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Engine Temperature Problems - Too Hot now to Cold!!

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Any help is much appreciated with my problem!!!

Here is the situation:

Dropped the boat in the water last weekend and as I was leaving the launch I noticed the temperature raising fast.  I was able to keep the temperature below 200F by keeping the boat at low speed and hatch open and made it to the dock.


Went for a nice spring swim and unblocked the water intake (or so I thought).  Took the boat out again and I was able to get up to plane (was not able to do this without reaching 200F) but the temperature was still high.  Went full out on the engine and the temperature spiked to high alarm limit, obviously I backed it off instantly and the temperature dropped to the 180's or so.   

I got two high alarms the whole time I was out but was able to drop the temperature down in a few sections by slowing the boat.


Alarm codes:

SPN110 FMI 15 - EngCoolant Temp

Data Valid but above normal range

-This code is still active but from what I read think it will stay that way until the boat goes through 3+ warmup cycles


SPN3266 FMI5 O2 2/2

Current below normal or open circuit

- not sure what cause an O2 code but its inactive now.. I bought an new O2 sensor so I might replace it if this keep coming up.


Went for a second swim and discovered there was still some blockage on the intake which I then removed.  After this I replaced the raw water intake impeller and took it out for spin. 


Motor runs nice and cold now, but wont get up to temperature.  Maximum temperature I get up to is around 95F.  When I a cursing around the lake the temperature actually drops about 5F.


I thought it might be a failed thermostat so I replaced it with a brand new one.  Same result, engine wont reach temperature.  I tested the old thermostat and it opened and closed at 160F as expected.


I have done some basic checks on the ETC (temperature sensor) and the resistance value matches what the temperature gauge is reading based on a cross reference chart i found for a similar boat (same brand but newer).


I am going to do some more checks with my DMM today to see if I can trick the ECM to thinking the engine is hotter (jumper ETC with a fixed resister), do some grounding checks, etc.  But I am at a lost for what could be causing this.


Engine: 2010 Indmar MCX 5.7L (date of manufacture)

Boat MasterCraft X2 2011

Hours: 450 or so.


I relize this is a Malibu Form...


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I disconnected the temp sensor ( (is there a difference between a sender and senor??)) and jumped two different resisters accross the terminals and the temperatrue on the dash corresponded to the temperature I was expecting for the resisters used (470ohm = 158F, 680ohm = 140F).  Based on this I think the wiring harness is good and most likely the ECM.

Next I pulled the sensor and placed it in some hot water ~160F and measured the resistance accros the RDT (temp sensor) and the value was in the range of what I was expecting.

So thermostat should be good, Temperature sensor appears to be ok and the link between the sensor and the dash is ok (assuming that means the ECM is ok too).

I will have to get in IR gun from someone to check to see if the block is getting warm...I am not sure what else to look for.

Is it possible that the cold lake water ~55F is just keeping things to cold??

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The temp sensor for the dash is probably not the same as the sender for the ECM.  You should check both.

A good rule of thumb for temperature is actually putting your thumb on parts of the engine.  If you can keep your thumb there for an indefinite period, it is below 125 degrees or so.  If you can't keep your thumb on it, it is above 125 degrees.  The faster you have to pull your thumb away, the hotter it is....

The intake manifold at the thermostat housing is an excellent place to check temperature.

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Hey justgary

I just fired up the engine and let it idle for about 5 to 10 minutes.  I was able to place my hand pretty well anywhere on the engine (beside thermostat, side of engine block, on top of exhause risers, beside manifold, etc.)  I could leave my hand there all day!  Its barely warm.  Engine is running smooth had no power issue the other day when I took it out..

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I think I have drawn out the cooling water circuit corretly, but I am not sure which of the lines to the exhaust risers actually exhaust the risers.  I would post the image but I am not sure how.



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On 5/14/2022 at 12:22 PM, justgary said:

The temp sensor for the dash is probably not the same as the sender for the ECM.  You should check both.

@brentjp you didn't explicitly say which sensor you were testing.  I'm guessing they are not the same, but could be wrong.  If there are two, then ignore the dash indictation for now and check the sensor that goes to the ECM.  It might be that you only have an indication problem and engine temps are fine.

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Hey formulaben.  There is only one sensor on this engine.  Either way the block is not getting hot or warm.  I let it idle for 5-10 minutes and I could place my hand anywhere and it was not hot (barely warm).  Its almost like the block is either getting cooled (i.e. water is being allowed to flow when it should not be) or the engine is not producing enough heat (does not make sense).

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