Coolant Temperature Needle moving slightly below 12 o'clock. Normal?

[RobE36E46]

Hello everyone,

I picked up a 2003.5 M3 in Montana in October 2024. One thing I immediately noticed was that occasionally the temperature needle would drop from dead center when I was cruising off throttle or going downhill. Car has 80k miles and sat quite a bit the last decade.

When I got home, I replaced the thermostat with a new Wahler. I boil tested it both it and the original thermostat, and noticed that the new one seemed to open at a higher temperature and quicker. I thought – “aha got it!”

Well it still does this some. Tonight I hooked up an OBDII scanner and found out the gauge will drop below 12 o’clock if the coolant temperature falls below 83° C. The first little dot below the mid-point is at around 73° C FWIW.

Cruising uphill on the highway, I’m at around 91° C. Car warms up quickly. If I get off the gas and cruise, my radiator outlet temperature drops quickly and the engine coolant will drop temperature to around 82-83 or so before it stabilizes and then the outlet temp drops rapidly. I assume this all a result of the 80° C thermostat closing. This seems good to me?

However, if I keep cruising temperature will slowly drop. I saw as low as 79° C going downhill on a frontage road off I-40 through the Sandia's for a few miles. This causes the gauge to start start dropping as I've seen most times I drive the car.

Coming from m/s52 and m54 engines where the needle absolutely doesn’t move from center unless there is a problem this is disconcerting. However, given it’s an 80 ° C thermostat and any coolant temp below 83° C causes the needle to move, it doesn’t sound nutsy to me to think that the S54 “just does this” because the gauge is less buffered.

Is this normal S54 behavior that is well known (cause race engine?) or do I have a potential problem here? Thanks!

[Slideways]

It is normal for it to be slightly to the left of center if the rad is getting good airflow. When it starts dipping closer to the first dot or you are reading 75C or below at operating temp, then the engine coolant temp sensor under the intake manifold is faulty.

[jayjaya29]

Normal behavior.

[ac427]

Hello everyone,

I picked up a 2003.5 M3 in Montana in October 2024. One thing I immediately noticed was that occasionally the temperature needle would drop from dead center when I was cruising off throttle or going downhill. Car has 80k miles and sat quite a bit the last decade.

When I got home, I replaced the thermostat with a new Wahler. I boil tested it both it and the original thermostat, and noticed that the new one seemed to open at a higher temperature and quicker. I thought – “aha got it!”

Well it still does this some. Tonight I hooked up an OBDII scanner and found out the gauge will drop below 12 o’clock if the coolant temperature falls below 83° C. The first little dot below the mid-point is at around 73° C FWIW.

Cruising uphill on the highway, I’m at around 91° C. Car warms up quickly. If I get off the gas and cruise, my radiator outlet temperature drops quickly and the engine coolant will drop temperature to around 82-83 or so before it stabilizes and then the outlet temp drops rapidly. I assume this all a result of the 80° C thermostat closing. This seems good to me?

However, if I keep cruising temperature will slowly drop. I saw as low as 79° C going downhill on a frontage road off I-40 through the Sandia's for a few miles. This causes the gauge to start start dropping as I've seen most times I drive the car.

Coming from m/s52 and m54 engines where the needle absolutely doesn’t move from center unless there is a problem this is disconcerting. However, given it’s an 80 ° C thermostat and any coolant temp below 83° C causes the needle to move, it doesn’t sound nutsy to me to think that the S54 “just does this” because the gauge is less buffered.

Is this normal S54 behavior that is well known (cause race engine?) or do I have a potential problem here? Thanks!

Does your coolant level remain constant?

[sapote]

Cruising uphill on the highway, I’m at around 91° C. Car warms up quickly. If I get off the gas and cruise, my radiator outlet temperature drops quickly and the engine coolant will drop temperature to around 82-83 or so before it stabilizes and then the outlet temp drops rapidly. I assume this all a result of the 80° C thermostat closing. This seems good to me?

However, if I keep cruising temperature will slowly drop. I saw as low as 79° C going downhill on a frontage road off I-40 through the Sandia's for a few miles. This causes the gauge to start start dropping as I've seen most times I drive the car.

It's normal for M3. The non-M M54 cars have the gauge coded with too luch hysteresis -- litterally locked the needle at 12 from 73C to 115C, and M3 gauge is coded with less hysteresis.

[RobE36E46]

Thanks everybody, glad I haven't gone (too) crazy!

ac427 - Coolant level stays constant though I have a minor seep near the thermostat housing that I need to address (It's so minor that it seems like it would take quite a bit of driving to drop coolant levels a notacible amount) . After I drive it, I have little dots of coolant all over the driver's fan shroud and I've traced that to coming from somewhere near the T-stat housing. I used the FCP euro "OE" (lol) O-rings on the coolant pipe, they were an absolute PITA to get seated, and my guess is they are leaking (other forum comments said this is common last year). I need to drain the coolant, and do another T-stat O-ring, but I wanted to confirm that I didn't need to do another T-stat while I was in there.

[Slideways]

Thanks everybody, glad I haven't gone (too) crazy!

ac427 - Coolant level stays constant though I have a minor seep near the thermostat housing that I need to address (It's so minor that it seems like it would take quite a bit of driving to drop coolant levels a notacible amount) . After I drive it, I have little dots of coolant all over the driver's fan shroud and I've traced that to coming from somewhere near the T-stat housing. I used the FCP euro "OE" (lol) O-rings on the coolant pipe, they were an absolute PITA to get seated, and my guess is they are leaking (other forum comments said this is common last year). I need to drain the coolant, and do another T-stat O-ring, but I wanted to confirm that I didn't need to do another T-stat while I was in there.

Those FCP "OE Supplier" orings seemed to be a size too large and were pretty loose when installed on the thermostat housing pipe. There is a good chance that one of them is pinched which is causing the leak. While it is possible to get them to seal by using an oring lubricant and being extremely careful when installing them, I would go with factory orings next time unless FCP updates those "OEM" orings with a smaller size.

[ac427]

Thanks everybody, glad I haven't gone (too) crazy!

ac427 - Coolant level stays constant though I have a minor seep near the thermostat housing that I need to address (It's so minor that it seems like it would take quite a bit of driving to drop coolant levels a notacible amount) . After I drive it, I have little dots of coolant all over the driver's fan shroud and I've traced that to coming from somewhere near the T-stat housing. I used the FCP euro "OE" (lol) O-rings on the coolant pipe, they were an absolute PITA to get seated, and my guess is they are leaking (other forum comments said this is common last year). I need to drain the coolant, and do another T-stat O-ring, but I wanted to confirm that I didn't need to do another T-stat while I was in there.

Sounds good. The only time i had the temperature needle going to the left was when i had an issue with my head gasket. I would have thought the temp would have risen if the system was losing pressure but the gauge actually went to the left of centre. Not as much as yours but maybe that slight leak os all that is needed.