I was helping my brother in law with his '94 Saturn (failed smog for "EGR Function") when we came across this symptom that I don't understand. There are many things I don't understand, but cutting straight to the chase . . .
When applying vacuum (sucking on the hose to the EGR valve, it holds vacuum nicely (sticks to the tongue) and moves the diaphram and piston in the EGR valve a good 1/2 inch or so. But only with the engine off. When the engine is running, no amount of sucking will build up a vacuum, and the diaphram barely moves.
My first thought was that the exhaust pressure must somehow be leaking into the vacuum chamber of the EGR valve, but this idea doesn't make sense to me. There's only a mechanical linkage, so what gives?
Well, intelectual curiousity aside, the real question is should we shell out the $80 for a new EGR valve? Is this a known symptom of a bad valve, or should we look elsewhere?
TIA