OK I,m sure this topic has been beaten to death and I've searched many previous posts but here is my issue. I took my boat in for a fuel issue but also changed the impellers and housing because I had noticed the port engine gauge was not moving at idle, but would jump a few psi if I brought the rpm over 1,000. The gauge still reads zero at idle but the mechanic told me he tested the pressure with a mechanical gauge which showed 4 psi at idle. The starboard gauge shows 5+ psi. When I run the boat the port gauge runs around 20 psi regardless of rpm up to 4,000 rpm. The starboard gauge runs 30 psi +/-. The other difference is the starboard gauge bounces +/- 4 psi showing apparent fluctuations, where the port gauge adjusts more slowly and doesn't, bounce noticeably. Another observation, when I stop the boat and chech header temperature, both engines are similar. Also water and oil temperature gauges read the same, 158 water &190 oil temp. What can I do before going back to the mechanic? I've read the sensors can be affected by dirt, but I don't know how to access or clean them. My other question is if my port engine is only making 20 psi at cruising, is this enough to keep the headers healthy? Thanks

Sounds like it could be a bad gauge. Is it a mechanical or electrical gauge?

I would swap gauges and see the results - stays the same or follows ( psi ) and then if it stays the same I would either swap wp sensors or just throw a new one in and see what happens.