for and how can he tell it was bad?my car runs rough until i get up to some speed. plugs and wires where changed also

Answer
Hi Robin, What the EGR valve does is create a way for unburned fuel to get sent back into the engine to be burned again when you go from cruiseing at 40mph or more and then let your foot off of the gas. It can also open at highway speeds to help lower the engine temperatures so the engine doesn't ping and create NOx, oxides of nitrogen.

     It is a calibrated vacuum leak because it opens a passage between the intake or vacuum side of the engine and the exhaust side. If the system is working correctly it will be hard to detect when the valve is open. But if it should stick open, at idle the engine will run rougher. If you have the kind with a vacuum hose going to it you can remove the hose and plug it and then tap litely on the EGR valve and get the valve to close. Then after driveing the car the roughness shouldn't return. When you remove the valve you can see that it is partially open if you know where to look which is what I suspect he did.

      The other kind of EGR valve relies on an electric motor to open and close the valve. It too can stick partially open due to carbon build-up and cause a rough idle. When that happens with eithor valve you can sometimes clean it but most times it is just better to replace it.

     The last thing that can happen is the spring in a vacuum EGR can become weak and will open at idle or flutter  causeing a rough idle.

    I hope that makes sence and answers your question. Bill