Humm not sure if it could be the same problem I had this weekend. I just checked mine and I had trouble codes 171 & 174 stored. I reset them with my Scan gauge II and looked around. I had a vacuum leak coming from the left side of the common chamber - Intake manifold. Turns out I have a leak in the lower gasket Well I just used some gasket sealant on it and I'll check tomorrow to see if it sealed tight.