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Author Topic:   Mercury OptiMax Water Pressure Problem
highanddry posted 05-20-2007 01:09 PM ET (US)   Profile for highanddry   Send Email to highanddry  
[I] noted lowering water pressure last summer [on a Mercury OptiMax motor] so installed new impeller and gaskets. Ran engine on the muffs and strong tale tale after about five seconds, all normal except zero water pressure indicated on the SmartCraft guages. OK, sea trialed the boat. Appears to run normal. Ran for a good three hours at all speeds from idle to WOT, all appears normal. No warnings, no reduced RPM. Max water temperature at trolling speeds is 147 and usually around 135 to 145. Water temp indicated at speed is under 100 degrees. Water temperature in the lake was 62 degrees.

Per the manual, the ECM should reduce power and provide a horn that water pressure is low. Hmmmm, 0 is low huh? However the engine ran and operated normal.

Does the ECM get its water pressure signal from the same place as the SmartCraft guages? If so, where do the signals originate? I have the manual and see where there is a water pressure guage but am not sure what sources the signal for the Smart Craft.

Something is goofy here and I don't think it is my water pump. An engine simply cannot run normally for three hours with zero water pressure.

I am sure I can figure it out but perhaps somebody can shorten my learning curve, thanks in advance.

There are no OptiMax certified mechanics or ECM readers here or I would avail myself of them and just pay.

highanddry posted 05-20-2007 05:09 PM ET (US)     Profile for highanddry  Send Email to highanddry     
More research and limited trouble shooting leads me to believe there is only one water pressure sensor and it is up on the aft side of the engine. There is a tee in the gray water pressure line that leads forward to the front of the engine where that line is plugged. What I think now is that the sensor is gone bad and is providing NO output--as opposed to low or high output. I believe in this state the ECM logic would ignore the --NO--output but it would generate an internal fault code readable by a shop that has invested in the equipment to do so. NO output is interpretted differently from either high or low output (my hypothesis) and therefore does not trigger the ECM to command a warning horn and engine RPM reduction. Per the service manual low water pressure will sound the horn and reduce engine power after 3 or 5 seconds depending upon RPM. Since I expereinced none of these and since I now believe there is only one water pressure sensor, the only real remaining option is that the sensor is kaput and that the ECM sees this and through it's logic ignores it since other parameters are within specification--ie--water temperatures, RPM, combustion temps etc.

I will purchase a new sensor tomorrow--well--order one. People here think I am from Mars, I have a boat.

lurkynot posted 05-21-2007 09:41 PM ET (US)     Profile for lurkynot  Send Email to lurkynot     
Nothing to fool around with when it comes to temp and aluminum . My 135 has been acting up, a spill over from last season when prolonged low speed trolling would elevate the temp read out on the smart craft gauge. When attempting to get on plane the routine was to power down an the temp would immediately drop 20 degrees or so and allow planing. This year intermittently the temp at prolonged intervals of low speed operation will reach 170 at times. I am going to replace both t-stats and either clean or replace the poppet valves, should do the latter if they are as cheap as some have suggested on this site and others. Good luck with your issue. I never remember having all these issue with old tech motors. Then again fuel economy will never be the same either.
Knot at Work posted 05-22-2007 02:13 AM ET (US)     Profile for Knot at Work  Send Email to Knot at Work     
If the engine ran and operated as normal, you could have a blocked vaccum line from the gauge to the sensor. Disconnect both ends and try to shoot a little compressed air through both ends. Also take a look at the zip tied lines around the steering and fuel bundle. Sometimes a nice kink occurs.

Happened to me, and after replaceing the line (144.00 samolians) she is good as new.

try the line first....

highanddry posted 05-22-2007 04:37 AM ET (US)     Profile for highanddry  Send Email to highanddry     
Thank you for the ideas. The engine is operating correctly and the temperatures are correct. At idle speed the thermostats control temperature which is why the temperature stabilizes around 140, that is where the thermostats regulate it--this is normal. At higher RPM the poppit valve helps control temperature and our cruisng temps are below 100 degrees which is again per Mercury--normal.

SmartCraft guages are computer controlled from the ECM, there are no pressure or vacum lines going to the instruments, it is all digital/computer controlled. The ECM I think, passes the signala to the SmartCraft guages?? Unplugging the water pressure sensor resulted in an immdediate fault display on the SmartCraft guage. However when operating, as stated, I get no warnings, no horns, no reduced power, no Guardian activation so the ECM must be seeing what it wants to see or is able to ignore erroneus imput--or something--beats me. I do know there is no way, and Mercury agreed, that the engine could operate at normal temperatures and normal operation for three hours at all speeds if it did not have sufficent water flow and water pressure. Mercury tells me the problem is probably the wire harnes to the guage or the guage--I think baloney. It was working fine last year. The only thing that has changed is I installed a new water pump impeller which seems to be operating normally.

BTW, there is a tee in the line to the block water pressure sensor that is plugged up at the front cowl. Removing the plug while the engine is running results in a steady stream of water from the little tube. The tale tale also seems normal--hmmm.

Knot at Work posted 05-22-2007 07:02 AM ET (US)     Profile for Knot at Work  Send Email to Knot at Work     
Looks like it is beyond my capabality to repair. Hell, I still have carburetors on my motor!

Good luck!

sosmerc posted 05-24-2007 11:14 AM ET (US)     Profile for sosmerc  Send Email to sosmerc     
You have confirmed that you have water flow and pressure, but you need to remove the water line from the sensor beyond the tee to confirm that you have water flow right up to the sensor. If you do, then I would disconnect the sensor and see if you get an alarm with the engine running.
I think you will get an alarm as the ECM needs to "see" the sensor.
With the sensor disconnected you can measure the resistance across the terminals with an ohm-meter.
If you give me your serial number I can look up the spec for this test.
email me at
highanddry posted 05-30-2007 12:44 PM ET (US)     Profile for highanddry  Send Email to highanddry     
Now the rest of the story. I took the engine to a Mercury dealer so they could read the computer and all was well. I had Mercury Express ordered the water pressure sensor through them and it was waiting. A manual water pressure guage confirmed correct water pressures and the sensor was replaced and now there is a water pressure readout at the SmartCraft guages--as there should be. The engine is happy and shiny best we can tell. Warranty should cover everything.


slimey posted 05-06-2011 02:28 AM ET (US)     Profile for slimey  Send Email to slimey     
I have replaced the water pressure sensor on my Mariner 115 Optimax 3 times in the space of 3 years. Each time the sensor has become erratic in its readings and eventually the readings would drop to 0.
At startup the present sensor reads OK but almost immediately the pressure drops off rapidly leading to the inevitable alarm horn and loss of power.
Water pressure is OK as the "peeing" telltale is strong and the impeller has recently bee been replaced after 300 hours operation.
I really don't see the point of having a water pressure sensor when the "peeing" telltale and water temperature sensor are sufficient to indicate that the cooling system is working OK.
Can the water pressure sensor be bypassed or replaced with a blank of some kind to fool the engine management system?
Thanks in anticipation.

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