2030 RAW water pump.

Hi guys,

Ive had the same engine for 15 years and had it prof. overhauled last winter. First season was fine but this year the engine suddenly started playing games.
Starting the engine when the boat is laying still is fine, but if I start at speed with sails the engine starts but no raw water coming from exhaust. If I reverse the engine with approx 2000RPM for about a minute or two the water starts coming.
Reversing obviusly leads a flow of water into the suction holes on the sail drive and perhaps 'primes' the pump.

Things I've checked:
Changed impeller,
Cleaned hoses,
Cleaned the saildrive holes and inside as much as I could. I didn't look bad at all!
Tried to let the pump suck from a bucket in the cockpit. This was only possible if I primed the hose and hence gravity feed the pump.
With pump running dry there is on slight suction at the inlet.
No water leaks from pump nor filet.
The heatexchanger is clean.

Any suggestions?
thanks

I don't know that pump, but there are a few things in a typical raw water pump that wear and can cause low suction. End plates, cam, impeller (cheap ones or sometimes they can spin the center like a spun prop hub), seals. Was the pump rebuilt or replaced when the engine was overhauled? There are other possibilities but I would check all those first.

Do you have the ball valve or the faucet type valve 90 degree at the upper gear unit? Sounds like you have a small air leak the impeller should be self priming. All it takes is a tiny air leak to lose suction pressure. Possible bad seal for the water pump shaft?

Did you replace the gasket with the impeller?