There should be an acceptable range that will be listed in your
maintenance manual. I'm sure that being in the green at full throttle
will not be harmful to the gauge. Even vacuum a bit lower than the
green should be sufficient to spin the gyros. Check the lines for a
pinhole leak, which could also be introducing dirt into the
instruments. I won't give you the ol' "I told you so" about having a
recent instrument rebuilt just yet!



In the last couple of months we have replaced both vacuum instruments (DG
and AI) as well as all vacuum hoses.

Immediately after this was done, I noticed our suction was lower than
before. It would top out in the 4.8 region -- at the very low end of the
acceptable scale.

At the same time we also noticed that our newly overhauled DG was precessing
excessively. It would precess 10 degrees every 15 minutes or so. Sometimes
less.

Before we pulled and returned the DG, I wanted to make sure that the lower
suction wasn't part of the problem, so today I had my A&P adjust the vacuum
regulator. It's now reading 5 at 2000 rpm, and almost 5.2 (the high end of
acceptable) at full throttle in a ground run. We left it at the high
setting to see where it settles out after a flight or two.

Question: Will this increase in vacuum help the DG's precession problem, or
is the DG toast?