Out on the boat yesterday, I noticed on the port engine that water pressure and water temp at speed were down quite a bit. The engine seemed to be running fine so we kept on since we were trying to beat the sunset in unfamiliar nighttime water.

Pressure at 50+ is usually around 25-30 psi with temp right at 180-185 depending on seawater temp. She was down to about 12-15 psi with less than 140 on the temp gauge. We had plenty of water out of the exhaust so assuming good water flow.

What woukd cause this? On the hose today, she did the same thing with temp. When I added the other engine to the water flow manifold, temp bottomed out. A slight rev would make it come up, but then it would go back down.

I cracked open the t-stat housing and all looked okay (I was thinking maybe a stuck t-stat, but that wouldn’t explain lower water pressure). Maybe the impeller is on the way out?

Anyone see this before? I did some searching and didn’t quite find an answer.

A stuck open or or something in it thermostat might lower temp and water pressure. More flow less restriction = cooler temp and lower pressure.

What was the results?
I do not have (raw) water pressure guages but am thinking of adding them. I have roller rocker 496s...can you recommend a gauge?
I'm assuming I can just T it off the output from the raw water pumps...agreed?

the same reply here as well. And will add the cork gasket to seal the t stat against the housing portion as well. It gets very crusty and rusty over time ( the cork gasket seal )

I do not think you have closed cooling is that correct ?? Have to make sure so the reply matches for a possible correct answer

Hey guys. I’m guilty of my own pet peeve.... not closing out a thread!

It was the thermostat. I replaced it and all was back to normal. I went ahead and did the other one for good measure.

Tom, for water pressure you really should rap into the block. Mine are plumbed into the thermostat housings (MPI engines), but on the Cig they’re in the block drains. This is where most will install them. You want to see what’s happening in the cooling system itself.

Where wp sensors are mounted depends if it's open or closed cooling. On my 496 (closed cooling) I mounted the fitting right after the output of the sea water pump, t ed with the factory sensor.

Merc 496's with closed cooling or ???

496s with open/raw cooling.
I think I want a mechanical gauge under the inside the engine cover/area so that I can confirm I have good flow/pressure when I start the motors and idle around. I believe I want to watch for signs of age/wear on my impellers/water pumps by making sure that my flow or pressure is sufficient.
Should I get pressure gauges (style, brand, and installation location recommendations) or flow gauges (same questions)?
THANKS