8.1 engine guys , water PSI question

[DaltonGang]

I have twin 8.1s. (2015 engines, 225 hours on both)
New manifolds, risers, port impeller, starboard pump and impeller, and cruising speed temps run at 160-163 all day long. No overheating at all. I have vesselview installed also.

One thing vesselview has always shown me was the water pressure on port is always pretty low compared to starboard (starboard seems normal). But I ignored it because all ran great and no alarms, etc.

I was 30 miles off shore and cruised all around over the weekend. When I got back I got off throttle down to around 1200 rpm and the alarm went off for 15 seconds. I went down to idle right away. Checked vesselview and it said fault code no water pressure on port.

Note, this is the side that is always low anyway.
It ALWAYS shows 0 at idle and sometimes slightly above idle. Then it starts rising when I pick up speed. If I go from idle to 1500 rpms, no alarm. If I goto 3400 rpm, no alarm. It's only around 900-1300 rpms will I get the alarm on port side.

I went ahead and bought a new OEM sensor & I'll try that first. That was my first thought when seeing all of this.

Again, new impeller, no scoring on pump I believe, temps are all perfect, water flow out of exhaust is identical to other engine. Everything is great at every speed except 900-1300 rpms.

So, does this sound like a sensor?
wondering if the raw water pump is cavitating at that RPM because of scoring that I didn't notice. Other than those 2 things, I don't know what else it could be.

 

[Espos4]

You could try swapping sensors between motors first before replacing the one suspected bad sensor.

 

[paulswagelock]

Sounds like a bad sensor or plugged sensing port.

 

[LG111]

If its not the sensor and as you mentioned the pump and impeller are good, it is possible the system is sucking air into the pump. Loose hose clamp maybe?

 

[DaltonGang]

If its not the sensor and as you mentioned the pump and impeller are good, it is possible the system is sucking air into the pump. Loose hose clamp maybe?

Good point. I know there are 2 air lines for that little air pump for draining and those goto the water pump I believe. One doesn't even really plug in. The quick connect fitting is shot I think. I'll have to look into that also.

 

[Cory Hart]

I have 2004 8.1's but I recently had the same issue. Sounds like a sensor to me. Port side alarmed and I limped back at idle. Prior to that, my port read lower pressure than starboard. So, I swapped them. While the low pressure moved from port to starboard, during the swap process , I blew both out with compressed air. I then fired it up, went for a quick cruise and was unable to recreate the problem. I did, however, replace the sensor that had alarmed, just in case. But I have that sensor in my arsenal in the event another goes bad while out on the water.

 

[DaltonGang]

This is what I'm hoping. Plus, I like doing the easiest thing first. But if the old sensor turns out to be ok, well then I have a spare on hand and I'll move to the next logical thing it could be. I think there is a test we can do where you run the motor and measure how much water it pumps out in 15 seconds or something like that. I think it has to be around 7 quarts so that's something I may look into if the sensor doesn't work. I'm not sure how to do it but I did read this is another good test for flow.

 

[DaltonGang]

Found this to do the water volume test.

volume output test on the water pump. To do this you need to remove the pumps output side hose, and if necessary, replace it with a longer one to do the test.



What you need to do is put the boat in the water (this will not work on a flushing device) and with the engine stone cold start it up. Bring the rpms quickly up to 1000 exactly and take the output hose and put it in a bucket to collect the water. After exactly 15 seconds remove the hose and shut the engine off. You do not want to run the engine any more than a minute as it will have no cooling water going to it.



Now you need to measure the water in the bucket, It can be no less than 9.5 quarts. If it is over 9.5 quarts we move on.

If it is under, the pump, and everything on the inlet side of the pump needs to be checked.

Through hull fittings, seacocks, and sea strainers can all clog up an limit water input to the seawater pump.

 

[DaltonGang]

Update to this. It's never the easiest thing that fixes is but this time it was! A new Merc sensor and all works perfectly again. A whole 3 minute job. But now I see the other side is reading low and the sensor may soon fail so I probably need to switch that one out also.