Mercruiser water pressure sensor

[Devil_Inside]

I am headed to the boat armed with a second, brand new, OEM pressure sensor and a digital pressure gauge certified to be accurate to 0.015psi, which I borrowed from a friend who works at a lab. I also plan on testing the output of my sea water pump as per Mercruiser's procedure. I will post my finding later today or tomorrow. Stay tuned.

 

[achris]

Standing by with baited breath.

 

[Devil_Inside]

UPDATE: I was able to confirm that I have LOW pressure and NORMAL flow - whatever that means

First I decided to test the water pressure with my friend's lab-calibrated digital gauge, and I discovered that I indeed only have about 0.5-0.7psi of pressure at idle.

I then proceeded to do a flow test and I measured over 12 quarts of water per 15 seconds @1000rpm. I repeated the test multiple times and got similar results. This rules out Bravoitis and any other potential restrictions.

The boat ran great with no alarms and no Guardian mode issues, but my pressure never went above 6psi even at WOT, and at idle I am always <1psi

So now what?

 

[alldodge]

pressure never went above 6psi even at WOT

Have never thought much of the idle flow test, but would be good if it was overheating at idle

You either have:
Drive inlet ports are clogged
Worn impeller and/or pump housing.
Air leak from drive thru to pump in one of the connections
Bravoits

Don't see it being after the pump because your not building any pressure

Use a piece of 1 inch clear tubing and place it at the thermostat housing and inlet hose. Then take it for a run in the water and look for bubbles. If no bubbles then no air leak and then look between pump and outdrive

 

[Devil_Inside]

Have never thought much of the idle flow test, but would be good if it was overheating at idle

You either have:
Drive inlet ports are clogged
Worn impeller and/or pump housing.
Air leak from drive thru to pump in one of the connections
Bravoits

Don't see it being after the pump because your not building any pressure

Use a piece of 1 inch clear tubing and place it at the thermostat housing and inlet hose. Then take it for a run in the water and look for bubbles. If no bubbles then no air leak and then look between pump and outdrive

I suppose anything is possible however I don't see how I could have a clogged inlet, Bravoitis, etc and still manage to get a flow rate of over 12 gallons / minute at 1000rpm. I am also not overheating - it does't matter if I am idling around or if I am WOT - temp doesn't change.

Since this is a fresh-water cooled motor I would have to find a different place to put a piece of clear hose...it is worth a shot I suppose.

 

[metsfan3197843]

I’ve never checked my pressure at cruise speed because I’m using a scan tool for the values but I can confirm at idle I’m at .7 psi also so perhaps that’s normal. I’m not getting any low pressure alarms are you?

edit-that’s with a new impeller this year

 

[Devil_Inside]

I’ve never checked my pressure at cruise speed because I’m using a scan tool for the values but I can confirm at idle I’m at .7 psi also so perhaps that’s normal. I’m not getting any low pressure alarms are you?

edit-that’s with a new impeller this year

I haven't gotten any alarms since I replaced the pressure sensor, but Mercruiser is telling me that the pressure at idle is supposed to be greater than 2psi and anything less than that could trigger an alarm (though I havent been able to find any confirmation)


P.S. My impeller is also brand new and so is my WP wear plate.

 

[alldodge]

The original post was because the motor went into guardian mode, so there was a low pressure issue either by sensor or actual. The sensor was changed and have same readings, so guessing the sensor was not the issue.

Fresh water = closed cooling and gets mixed up quite often with folks boating in fresh water. Just something we will always deal with, because folks have not seen a HX filled with "fresh water", always AF

Tubing can be connected to inlet of the HX

Use a Tee fitting so you can have both sensor and mechanical gauge installed.

 

[Devil_Inside]

The original post was because the motor went into guardian mode, so there was a low pressure issue either by sensor or actual. The sensor was changed and have same readings, so guessing the sensor was not the issue. - That's not entirely correct - the thread was started because the motor went into guardian mode due to a pressure sensor which was reading 0.0psi. After I replaced the sensor I am no longer going into alarm, however the pressure readings remain "low". I have not been able to find any published values those values are supposed to be - according to Mercruiser customer support, the magic number is >2psi at idle, but in reality folks seems to be getting anything from 0.7 to 5 psi at idle.
The only reason I keep re-hashing this is because I want to prevent my engine from going into Guardian mode because of the same issue again. Frankly, I couldn't care less what the pressure value is - all I care is if my engine is operating at the correct temperature.

Fresh water = closed cooling and gets mixed up quite often with folks boating in fresh water. Just something we will always deal with, because folks have not seen a HX filled with "fresh water", always AF - Point taken - closed cooling it is

Tubing can be connected to inlet of the HX - I need to look at the routing of the hoses but that sounds like a good spot

Use a Tee fitting so you can have both sensor and mechanical gauge installed. - I could do that, but I feel like I already proved that the replacement pressure sensor is reading accurately. I had some doubts initially, when I compared it's values to an inexpensive mechanical gauge, but it turns out it was the mechanical gauge that was reading inaccurately. Unfortunately we are splitting hairs anyway, since we are dealing with such low pressure values (for ex. 0.7psi vs 1.5psi)

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