Let's say you have a truck that may be idling rough because the EGR valve is stuck open, but you're not sure. To test and see if it is the EGR valve, could you just put a nice thick vicegrip on the line going from the valve to the intake and clamp it shut (assuming it's rubber), and see if it runs better? That way exhaust doesn't get into the air intake and cause the engine to stumble. Or would it be stumbling because it's creating a vacuum leak by keeping the valve open?
Anyway, I could very easily be wrong, I'm just musing.
S-Mart Corporate Security Division
Necronomicon Retrieval Specialist