Is it the gauge, or something else?

[TBalch]

Several months ago, I began to experience some odd performance from my engine. The power settings I learned to use during instrument training - high RPM, 18.5”, 10 degrees flaps - no longer gave me 100KIAS. Airspeed was usually considerably lower, and in the descent, when 12.5” previously produced 100KIAS and a 500FPM descent, I had to increase power - a lot. At that time, my IA plumbed his pitot-static tester into the MP gauge and found that the MP gauge readings correlated with what his vacuum gauge was reading.


Recently, at the end of a X/C and a long, slow descent, I noticed it happening again. On the downwind I had to make the MP gauge indicate 22” in order to get 100KIAS. After landing and exiting the runway, I closed the throttle, set the brakes, and took a picture of the panel. With the throttle closed, prop at high RPM, tach indicating 670 RPM, the MP gauge was reading 23” – cruise power! When I shut it down, the MAP gauge slowly returned to ambient pressure.

The prop governor has recently been overhauled, so I don't think it's that. Opinions welcomed.

 

[Silvaire]

Not to be smart but the answer to the question is "Yes" it is the gauge, or something else.

 

[455 Bravo Uniform]

Gauge may be fine, fittings or line might be leaking. Engine vacuum is escaping before getting to the gauge.

 

[Dan Thomas]

If it takes time after shutdown to return to ambient, there's probably a blockage in the line somewhere. Plugged fitting, hose liner peeling loose and being sucked in, kinked hose maybe. Might be both that and a leak.

 

[Magman]

During shutdown , gases often reverse and deposit residue in the gauge. Some installations have a tiny hole in the line near the cylinder to reduce the build-up.

With the gauge on the bench put some fuel in the port. Allow to sit for a little while and dump out. Repeat a number of times. Hopefully you are cleaning the crud out.

Instrument guy showed this to me . He said some competing shops called this an overhaul!