I have a volvo penta 195cv aq151c 1990 it looks like its…

Hello and welcome to Just Answer

my name is ***** ***** I've been an automotive engineer for the last 22 years and I'm going to help you with your issue

1st thing to ask is why do you think the engine is overheating? If the temperature gauge is showing hot then I'd next use a thermal probe or infra -red gun to confirm the actual temperature of the top coolant hose and thermostat housing

as if its normal ( 85-90 deg C) then you know its likely the temp sensor is faulty or the wiring to it is damaged

if it shows the temp really is high I'd next say to flush out the whole engine with a acid / flushing treatment and then try fresh coolant

I'd also suggest ensuring that you've got all the air out of the system as this sort of episode can leave pockets of air trapped and prevent the coolant from circulating properly.

So check that the small hoses that run from the top of the engine and radiator that run back to the header / expansion tank are free flowing and not blocked

If these are OK then remove the tank pressure cap and run the engine from cold with the heater set to full hot

As the engine starts to warm, squeeze the large hoses running to the radiator to help displace any air - all the time keeping an eye on the level in the header tank, making sure it doesn't drop below the minimum and open any bleed screws built into the top cooling hoses - closing them off as soon as they start to gush water

Once the engine is up to temperature the tank should be quite full. Switch the engine off and leave it to cool

Then top up to the mark once its all cold and replace the cap