I'm trying to understand my cooling system, and maybe I'm the one that's broken

[dukeofchen]

It's worth changing the coolant temp sensor located under the intake manifold. It is the sensor that is supplying signal to the needle in the cluster gauge. My sensor went bad without throwing an error code and it would dip down to the first dot under the same overcooling conditions you are describing.

I have overcooling symptoms on my car at highway / downhill speeds and hoping that this fixes it. In theory makes sense with increased airflow / heat dissipation and relatively low engine loads. Prior to this my gauges always stayed pegged dead center.

Worth noting for OP, I think these symptoms may have emerged from when my water pump failed on me. Might be a stretch too far, but the overheating (gauge in the red) may have been what damaged the sensor.

Will report back if it works for me.

[t3ddftw]

Glad someone is getting real world use out of the coolant temperature display on the BlueBus

I had similar issues with my car when I lived in Las Vegas. Generally speaking, I could cool the engine off by turning the A/C compressor off. Does that have any impact on your overheating?

I hadn't thrown parts at it, but I was going to go for an aftermarket radiator and OE everything else.

-Ted

[Slideways]

I have overcooling symptoms on my car at highway / downhill speeds and hoping that this fixes it. In theory makes sense with increased airflow / heat dissipation and relatively low engine loads. Prior to this my gauges always stayed pegged dead center.

Worth noting for OP, I think these symptoms may have emerged from when my water pump failed on me. Might be a stretch too far, but the overheating (gauge in the red) may have been what damaged the sensor.

Will report back if it works for me.

Yeah, I replaced the thermostat first thinking my first replacement temp sensor couldn't have only lasted a few years and 15k. It turned out to be that sensor behind the thermostat and under the manifold. It's a $25 part and doesn't require draining the coolant.

Usually when a thermostat fails and is overcooling the engine, it will stay stuck on the first dot while warming up. A major fluctuation in temp when fully warmed up seems more like a sensor issue. The sensor might even fix the getting hot at the second dot issue. It would be nice if it threw an implausibility code, but I guess that is a fault which is rarely picked up by the DME.

[heinzboehmer]

It's a $25 part and doesn't require draining the coolant.

How do you replace that part without draining the coolant? Are you saying you don't have to drain it completely? Because I can't think of a way to replace it without at least partially draining the system.

[Tbonem3]

He's not talking about the one on the bottom rad hose.

[heinzboehmer]

He's not talking about the one on the bottom rad hose.

Yes I know. But the sensor is on the bottom of the pipe that goes to the rear of the thermostat housing. I haven't personally replaced this sensor, but because of its positioning, I'm assuming some coolant will begin to drain once removed.

[ATB88]

Yes I know. But the sensor is on the bottom of the pipe that goes to the rear of the thermostat housing. I haven't personally replaced this sensor, but because of its positioning, I'm assuming some coolant will begin to drain once removed.

When I did mine, I just did it very quickly. Unthreaded the current sensor until it was juuuust about to come off, had the fresh sensor ready in the other hand, and did a split-second ninja-switcheroo. Lost a bit of coolant, but not much, was very easy to clean up and barely needed to be topped up afterwards if at all.

Also OP's thread title really encapsulates how I feel when posting most of my own help threads lol :'[

[heinzboehmer]

When I did mine, I just did it very quickly. Unthreaded the current sensor until it was juuuust about to come off, had the fresh sensor ready in the other hand, and did a split-second ninja-switcheroo. Lost a bit of coolant, but not much, was very easy to clean up and barely needed to be topped up afterwards if at all.

Makes sense. Might need to do this soon. I've been experiencing a similar overcooling issue while on the highway. Coolant stays rock solid at 12 o clock when driving around town, but as soon as I get on the highway and get more air flowing through everything it drops to 11-10:30ish.

[herwawan]

So much knowledge, thanks, guys! I'll try to address it all.

About replacing all parts with OEM: as much as I see your point Obioban and those who support that - and although I do agree to a certain point - I’m not going to throw another grand in new parts before labor when I just replaced them already.

Plus, it’s not like Whaler just came out of the wood, and it seems like Nissen and Geba aren’t bad either (thanks for the insight, Tbonem3 & oldFanatic). To give a bit of context on the reason why none are BMW parts: the work for all of those parts was done in different indy shops. I usually have my one spot that I trust but sometimes, you just gotta go with what’s close.

I think the best lead that came up a few times here is the coolant temp sensor. I checked the couple last code reports (from the PPI I did before buying it and when I did the Inspection II shortly after), and both times, the DME had stored “69 Engine temperature, signal implausible (105)”. Both times, it said the error wasn’t present at the moment.

Seems like we’re onto something! As oldFanatic says, the only thing that gives me the impression the cooling isn’t top-notch is my gauge (via the reading from the BlueBus indeed Ted!).

I am really curious about the ability to change it without leaking much coolant, yeah.

When I did mine, I just did it very quickly. Unthreaded the current sensor until it was juuuust about to come off, had the fresh sensor ready in the other hand, and did a split-second ninja-switcheroo. Lost a bit of coolant, but not much, was very easy to clean up and barely needed to be topped up afterwards if at all.

Also OP's thread title really encapsulates how I feel when posting most of my own help threads lol :'[

Love the ninja approach to it! I need to look into that, I don't really visualize where it is.

To cover the other questions/suggestions:
Aux fan: yes, it works. I hear it a lot, and the sensor switch was replaced when changing the radiator
Clutch fan: I don’t hear it so much. I did check it though, it’s pretty stiff when cold and engaged when engine hot. It's just very quiet I guess (I’m used to the old M30 motor with its truck-style noisy clutch fan).
Bleeding: it’s been done properly every time and symptoms remain steady before/after each
Coolant: for the last one, I used the BMW mix which came with the radiator kit (antifreeze coolant)
Expansion tank: I did check the level, everything’s good and consistent here
Water pump: if the temp sensor doesn’t do the trick, it would probably be the first part I would look into…
About the AC: I almost never use the AC (it’s a vert so might as well use that!) but it’s not blowing cold much anymore anyway, gotta recharge it.

[old///MFanatic]

I’m not going to throw another grand in new parts before labor when I just replaced them already.
Plus, it’s not like Whaler just came out of the woods...
I think the best lead that came up a few times here is the coolant temp sensor. I checked the couple last code reports, and both times, the DME had stored “69 Engine temperature, signal implausible (105)”. Both times, it said the error wasn’t present at the moment.
Seems like we’re onto something! As oldFanatic says, the only thing that gives me the impression the cooling isn’t top-notch is my gauge (via the reading from the BlueBus indeed Ted!).
I am really curious about the ability to change it without leaking much coolant, yeah.

And apparently Wahler is thee OE manufacturer anyways.

Yep that sensor sounds like a good place to start, especially with those codes stored. CLEAN the connector for that sensor also with Electrical Contact Cleaner.

[jbfrancis3]

You are smart to not just throw parts at it; my opinion is this was questionable advice given this otherwise sterling group. There are two things you should do which have already been partially covered:
1) Remove the thermostat and test it. Pot of water on the stove with a cheap thermometer in it. Watch for it to open as water approaches 180F. Turn off heat and watch for it to close. In my mind, and call me wrong, but this is the only mechanical piece that can cause this type of fluctuation you are reading.
2) Sensor as others have chimed in. You could try testing on the stove with a multimeter but depending on the cost, may be best to throw a part at it in this case. I'd be curious if the DME code you read could be thrown by too high water temp [caused by improperly functioning t-stat]. In other words, just because you get a code, doesn't necessarily mean its an electrical component.