Questions regarding Water Temp Sensor / Engine Temperature - 350 Magnum 1995

[DHutch]

Hello
I have been dealing with an Engine Temperature issue now for a second season.
Have posted here before and had great feedback.
Brief history, sorry for long post - Purchased a used 1995 Direct Drive ski boat in fall of 2022. It has the 350 Magnum Tournament Ski Engine - Carburetor version.
At the time of purchase, I knew the temp gauge did not function. I replaced this last May. New Quicksilver, exact same model that was originally on the dash. Running temp on gauge last season was almost 180F. But it stayed there all day long. Also checked with IF Temp gun the risers, elbows and T-stat housing. All readings seem good. Ran very well all last summer. Boat primarily used for skiing on a small lake.

Fast forward to 2024 - I removed water pump for inspection and replaced both O-Rings seals in housing. Impeller was perfect, did not replace. Wear plate looked new, no marking, no signs of issues. Also replaced T-Stat (160F) and the plastic collar inside housing.

Yesterday was first use of boat this season - again +180F on gauge when running.
I ran the boat at least +10 mins constantly at 3100 RPM / 32 MPH - temp stayed just over 180F.
I took several temp readings with the IF Temp gun immediately after coming off plane:
Both Risers - 70F & 77F
Both Elbows - 95F & 118F
Exhaust hoses - 75F & 75F
Mufflers - 85F & 85F
Measured T-Stat Housing near both temp sensors - 98F & 97F
Valve covers - 164F & 165F
Oil Filter - 160F
Front of Block near the circulating pump water passages - 150F & 150F.
More info - whiling running, dashboard does not sound any warning signals. Oil Pressure constantly 45 psi. When slowing down to idle - the temp does momentarily drop down to 120F-130F and within 30-45 seconds up to 160F quickly and then slowly to 180F.

Do any of these readings and info seem concerning?

Is it common or possible the water temp sensor can fail to give higher readings to the dash temperature gauge?

Also, this am for info I tried metering the water temp sensor for resistance with the engine off and cold. This is the sensor with the "Tan" colored wire - I cannot even meter any resistance from the sensor's post to ground - this seams not right. Should I be reading at least some resistance? I read somewhere approx 400 ohms.

Thanks for reading, any feedback would be appreciated.

 

[alldodge]

What's your serial number?
Does your thermostat housing have check balls?

 

[dubs283]

Taking the sea water pump off, disassembling, inspecting and re-installing the old impeller is madness

The temperature numbers you've posted are in accordance with spec, or at least not exceeding normal observation however, I'd be curious to know the temp reading at the sender itself with an ir temp gun. A comparison between noted values at both sender/guage will tell you if guage readings are accurate

Its possible there is a blockage somewhere before/after the pump. A full inspection of the cooling system may produce a finding as to such

 

[DHutch]

Thxs for the replies
Serial number is 0F621724 - no it does not have the check valves.

All hoses checked for blockages including the trans oil cooler. Found none.
Also - FYI - last week started boat on driveway with a "fake a lake". Originally had some issues with this & the garden hose but once figured out had water flow out both exhausts.

 

[DHutch]

also - the temp readings i took of the T-Stat housing were right at both sensors - "Measured T-Stat Housing near both temp sensors - 98F & 97F". I aimed the gun right at the sensors threads entering the housing.

 

[alldodge]

0F621724 and no check balls
So you have this type housing

Measured T-Stat Housing near both temp sensors - 98F & 97F

I cannot even meter any resistance from the sensor's post to ground - this seams not right

Your getting 180* at gauge and measuring 98* at sensors
and your sender should read between 240 to 33 (cold to hot) ohms to ground

Remove the sender and connect meter. Put in pan with cold water and heat up on stove watching the meter

 

[DHutch]

Update on my Water Temp Sensor / Engine Temperature issues....

In lieu of testing the T-Stat housing temp sender on a stove, I simply bought a new sender & gauge - both of these new Mercury Quicksilver OEM.

These made no difference. I also pulled off the hose from sea water pump o/p side at the T-Stat housing and tested water flow here with clear hose & 1000RPM. I'm getting easily 7.5 quarts plus for 15 seconds and no air bubbles.

Just to re-cap:
Engine runs consistently just over 180F on temp gauge at speed including WOT.
When coming off plane, briefly drops down to 120F-130F and within 30-45 seconds up to 160F quickly and slowly back to 180F.
Exhaust risers & elbows not hot
Exhaust pipes not hot
I cannot even measure even 100F on T-Stat housing with gun.
No audible alarms - ran the boat yesterday +30 mins at various speeds.
No unusual sounds from engine - no belts squeals.
T-Stat replaced with new 160F
Raw Sea pump removed and disassembled / reassembled
Tested Sea pump flow - no air detected & +7.5 quarts per 15 seconds
New temp sender & dash temp gauge

One item I did notice which I wondered if normal:
Both small hoses from T-Stat to exhaust manifolds are cool to touch.
But the large hose for the recirculating pump is hot - not scalding but hot to hold (i didn't take a temp reading with my gun).
I'm curious as to why the large hose would be hot? Shouldn't the water in the large hose be cool lake water from the sea pump flowing into the engine block?
Can some explain how the water actually flows from the sea pump to T-housing and through the block?
Also, what are symptoms for recirculating pump fail?

Thanks for reading, any feedback would be appreciated.

​

 

[alldodge]

With Stat as in post 6 the large hose can be slightly warm to 180* while running.

The cool raw water comes in from drive to port side Stat most front port. If Stat is closed then the circulating water pump just moves current water in the block, there is very little cool water going into block

When Stat opens hot water exits the block thru the Stat to exhaust

 

[DHutch]

With Stat as in post 6 the large hose can be slightly warm to 180* while running.

The cool raw water comes in from drive to port side Stat most front port. If Stat is closed then the circulating water pump just moves current water in the block, there is very little cool water going into block

When Stat opens hot water exits the block thru the Stat to exhaust

View attachment 399984

thank you for the diagram

Questions - if the recirculating pump is not functioning properly for whatever reason, is it possible the cold water from the sea water pump is now only cooling the exhaust risers and not enough water is being pushed through the block?
Could cold water go directly to the small exhaust hoses and may not even mix with water at the temp sender?
I'm just wondering if the recirculating pump is not pushing water through the block at the correct rate?
What I find weird is how or why I can get a temp swing from 180F suddenly down to 120F when I quickly slow down off plane.

 

[alldodge]

It could be the Cir pump has problems.
could remove large hose and use a long thin screw driver push up in side. With belt off Cir pump see if you can stop the pump from turning by hand