OBD coolant temp - where is that being measured?

[gmushial]

Title says it all: after doing the water pump, I opted for a fan delete, but have been using Torque Lite to monitor the coolant temp... but I'm wondering where that's being measured.. ?? [I'm seeing numbers between 192 and 210F... how do those look? Or should I be looking at the lower temp switch for the radiator??]

[328 Power 04]

It comes from the temperature sensor in the head, which sends temperature to cluster and to the DME. 192-210deg F is normal.

[bluptgm3]

In the base of the head under the intake manifold.


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[gmushial]

Gents - thank you... and for my purposes, that sounds like the perfect location to take a temp - I was afraid it was at the radiator or somewhere equally worthless... though it does raise a question: if this is used by the DME, how so? Wouldn't charge temp be more important, or is that also factored into the equation?

[bluptgm3]

Both intake air and engine temperature are used by the ECU/DME


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[Vintage42]

It comes from the temperature sensor in the head, which sends temperature to cluster and to the DME. 192-210deg F is normal.

Good to know where that OBD2 sensor is. And to know what normal is. My OBD2 ScanGuage showed temps varying from 170 to 200 for a couple of years, depending on driving conditions, until I replaced the thermostat. Now its a steady 206.

[rf900rkw]

On the M52TU and M54, the sensor measures the coolant entering the engine.

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Good to know where that OBD2 sensor is. And to know what normal is. My OBD2 ScanGuage showed temps varying from 170 to 200 for a couple of years, depending on driving conditions, until I replaced the thermostat. Now its a steady 206.

You have a totally different engine with a different cooling system. Your sensor measures the coolant temp exiting the engine.

[gmushial]

On the M52TU and M54, the sensor measures the coolant entering the engine.

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You have a totally different engine with a different cooling system. Your sensor measures the coolant temp exiting the engine.

So just downstream of the blend with the cooler water from the radiator, ie, this was an actively controlled/selected temperature... or the best it could do? [Where is the actual sensor, btw??] ... I'm still a little iffy on not running with the fan - but given your work on understanding these systems: I though I'd give it a shot... it was 104 today and I was driving in town... so my take is that if could handle that, then I'm probably good to go.

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Good to know where that OBD2 sensor is. And to know what normal is. My OBD2 ScanGuage showed temps varying from 170 to 200 for a couple of years, depending on driving conditions, until I replaced the thermostat. Now its a steady 206.

I'm running with a new thermostat - or at least a month old... but my impression is that the t'stat in our cars is not like the old school ones which opened per water temp, but are electrically controlled by the engine management system... I'm sure Randy will correct me if I'm wrong.

[328 Power 04]

Correct, electronic control, some of the maps controlling it are below

MS42 coolant temps.jpg
X axis is rpm
Y axis is load mg/str
Z axis is target coolant temp


There are many other condition based maps to modify these.

These serve dual purpose, to help oil warmup (which I believe the above maps are doing more of), and to control engine temp (including other maps).

[rf900rkw]

All Z3 motors have the thermostat on the inlet side. It mixes hot coolant exiting the engine with cooler coolant from the radiator to supply a target temp to the waterpump.


M/S 50/52 the waterpump pushes the coolant into the block. From there is rises up into the head at places and rates defined by the orifices in the head gasket. The flow migrates up and back. At the rear of the head, there is a port that collects the coolant had pipes it back to the front and into the thermostat cavity where some is sent to the radiator while some is sent on another loop.
This rear to front pipe is cast into the head along the lower edge on the intake side. The coolant sensor is in this passage. It measures exit temp, even though it is located in the middle of the motor.

M52TU/M54 The waterpump pushes the coolant into a passage that goes back up through the head gasket and then through a passage cast into the head just below the intake ports. The coolant is then introduced into the engine proper at the rear of the head. The coolant travels forward through the head to the thermostat cavity, where it is again graded and mixed. The bolck has no direct coolant flow, relying purely on thermosiphon to transfer coolant/heat. The coolant temp sensor is again in the passage under the intake. But it is now measuring the temp of the coolant going into the engine.

M44 Will work similar to the M52. I have not bothered to subject a head to detailed inspection of the flow paths, which often entails a bandsaw.

[gmushial]

Correct, electronic control, some of the maps controlling it are below


X axis is rpm
Y axis is load mg/str
Z axis is target coolant temp


There are many other condition based maps to modify these.

These serve dual purpose, to help oil warmup (which I believe the above maps are doing more of), and to control engine temp (including other maps).

So the 210F numbers I was seeing was probably an artifact of the 102C part of the map?

[gmushial]

All Z3 motors have the thermostat on the inlet side. It mixes hot coolant exiting the engine with cooler coolant from the radiator to supply a target temp to the waterpump.


M/S 50/52 the waterpump pushes the coolant into the block. From there is rises up into the head at places and rates defined by the orifices in the head gasket. The flow migrates up and back. At the rear of the head, there is a port that collects the coolant had pipes it back to the front and into the thermostat cavity where some is sent to the radiator while some is sent on another loop.
This rear to front pipe is cast into the head along the lower edge on the intake side. The coolant sensor is in this passage. It measures exit temp, even though it is located in the middle of the motor.

M52TU/M54 The waterpump pushes the coolant into a passage that goes back up through the head gasket and then through a passage cast into the head just below the intake ports. The coolant is then introduced into the engine proper at the rear of the head. The coolant travels forward through the head to the thermostat cavity, where it is again graded and mixed. The bolck has no direct coolant flow, relying purely on thermosiphon to transfer coolant/heat. The coolant temp sensor is again in the passage under the intake. But it is now measuring the temp of the coolant going into the engine.

M44 Will work similar to the M52. I have not bothered to subject a head to detailed inspection of the flow paths, which often entails a bandsaw.

After spending 40+ years in the sciences: one very much has to respect that level of knowledge and understanding - my hat's off to you.

Though a question: the thermosiphon: this is using heat flow through the metal of the block? Or they're setting up convection currents exchanging coolant between the head and block? Or... ??

Have you thought about the failure modes: ie, I assume the t'stat is using a bi-metal element to open and close the flow.. is this a activate to open, ie, without power stays closed? Likewise, if they use a heating element to open it: is this to-open temp reachable via coolant temps? Or is it a case that once you lose power to the element or it fails, all that's going to happen is continual recirculation of coolant through the head/waterpump circuit? [though I thought I remember people here talking about losing heater heat, and that was caused by a failed t'stat ??]

Also, where does the heater circuit figure into this [I'm thinking in M52TU context] - it's taking heated coolant from the back of the head (coolant temp controlled by the t-stat) and returning it to the radiator via the expansion tank?

But again: many thanks... likewise an impressive/totally professional level of knowledge... well done.

[rf900rkw]

Thermosiphon via natural coolant flow. there may be some minor forced flow via pressure differentials, but it will be minor compared to the forced flow of the earlier systems. They basically used the block as a manifold to feed coolant to the head.

The thermostat is still a wax bulb design. There is now a heater in the wax element so that the computer can drive it open at an earlier point. If the heater fails, you still have the mechanical wax doing the regulating at the base (highest) temp. If the wax bulb fails, then it will be just like any other thermostat failure; stuck open, stuck closed, random, etc.


The heater circuit. Remember the two plastic pipes under the manifold. One take coolant from just before the hot exit coolant reaches the thermostat cavity. Through the heater, then back to the waterpump cavity with the expansion tank in series.

[gmushial]

Related: I did a 530 mile drive yesterday... and it seems that not having to spin that fan saved me 3+ mpg... I'll keep a careful log of this and see if this continues, but before my best single tank mileage (actually best 3) were just over 30.0 mpg [m52tub25 5mt] - and that was using E0 Arizona gas on long straight interstate stretches... but yesterday [using miles from the odometer, which didn't disagree with the mileage numbers from two dozen other trips over the same route], and the gallons number from the pumps: 34.4 mpg going down to the BayArea, and 33.9 coming back... and these were confirmed indirectly, in that I was able to get an extra 30-40 miles per tank before having to refill. ?? :-) ... back on topic: had exactly zero problems with coolant temps, and on the way back the ambient temps were in the 106degF range.