Correct me if I am wrong concerning the EGR valve ('86 305 non-ECM). When checking if the EGR valve is fully operational I was told to apply vacuum to the EGR valve (~5 in Hg) and hold up a rag to my exhaust pipes. After this if my engine stumbles my EGR is fully functional, if it does not stumble my EGR needs to be replaced because the pintle is not opening. Why would the engine stumble if my EGR is fully functional, and not stumble if it was not functional? I do not understand the dynamics of this, if anyone does can you please explain it to me.