Check the cam inside of the raw water pump for wear. Sounds like a lack of water flow. If the cam is worn it wont compress the impeller blades.

Let's back up..........

Is this a new too you boat or did you have the engine rebuilt?? When did this issue start??? Did it ever maintain temp at WOT???

No idea if those gauges with that size hose will work to get water psi. Normal water psi gauges use 1/8 tubing.

This seems inaccurate to me, isn't the the impeller and the cam on the same shaft? So if the cam is worn, the impeller is still spinning the same rpm as the pulley. If the cam was wore he would have low fuel pressure

Ediit: I don't think the 496 came with a fuel pump mounted on the water pump, are you just taking about the "shape" on the inside of the housing when you say cam?

The out of round portion of the raw water pump impeller housing is the cam plate, or cam.

Thanks for the responses. I’ve had the boat for 15 years. Seems like it used to overheat on long, maybe 15-20 minute WOT cruises. Also had a cracked piston ring and had engine rebuilt by a reputable marine builder, Dean Kelly on Grand Lake, OK. Engine ran great after that but I’ve babied it to some degree. One day it started overheating even at low rpm, maybe half throttle. I stopped, changed the impeller, same problem. On further inspection the rear plate of the RWP had a groove, I was told was caused by use of incorrect impeller. I bought a new OEM Volvo penta raw pump with impeller. It helped, but didn’t eliminate the problem, only moved it to occur at high rpm.

Today I pulled and tested the thermostat suspended in a pot of water along with an electronic thermometer (on my multimeter). I confirmed the thermostat tstat I purchased last year was an OEM 140 degree tstat. Volvo Penta has changed their recommendation to a 160 degree tstat for all 8.1’s. The tstat began to open at 157, and was fully open by 160. So much for a 140, ha, but 160 shouldn’t be a problem.

I performed a raw pump flow test by starting engine with hose to thermostat housing removed. VP service manual says to hold hose at flame arrestor level and observe water exiting hose. Says if water head is 1 inch all is good. Said to look take action if below half inch of head. I couldn’t get it level with flame arrestor without flooding electronics, but had a solid 1inch of head when held 5 inches above flame arrestor. I decided not to replace impeller (again).

i also hooked up a garden hose to flush the block and heads and check flow rates via the circ pump hose exiting the crossover tube. Seemed to flow well. But coudnt isolate each bank that way. No grit, chips, discoloration, etc.

im thinking about filling the block and heads with either 30% vinegar or evaporation-rust and let it sit a day or two. No sure about how that would affect circ pump seals or ceramics, or rubber, or head gasket material.

i think that answers questions above and then some. Tomorrow I go to the lake with tstat removed. Not the end solution but perhaps one more pies of puzzle.

you say 8.1L,isn't the motor closed cooled?if it is,take the heat exchanger to a radiator shop and have it cleaned.I chased an overheat problem on a big tub and thats what fixed it.good luck,keep us informed.

Thanks, but it's open cooling, "seawater cooled", no exchanger.

I emailed CRC, the manufacturer of Evapo-Rust, asking if their product would harm the brass, ceramic or rubber on the circulation pump, or head gasket material. They said that their product would not harm any of those materials, only rust. I plan to warm up engine, remove thermostat housing, drain block and heads via the winterization hose, then raise same hose above engine and pour Evapo-rust in until liquid appears in top of thermostat housing. Block and heads should be full. Let sit for 3-4 days then flush. Product won't arrive until Monday, so maybe I get a short vacation from the boat. Still planning to run it tomorrow with no thermostat. Not sure that really tells me anything, but why not.

Gonna try and find a clear hose to run the VP suggested head gasket leak test. From VP service manual:
Replace water hose(s) between thermostat housing and exhaust manifolds with clear plastic hose. Operate at rpm at which overheat occurs.
No bubbles evident, head gasket not leaking. Bubbles evident, head gaskets leaking.
They then suggest a compression test and to check spark plug condition. I did all that and a leak-down test first, but engine was cold. Gonna put the plastic hose in anyway, since I'm running out of ideas.

Has anyone used some kind of aftermarket "cup" to install on outdrive by water intake holes to help force water in at speed and increase water pressure? Buddy of mine said he did it on a bass boat years ago. I can't find any kind of commercial product.