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?