I spent all day at the boat, the trim tabs have nothing to do with my problem, only that when they are up the engine rpms go up from 4500 to 4800 which is where I have my problem. Only on port engine, starboardengine water pressure is normal. Engine temperature stays perfect at 154 degrees. I payed close attention on a test drive to the water pressure based on all your great feedback and the engine water pressure is fine up to 3500 rpms, it's over 8.2 at 3500. Then as the rpms increase the water pressure goes down to 6.5 at 4500 and then falls to the low of around 6.2 when I go WOT, 4800 rpm and alarm and guardian mode shuts that engine down but it does stay on, I just go back to idle to stop the alarm. So I replaced the water pressure sensor and it didn't make a difference. THEN I saw that the connection from that engine to the water heater(which doesn't work) had a huge leak in the connection, the connection on the water heater corroded off. So I took the hose and bypassed the water heater by taking the hose that came out of the engine to the water heater and looped it back to where it goes back into the engine. I was sure that was the fix so I went out for a test drive and there was no change! I was so disappointed. I replaced the water intake pump 6 months ago with a stainless steel hardin pump so i know the impeller is good. Also had new intake hoses installed when I bought the boat two years ago so they are good. Also, checked basket on the intake line and it was clean. I don't have any way of getting the code unfortunately. Have a survey on Monday so looking for a hail mary that I can fix it before that. Any other ideas? Thank you so much
alarm on port engine 350 mag mpi v drive
- Thread starter tugger
- Start date
- Sep 25, 2016
- 1,257
- Boat Info
- 300 Sundancer 1994, trailered tri-axle LoadRite roller
- Engines
- Mercruiser 5.7 260HP Alpha One Gen II, twin
Look I am a retired marine mechanic and ASE certified Master HD truck mechanic. Your starting the thread over because you are looking for that "hail mary". But diagnostics doesn't work by wanting to find something easy.
Each thing you test needs to prove or disprove something specific. Details are important.
So you are operating at 154 degrees, to me this indicates you are a raw water cooled engine with no heat exchanger right? Because a fresh water heat exchanger engine would usually run around 180 degree thermostat.
At lower RPM you have higher pressure than at high RPM.
Let's explain how pumps work. These are positive displacement pumps, each revolution will move a constant volume of water. More revs per minute, more water per minute.
Pressure is the measure of resistance to the flow of a fluid. With no restriction at the discharge of a pump it will move the greatest volume of water but will measure 0 pressure. Pressure is a poor measure of adequate cooling flow. But the sensors are much cheaper then a true flow sensor so that's what OEMs use.
A given positive displacement pump, with a given resistance to flow, the higher the RPM the greater the PSI should be. You are inverse to this, the pump is moving less water at higher RPM.
The causes of less flow at higher RPM:
1. Restriction to flow on the INLET side of the pump.
back flush the line from the pump to the hull fittings, check for collapsing suction hoses at high RPM
2. Pump cavitation, pumps dont move air well, look for valves, strainers, hoses that are letting air be sucked into the INLET side of the pump
3. The impeller is failing, missing or cracked blades will fold over at high rpm and fail to move as much water.
It does not matter that the impeller is only 6 months old, any number of things could cause premature wear or failure. You could have sucked up a piece of debris and restricted water flow. You cant rule out the most likely cause of low flow.
One of Patrick's suggestions is to back flush the line from the pump to the hull fitting. I can't really access the line at the pump because it is too hard to access but I can get the hose off at the hull fitting. My question is can I access it there and hook up a vacuum and suck out anything that might be in there? Thank you