On a snap throttle test, if the vacuum goes to zero and is very slow to rise back to whatever is normal, you may have a clogged exhaust.
This is just generic stuff, if the needle is jumping around it may be a problem with one cylinder, if it is low and steady, it might be a problem that is common to all cylinders. I'd check for vacuum leaks.
You may need to do a cylinder balance test, you might locate a problem cylinder.
Tuesday, January 6th, 2009 AT 3:41 PM