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Actual engine temperature


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Rule No. 7,864 of megasquirting your GT6......
Do NOT install an EFI coolant sensor in a sedan thermostat cover and shut the bonnet!

Moving on.....
With only having ONE coolant sensor (now) I graphed the water temp twice from "cold' to hot at idle speed. Once with the sensor located under the tstat and once with the sensor teed into the heater hose leaving the back of the head.

Note: I have triple checked that my sensor is calibrated and accurate against a thermometer! (tsat in the same container at the same time as the sensor and thermometer)
So why do I get a "blip" in the temp around 76/70C as if the tstat is opening????????

The peaks and troughs between 80 and 86c are from the fan coming on. I have programmed the fan to come on at 84 and off at 82. I am guessing that the cold radiator water reaches the back of the head quicker and that is why the temp only max's out to 85 instead of 86 with the sensor up the front.

I triggered the fan so it would run continuously to simulate fast coasting. Interestingly the temp dropped way past where I expected and bottomed out at 72C at the back of the head and 76C at the front. I had always "assumed" that the temp would not go below the tsat specification (82).
Interestingly, these low figures are close to the figures of where I get that early blip in temperature. Maybe I will have to calibrate my sensor a fourth time???!

Does any "know" what temperature a Triumph engine "should be" running at, NOT what thermostat rating should be used?


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I've not studied the flow patterns in detail but it seems to me that the standard sensor location is relatively low in the housing and possibly seeing more flow than the wax capsule in the thermostat. It may simply be that there's a few degrees of temperature difference between them. Alternatively, the sensor is in a relatively bulky brass housing that adds a good few seconds lag on the temperature. Could it be that the wax capsule is seeing the 82degrees while the sensor is lagging behind?

The thermostat will exhibit a certain amount of hysteresis (stickiness) so I can certainly believe it closes cooler than it opens.

I'd also say that 84 is too cold for the fan threshold. An 82 degree thermostat is intended to open early enough to keep the running temperature somewhere in the upper 80s. The fan should cool the water in the radiator to around or below the thermostat temperature, but it doesn't need to do that unless the in-engine coolant is getting rather hot. So while 80 degrees would be fine (perhaps a tad high) for a rediator mounted fan switch, if you're controlling your fan off engine temperature I'd go with 90 to 95. For reference, a typical late 1990s Rover with an 88 degree thermostat would have the fan threshold set at 104 (engine).

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I think you maybe over analysing this.

From here it looks to be under control. As mentioned above, your current fan switching suggests you are trying to control it to too narrow a window.

Because you now have data, you are seeing things that the rest of us are oblivious too. Dead handy though to fault find if you do have a problem.

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Thanks for the feed back guys.
My intention is to get a base line before I start modifying the cooling system with a bypass and thermostat in the bottom radiator hose.
I think the sensor and the tstat bulb are within an inch of each other so I can not see any scope for a lag in time.

I have based my programmed cooling window on that the 82 stat opens at 82 and is fully open at 86, therefore I assume the design is to keep the engine temp in this range

What I am surprised/ concerned with is the difference in temperature from low 70s to high 80s despite using a 82C tstat.
I was hoping/expecting to see a difference in temperature from the back of the head to the front given the ancient design, ancient manufacturing techniques and decades of rust build up, giving me good reason to pursue the bypass system but what I am seeing is a massive temperature variation across the WHOLE engine from fast driving to idling. Perhaps this in itself is a very good reason for further experimentation.

So, what consistent/constant running temperature should I be aiming for? 75, 84, 90C??????


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An inch difference is more than plenty if it's in the right direction! The temperature difference between the middle of the flow and the edge of a rough channel can be large. The thermostat bulb is intentionally located where the hottest water ends up.

Anyway, the lag in time isn't only down to that but also the heat transfer within the sensor. There are actually plenty of lags going round - you can see some of them on your traces, where the fan cuts in but the temperature keeps rising for quite a few seconds (although much of that is the bulk temperature in the radiator taking a while to drop)

It's a common misconception that a thermostat that fully opens by 86 is intended to keep the temperature at 86. It's not. The aim is to keep the temperature somewhere in the 90s by flowing enough coolant through the radiator, and the thermostat is purely there to get hot quicker. (There are also flow behaviours that can mean removing the stat has unexpected results, but the reason for having the stat instead of a restriction is to speed the warm-up). In the summer, you want the stat open fully all the time once warm. In the depths of winter, this could result in over-cooling, so you pick a stat that opens closer to the desired temperature - perhaps an 88C one. Running the fan too much simulates this winter condition, where the radiator doesn't get hot.

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Hello Rob,
how does a 82C thermostat, fully opened at 86C keep the temperature in the 90s?
Surely once the tstat is fully open, temperature control is lost and in the hands of the ambient surroundings? ie no control
Aren't we trying to control/maintain a specific engine temperature despite ambient weather and engine load?

I am more perplexed that a 82C tstat will allow the temp to drop to the low 70s.


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FWIW I prefer to keep the engine temp lower for a variety of reasons. I found a land rover 76 degree thermostat (IIRC) which worked well for me. In reality, the thermostat is open a bit most of the time, and temps are hotter than this in some places - a lot of heat comes off the manifold. I also lagged the exhaust manifold to reduce this - the car was also less powerful when it warmed up, though not as badly since I put the air intake back to the front of the car . . .

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what i tink is that cos you got a digi read oot noo,
then you can see the swing faster, as it aint going thru a VS unit.

the OE gauges react slowly, so they wont be swing aboot alott.

On my own digi read oot, the thing swings frae 78 to 85.
butt , the car gauge stays central

Also, a termostat will let water thru moer if the water suddenly heats up.
but cos the gauge is always lagging behint, then it shows up a lower temp   after the event.

As when it opens, then the cold water in the rad will then go thru engine.
and cool it, thus bringing yer digi sensor down

the colder the outside air, the moer this will swing, mainly to the lower  temps.
as we must all have seen driving our cars in winter,or cold air

On me own, I can switch the heater off,  and run it along the road.
then switch the heater back on, and in a few secs, the digi read oot on ECU has dropped 4 degs
but soon goes back, but agen, temp gauge no sees this drop.

And this wid just aboot a pint an a bit int heater core


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Exactly - the thermostat isn't regulating the engine temperature at all, merely speeding up the warm-up. We are in the hands of the surroundings but we can tolerate a certain amount of variation. Older cars were generally set up to run colder than optimal because that gave more headroom for this variation. Modern cars have better thermostatic control (increasingly we're seeing flow control valves instead of thermostats, electric pumps, etc.) and rely on the radiator fan to remove a lot of the ambient uncertainty.

If the coolant was being held at 86 it wouldn't boil, and you wouldn't need a pressurised system.

If you look at the design of the GT6 water transfer housing, the temperature sender is located very close to the feed for the inlet manifold heating. It's offset away from the engine ports, where the hottest water will be, by more than the thermostat. I could certainly believe that the hottest water is going up and out of the thermostat while the water that the sensor sees is coming at least partly from the pump, which feeds in some cold from the radiator bottom hose. That would explain why your sensor is showing 76 while the water flowing past the thermostat is still hot enough to keep it open.

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Hi GT6 M,
this is exactly my point.
Now we have the technology, we can see what is actually happening. "Everyone" says/thinks that their temperature is "rock solid" because the gauge doesn't move.
Well it certainly appears that my temperature is a swinging as the sixties! We can assume that our engines can withstand this but I would like to experiment and see if I can get it a bit more mundane and steady. At least I will be able to measure any changes accurately.


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Hi Rob,
I though you "had it", but on further consideration, perhaps not.
Why would a thermostat housing be designed to let in cold water direct from the radiator? That would defeat the purpose of measuring the water temperature coming out of the engine.
Also, if my early "blip" was the tstat opening at 82 but the sensor under the tstat measuring the sudden inflow of cold radiator water "directly" onto the sensor, I would not get the same blip when the sensor is in the heater hose being fed directly from the head.

I have attached a photo showing where I have installed the sensor on my 2500 engine, not GT6, if there is a difference in the thermostat housings.

My experimentation and documentation of the cooling system is to get a baseline so I can measure any difference when I add a bypass and move the thermostat to the bottom radiator hose like modern cars.
I have been surprised with the results I have found  and am now second guessing my megasquirt and engine. So I am very interested in hearing from other people and experiences, and I appreciate your and others input.

Many thanks,

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That isn't the standard location for the GT6 sensor but not far off. I assume the single wire sender is the gauge one, and the two wire is the one you're graphing? That looks to be where the GT6's inlet heater hose would be. The sender is normally on the other side of the housing, directly opposite that.

The thermostat cavity is part of the coolant circuit when the thermostat is closed, so it has openings to both the hot and cold sides of the engine/pump circuit. Admittedly you would expect the flow to be from the hot to the cold (where it's on the inlet of the pump) but I think the shape is such that the sensor is much more "in" that flow than the stat. Now, at this point I'm having to guess and speculate, but given that the cooling passages are moderately wide, there may be a semi-laminar flow. It's possible, then, that the stuff which takes the simple circuit through the bypass hole (and hence past the sensor) is from the middle of the flow. It's also possible that the slower-moving edge fluid, which picks up a lot more of the engine heat and is therefore hotter, follows round the top edge of the cavity to where the thermostat is. This is good, in that it opens the thermostat quicker, but bad in that it's not circulating too well. Of course, during warm-up we don't mind that, and once the thermostat's open the patterns might change.

The drop in bulk coolant temperature when the stat first opens is a normal characteristic of top-hose-stat engines, and is actually the reason most modern engines are bottom-hose-stat designs. In fact, many engines exhibit repeated dips - more  of them the larger the radiator is compared to the block coolant capacity.

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