Volvo D4 300

Call the service company back and tell them you want the A-Team not the B -team to come to your boat!

Or look around for a different Volvo service company . You do not need to change the sea cocks and strainers that’s for sure!

Make sure they’ve changed out all impellers , even if they look good.
pull the hoses off where they can on the various oil coolers to check for debris in the coolers and heat exchangers.

check the raw water intake strainer or clamshell on the bottom of the boat for sea weed and debris short haul the boat or get a diver to do that.

If the heat exchangers can come out those should be pulled and take it to a shop and be cleaned out a good radiator shop can do that.

But otherwise I really don’t know Volvos very well.

Perhaps some others will chime in that really know your motors.

Overheating at a higher engine speed is a very common issue in D4 engines of that age. The heat exchanger and intercooler become partially blocked with salt deposits and scale reducing the cooling effect. Coolant temperatures rise at a higher engine rpm and as soon as the engine speed is reduced, the engine coolant temperature drops back to normal levels. There is no need to remove the heat exchanger and intercooler to clean them. Simply carry out a 3 to 4 hour chemical flush procedure of the complete sea water cooling circuit using a marine descaler. It will clean the entire system and the the engine will regain 100% cooling efficiency. Remove all the engine pencil zinc anodes and the sea water pump impeller before carrying out the chemical flush.

I had a client with a similar symptom on a D4 260hp. Oveaheating at above 3000 rpm.
After he cleaned the heat exchangers, and changed impellers (a very useless move) since an Imp or works or it does not. No middle ground.
Result was he had a fail exhaust manifold, which then failed and caused even further problem, including cracking a valve sleeve in the head.
Check Impellers, clean heat exchangers but if symptom continue, do not move and test the exhaust manifold ASAP, for failure before it is too late.

The manifold failure is a common issue with early Volvo D4 - D6 engines era 2005-10 seemingly effecting about 10% of the engines.

That's what I thought until I had an issue with 3406 Cats that were overheating and distorting the fiberglass exhaust tubes. The engine temps never came up under normal operation, but the exhaust tubes kept distorting and leaking water at the silicone hose. Finally found that the impeller vanes had taken a "set" in the direction of rotation to the extent that the vanes had no tension or wiping pressure action in the bore of the pump. Every vane was there and looked to be in good condition, but instead of sticking straight out when the impeller was out, they were curled over.

Ahh! The dreaded impeller dysfunction , better known as I.D.
I hate when that happens !

No, no SS liners, and what you say is what happened. A little drool at the silicone hose, and snug up the T-clamps, then OK for a while. "E" engines, and no alarms, no overheat, just no water flow out of the top of the spray ring.