IO-520D Manifold Pressure Question

Did a pre-buy on an aircraft with a IO-520D. The field elevation was 4700' and air temp was around 90. The engine cowl was on. Prior to starting engine the manifold pressure was reading around 24". After start the gauge dropped to 0". It came up to around 10" during runup at 1800 rpm. The plane is out of annual so I couldn't fly it to see where it would be at normal operating power. Just trying to figure out why it dropped to zero?

Sounds normal. Most manifold pressure gauges start at 10" so even though zero sounds low, at 4,700 msl it may be about 6" or less.

Thanks, I was thinking there may be a leak in the line.

If there was a leak in the line, the number would be higher not lower. With the throttle closed the engine is sucking harder causing the reading to be lower. When the engine is not running the MP gauge is reading ambient pressure, as you noted around 24" at your elevation. An aircraft manifold pressure gauge shows absolute pressure above zero, with a perfect vacuum being 0".

This is unlike an automotive suction gauge which shows 0" when the engine is not running. When running the suction becomes greater, thus a larger number.