Cooling Water Pressure Reading

JimGuy341

Installed vessel view mobile on twin 350 mag engines w/ bravo 3 drives.  Temps look good...any idea why the cooling water pressures are so different?  The engines are raw water cooled.  Impellers were replaced in 2019.

Grahamu

The higher temperature would suggest some sort of restriction. Try changing the thermostat.

JimGuy341

Thermostats were replaced this year.  They are rated at160 F so I figure 162 F isn't high (I could be wrong).  I figure the temp variation between the engines is normal.  

JimGuy341

Additional info:  Twice this season the STBD thermostat got stuck open by pieces of grit.  We don't get into shallow water so it's not sand.  A local mechanic suggested I run some barnacle buster through the engine.  Could these issues be related?

Grahamu

Your profile says you are in Michigan, I have never heard of barnacles in fresh water. More likely Zebra Muscles. I would still change the thermostat after being stuck open twice also check your hoses for kinks. I still think you have a partial blockage somewhere after the seawater pump.

earl1z19

The 4 degrees really isn't an issue. The difference in pressure is probably do to scoring inside the raw water pump. Even with new impellors the scoring is probably an issue. How many hours on the raw water pumps? ( the pumps themselves and not the impellors)

davidbrooks

I swapped out both of my thermostats this year and i level out at 163 on both engines.  If you get above 185 then that is time to get a little concerned. As to the pressure.  There are lots of places where you could have a partial restriction.  To me that would explain the higher pressure and slightly warmer engine.  I would start by pulling off sections of hose and backflush with a garden hose. You could also swap the pressure sensors between engines just to rule out a sensor that may be going bad.  The good news is that they arent expensive.  

LaRea

Your water pressures seem really low.  

For my 496/Bravo 3 powerplants, I'm usually seeing 20-25 PSI at cruise speed.  Your engine is raw-water cooled, so mine has extra back-pressure from pushing water through the heat exchanger.  But still, 5 PSI seems low.  

earl1z19

LaRea said:

Your water pressures seem really low.  

For my 496/Bravo 3 powerplants, I'm usually seeing 20-25 PSI at cruise speed.  Your engine is raw-water cooled, so mine has extra back-pressure from pushing water through the heat exchanger.  But still, 5 PSI seems low.  

My 8.2'S read 20-22 psi at 3500 rpm as well. Mine both exceed 10 psi at anything above 1500 rpm  so I agree on the low pressure. My motors also run a consistent 169 degrees

JimGuy341

earl1z19 said:

The 4 degrees really isn't an issue. The difference in pressure is probably do to scoring inside the raw water pump. Even with new impellors the scoring is probably an issue. How many hours on the raw water pumps? ( the pumps themselves and not the impellors)

I'm guessing the pumps are original.  That would put them around 500 hours. 

JimGuy341

earl1z19 said:

LaRea said:

Your water pressures seem really low.  

For my 496/Bravo 3 powerplants, I'm usually seeing 20-25 PSI at cruise speed.  Your engine is raw-water cooled, so mine has extra back-pressure from pushing water through the heat exchanger.  But still, 5 PSI seems low.  

My 8.2'S read 20-22 psi at 3500 rpm as well. Mine both exceed 10 psi at anything above 1500 rpm  so I agree on the low pressure. My motors also run a consistent 169 degrees

Sounds like both pressures are running low but as long as the engine temps are good, do I need to consider making any changes?

oscar1

I will do the pressure sensor before anything.

YYZRC

The same gunk that messed with your thermostat may also be in the water pressure sensors. The pressure sensors are very finicky and IMO pretty useless. I would change impellers next season (2 years use is reasonable) and see what the readings are. 

earl1z19

JimGuy341 said:

earl1z19 said:

The 4 degrees really isn't an issue. The difference in pressure is probably do to scoring inside the raw water pump. Even with new impellors the scoring is probably an issue. How many hours on the raw water pumps? ( the pumps themselves and not the impellors)

I'm guessing the pumps are original.  That would put them around 500 hours. 

I would check the pressure sensors to see if they are the issue 1st. With 500 hours on the brass pumps they are probably near the end of their service life. You can do a search on here for threads about the pumps and their lifespan. Alswagg says the are good for about 500 hours or so. The wear on the surfaces on each end impedes flow and reduces pressure. In most cases replacing the impellor only makes the situation worse as it is not conformed to the grooves and wear on the end plates and water just sloshes around in the pump and doesn't get forced into the motor.
Take a look at all the parts in the cooling system to trouble shoot. The sensors are probably the cheapest to replace and the pumps the more expensive items.
Take things apart and inspect their condition at the very least when you are done you know exactly what you have on your boat

davidbrooks

if you just swap the sensors between engines you can see if the readings stay with the engine or changes due to the sensor swap.  

JimGuy341

Thanks for the feedback.  I'll start with sensors next spring...

kordo

I had a similar problem, it turned out to be a twisted intake hose.  You can check it if you raise the drives and look in from the bottom.

Spyderweb

FWIW, my Mag 350 freshwater cooled engines run at 8-10 psi at 3800-3950 rpm with temps 154 and 156.

stuckmoto

Spyderweb said:

FWIW, my Mag 350 freshwater cooled engines run at 8-10 psi at 3800-3950 rpm with temps 154 and 156.

Same here-350 MAG runs around 10psi and never see over 156.

JimGuy341

stuckmoto said:

Spyderweb said:

FWIW, my Mag 350 freshwater cooled engines run at 8-10 psi at 3800-3950 rpm with temps 154 and 156.

Same here-350 MAG runs around 10psi and never see over 156.

This is good to know.  At least now I have a target.  

Spyderweb

I thought I had a problem when mine read 10 and 20.  Was told to swap sensors, and then learned it was a bad sensor.  Problem was I thought it was supposed to read 20.  Got new sensor and it read 10.  Thought it was bad, returned it, and next new one also read 10. Did some research and learned 10 was the good number.  Now they both read 10.
Definitely swap sensors 1st as @davidbrooks recommends.  Very easy 10 minute job.

rasbury

Mine will show 170 but the water is already 90.