Engine running cool

[ghebbns]

Hi all,
I have a 25xp in my 1990 Mk1.5.  The engine has always been very consistent at 165 degrees once warmed up.  The last couple of time I have been out, it doesn't quite reach 160 on the gauge.  Everything seems fine - the rpm/speed ratio is still the same.  I haven't done anything to the engine lately except change the transmission cable.  Is this something I should be worried about?

Thanks

Edit: Was just doing some reading and found someone that said by turning off the valve to the hot water heater, their engine runs about 10 degrees cooler.  I did just turn off this valve, so could that be it?  Why would this cause the engine to run cooler?

[KWKloeber]

You can't tell what's causing it unless you verify the temp at the engine with a laser thermometer. The gauge or sender could be causing incorrect readings. Or it could be a faulty thermostat.

A 5 deg variance us no big deal. But a diesel should operate as high as allowed - which is 160 in salt and 180 in freshwater.

What valve to the water heater - what's the link to that info? I'm not familiar with that plumbing. The coolant to the WH should NOT be stopped (coolant should always circulate.)

[Stu Jackson]

I agree with Ken, what valve?  You need to assure that if there is a valve, it doesn't stop the water from getting back to the engine, because the heater is in the loop for the freshwater coolant.

[ghebbns]

Thanks for the quick replies.  You are correct - I didn't think about it enough.  I have shut off the water to the hot water heater as I don't use it, not the coolent circulating from the engine.

I have a laser thermometer and will check.  What is the best area of the engine to measure?

[scgunner]

If you were to shut off the water to the water heater I'd think the motor would run hotter not cooler. You'd have less coolant and less coolant circulation, things which generally make a motor run hotter. An example is if your car is overheating you can increase cooling capacity by turning on the heater and fan, while not fun for passengers it will dissipate more heat.

[mark_53]

I don't know where you are located but water temperature has an impact on engine temperature.  Are you in cooler waters?  Also, you can cool your engine by running the hot water in your galley sink.

[KWKloeber]

I don't know where you are located but water temperature has an impact on engine temperature.  Are you in cooler waters?  Also, you can cool your engine by running the hot water in your galley sink.

Why would seawater temp affect the closed coolent system?

If the thermostat is not faulty, the coolant doesn't leave the engine and touch the heat exchanger unless the tstat reaches 160F (or 180F.)  There is very tiny amount (1/8" hole) that bypasses the Tstat, but that is AFTER the Tstat and after the temp gauge sender.  The slight return amount doesn't affect the engine temp, it merely keeps a small amt of coolant circulating thru the exhaust manifold so exhaust doesn't create hot spots.

Seawater temp DOES affect the temp of water to the muffler and out the exhaust. 

[KWKloeber]

gh
Read the temp in a couple places, but primarily at the TStat cap where your temp gauge sender is also "reading" the temp.

If you were to shut off the water to the water heater I'd think the motor would run hotter not cooler. You'd have less coolant and less coolant circulation, things which generally make a motor run hotter. An example is if your car is overheating you can increase cooling capacity by turning on the heater and fan, while not fun for passengers it will dissipate more heat.

Kevin

That's true if running a heater (an AIR heat exchanger) which is constantly removing heat from the closed system.  So the heater is cooling in addition to your radiator (or boat Hx.)

There would be some initial additional cooling.  But once the water heater gets hot, you'd need to run potable water so that heat is constantly transferred from the closed coolant loop.  If no potable water is run (i.e., turn off the fan on the car heater,) the closed system gets to equilibrium and there's no additional cooling.  Regardless though, the TStat controls the engine temp and things downstream from that can only make the engine hotter, not colder than the TStat setting -- The TStat doesn't know and it doesn't care what else is connected after it.  It only knows to open up at 160F or 180F and close down when the engine coolant temp falls below 160/180.

The one caveat to that is that the gauge sender is above (just downstream of) the Tstat (valve.)  So I suppose there could be a very slight difference between the tenp that the sender sees and the temp that the TStat sees.

[mark_53]

I don't know where you are located but water temperature has an impact on engine temperature.  Are you in cooler waters?  Also, you can cool your engine by running the hot water in your galley sink.

Why would seawater temp affect the closed coolent system?

I'm not sure I understand your question but cooler seawater will cool the engine more than warm water.  Agree a faulty tstat is a possibility though.

If the thermostat is not faulty, the coolant doesn't leave the engine and touch the heat exchanger unless the tstat reaches 160F (or 180F.)  There is very tiny amount (1/8" hole) that bypasses the Tstat, but that is AFTER the Tstat and after the temp gauge sender.  The slight return amount doesn't affect the engine temp, it merely keeps a small amt of coolant circulating thru the exhaust manifold so exhaust doesn't create hot spots.

Seawater temp DOES affect the temp of water to the muffler and out the exhaust.

Running hot water through the sink removes heat created by the engine. If for some reason the hot water system was running or functioning properly the hot water tank would be a heat sink to absorb engine heat.  It would take longer to get to temperature or if sea water is cool enough, it wouldn't get up to temperature.

[KWKloeber]

Caution - long post, TMI so ignore this if you're not counting calories.

(1) I'm not sure I understand your question but ...

(2) cooler seawater will cool the engine more than warm water.

#1) The question was - because we have two separate systems that don't handshake when engine temp is below the TStat temp (i.e., TStat is closed, no circulation,) how can SW temp affect the other system's temp (and vice versa)?

#2) Nope - It's the law of thermodynamics -- conservation of energy.  The TStat regulates flow so that the heat removed equals the heat generated.  Temp remains constant.

SW temp affects engine temp if the TStat is stuck partially open (i.e., constant flow) -- but not if everything operates correctly.  it may not change your mind but here's how it works:

(Let's use a 160F TStat for salt) if everything operates correctly the engine coolant in the closed system is not affected by the SW when the engine is below 160F -- because no coolant flows past the Tstat to the Hx (ignoring the small 1/8" hole in the Tstat.)  Therefore cold SW cannot lower the engine below 160F. 

No flow - hot engine coolant doesn't shake hands with the SW in the Hx.  Conversely stated, if the temp is below 160F engine coolant doesn't leave the block to get cooled down.  So cold SW can't lower the engine temp below 160F if coolant doesn't leave the engine.

With colder SW, the HX does it's job more quickly (more BTUs transferred every minute of flow) and warmer SW forces the Hx to try harder (less BTUs transfered every minute of flow.)  The effect is that the TStat cycling (open time) is longer w/ warm SW and shorter w/ cold SW.  But the same BTUs are removed either way, so the resulting temp is the same.

High SW temp can overheat an engine (the 2" Hx) so in that aspect colder SW does affect the MAX not the MIN engine temp.

Analogies -

Your car engine doesn't care whether it's 40F or 60F outside, it runs at the same temp.  Additionally (unlike a boat) there's cooling from cold air blowing past the engine.  The car still doesn't run cooler than the TStat setting when in colder weather.

or

Your home fridge is the *same as* our diesels.  It has coolant (Freon,) a thermostat, a Hx (the condenser,) and flow thru the Hx (the room air.)  If we set the fridge to 35F, whether the room is 50F or 75F, the food is still 35F.  The compressor and condenser work longer or shorter times depending on room temp, but inside the temp remains 35F.

Cold SW does suck some BTUs from the gear cover, via the SW pump case and shaft and bolts -- but that effect is immeasurable and inconsequential.


[/quote]

Running hot water through the sink removes heat created by the engine. If for some reason the hot water system was running or functioning properly the hot water tank would be a heat sink to absorb engine heat. 

[/quote]

Yep, that's what I said -- but of course that cooling depends on the temp of the potable water not sea water.  It's a heat sink (heat exhanger) while the WH warms up to the engine temp (or while potable water runs.)**
The difference is that the coolant to the WH flows 100% of the time [no TStat.]  That's bigly different than how the Hx and SW work.

There would also be very minimal cooling when potable water isn't running (so long as the galley cabinet temp is < the WH temp.)  WH insulation isn't 100% efficient so there's always heat transfer thru the WH case (and hoses) to the air.  Although the effect is immeasurable -- if the Hx is removed from the equation, cold ambient air around the WH would not keep the engine from overheating. 

It would take longer to get to temperature or if sea water is cool enough, it wouldn't get up to temperature.

[I presume that you are saying this in relation to the Hx, not the WH, so I seperated it.]

Not due to the SW itself.  Likewise to how the Hx works once the engine is warm, SW doesn't affect the time it takes to raise the engine to 160F.  (Other than the 1/8" hole in the TStat) coolant that doesn't flow to the Hx  -- can't lengthen the warm up time.

However, SW temp might or might not relate to ambient temp -- and that affects warm up.   
A cold engine compartment, say 30F vs 80F, would lengthen warm up.  Both the block starts off colder and (until the engine compartment warms up) there's greater heat loss per minute from the block to the air.

Nevertheless, the SW itself doesn't affect the warm up time.
Anology - an outdoor thermometer that reads -20F is not what causes frostbite.  Correlation is not causation.

**Ambient temp (WH and potable water tank temp) would also affect the WH acting as a heat sink.

[ghebbns]

Hi all,
Just an update. Was getting the boat ready for hurricane Teddy today and took her out for a little spin to get the engine up to temp. I used the digital thermometer and found the temperature on the tstat was 159.2. I would say this is within the proper range. Not sure why the temperature gauge is a few degrees lower than it used to be, but I think things are ok.

[KWKloeber]

gh

Great news.
There's a few things that can affect temp readings.

Gauges do go bad.
Senders do go bad.
The readings are +/- 16F (per Teleflex.)
If resistance increased in the circuit the gauge reads lower. A loose or corroded connection at the gauge or sender terminals or poor ground at the sender could cause a low reading.

Assuring 0 resistance on the temp sender is problematic- it's not like the oil switch that screws right into the block. You have a separate cap, a gasket, bolts that could make poor connection either at the bolt head or threads. There's things one can do to make a good connection but the easiest is to bond the sender body.  Below is what I put on harness reworks - Eazypeasy to make.

[Ron Hill]

gheb : Now that you know what the actual engine temperature is - I'd cut off the old ring connector on the temp sender wire, inspect as much wire as you can for cuts/breaks, clean the contact on the sender threads and install a new ring connector.  That just might get your gage reading correctly again!

A thought