Let's assume you have already checked for
small vacuum leaks in the rubber tubing on the intake manifold and replaced the PCV valve (if applicable)...
This could be cause by
a small vacuum leak or by a dirty air filter or mass airflow sensor (MAF).
A small vacuum leak will cause the engine rpm to be...
Not exact matches
Could be a
small, infrequent
vacuum leak too I think.
An easy way to check for the
leak is to pull a
vacuum line off the intake and suck a
small amount (1 - 2oz) of transmission fluid into the intake via that
vacuum line.
Having just replaced the PVC valve on a 2013 TSI and still getting a P2177 code I started looking for
vacuum leaks and would like to know if it is normal for a good PCV valve to expel a
small amount of air through the vent hole?
There is a poor - man's boost -
leak test that involves plugging the intake (a pint - sized paint can works well I think), pulling the
vacuum reference line to the bypass valve (the
small hose), blowing into it, and then covering it with your thumb.
Inspects
vacuum hoses and fuel vapor lines and EVAP
vacuum lines for breaks or bends (these lines can be very long so time should be taken when inspecting them; it may be necessary to use a smoke machine to find
leaks successfully, especially
small leaks that are difficult to see)