[NoTempoLimitN54]

I have a P3Cars gage on my E92 335i. have it set to show coolant temp in ºC because I find it faster to interpret (above 100º = above boiling). My N54 is 99% stock with just an AFE filter and air scoops. The only cooling "mod" is that I'm running 33% BMW coolant / 66% water + Motul Mocool. My engine has the stock oil cooler.

My car runs in the normal coolant temp range, from a low of about 95º C [203º F] (fast cursing on highway) to 107º C [225º F] (idling at lights, slow low-load driving, parking, etc.).

Today it in the San Fernando valley it got up to 103º F [39.5º C] and I was of course running the air conditioning. What got my attention is that my P3Cars gage showed coolant temp at 80 - 85º C while driving on the freeway. That's the lowest temps I've seen while the car was warmed up. Oil temps were around 230º F [110º C] rather than the usual 240-245º F. This is while driving at 50-60 mph in moderate traffic with an ambient temp above 100º F.

I've noticed that my coolant temp runs lower with the AC on. I know that the DME is speeding up the water pump. But I've never seen temps that low except on warm-up.

When I got back to my parking spot (underground garage), I turned the AC off to see how the coolant temp would react. It went down to 76º C. The electric cooling fan was running after I shut the AC off, and stopped when the engine was stopped.

What do you guys think? Anything to worry about or "normal vehicle operation"? Perhaps a thermostat stuck open?

[Rickygst]

check to see if you have any codes, the t stat could be going. If no codes just keep an eye on it over the next few days or weeks. how many miles on the car

[NoTempoLimitN54]

Originally Posted by Rickygst

check to see if you have any codes, the t stat could be going. If no codes just keep an eye on it over the next few days or weeks. how many miles on the car

No codes as far as a MIL light. I'll have to hook up my laptop to check for any hidden codes. 69.3k miles and running great. I've gone nearly 20k miles without any major service, just routine maintenance.

[JamesM3M5]

The ECU will regulate both the thermostat opening and water pump speed to maintain the proper temp. The two wires going to the thermostat are for a tiny heater that will heat up the wax plug and open the thermostat earlier than its advertised temperature - same on lots of different cars.

If you unlock the cluster you can also see the temps change under differing conditions. Mine will maintain about 87C when driving hard on track and 101C most other times. I haven't watched it too much, but I was just interested to see what it would do.

[jimmer411]

Doesnt running the AC turn on the fan? Where normally the fan only kicks on at a certain temperature? That could explain the lower coolant temps.

[NoTempoLimitN54]

Originally Posted by jimmer411

Doesnt running the AC turn on the fan? Where normally the fan only kicks on at a certain temperature? That could explain the lower coolant temps.

That's what I was thinking... but 80º C while driving 50+ mph in 100º F+ ambient?

Driving my car again today, I'll be keeping an eye on the temp for normal warm-up.

[ajsalida]

Some interesting reading here (see attachment), esp last section on heat management

[NoTempoLimitN54]

Originally Posted by ajsalida

Some interesting reading here (see attachment), esp last section on heat management

Thanks!

Update: Drove the car today and everything was as it normally is, except curiously I got low coolant warning. The coolant was a bit low, took about a pint (0.5 L) to fill it. Not enough to affect cooling, just enough to set off the level sensor.

I'm thinking that perhaps when the ambient temps are above a certain level, and the AC is on, the DME runs the water pump at full power, which brings coolant below the normal temp. This would provide a margin of safety in case something goes wrong. Normally this engine runs very hot (100 - 108º C) which does not leave much margin of error.

[ajsalida]

I stumbled across that doc while trying to find out at what water temp the fan comes on. Well turns out the answer isn't simple and depends on numerous factors, including what "mode" the ECU thinks you are driving in.

I had no idea what could be done with an electric water pump vs mechanical driven off a access. belt. Prior to this I was certain based on logging ECT on Cobb my pump was going out. But then that table in the back chapter told me it was dead on the thresholds for various stages of engine operation.

I still may have a pump going out but the temps that were scaring me are actually normal for the various operating conditions ECU is targeting.

[JamesM3M5]

When the water pump starts to go bad, you may not get an overheat or Check Engine light. My car would suddenly stop responding to the throttle for a few seconds at a time very sporadically. Took a few months afterwards to completely fail and throw the "Water pump speed deviation" code.

My engine runs at around 101C until you start beating on it or during other operation modes. The hotter the engine is, the more fuel efficient it is. Once you start flogging it, the fan and water pump speeds kick up and the ECU heats the thermostat to open it earlier than the standard 97C temperature.

[ajsalida]

Yeah that is exactly it. I was monitoring ECT's and they were shooting up fast in light load situations, then going way down under high load, counter-intuitive. ECU was trying to be fuel efficient.

Amazing what freeing up the pump from being driven by an accessory belt lets them do. In any case will continue to monitor but I hope pump is not going out. No codes and no episodes of exceeding temp thresholds.

[NoTempoLimitN54]

This is the reason they got rid of the coolant temp gage. Think about it: We know a lot about engines and the fluctuating temps still freak us out until we figure out what's going on.

I like having the oil temp gage, but I like the backup of having my P3Cars gage for coolant temp. After watching for awhile you'll have the ECU's temp regulation game figured out.

I can't believe some of these new BMW's don't have either.

[ajsalida]

You know that is a really good point. Never thought of it that way but you're probably right.

[NoTempoLimitN54]

Originally Posted by ajsalida

You know that is a really good point. Never thought of it that way but you're probably right.

Thanks... I still want to know the coolant temp. I feel better knowing even if I have to get used to an new scheme of operation. If my water pump fails I don't want to find out by getting the warning and having seconds to shut down. What if I'm in the left lane going 80?

The coolant temp yellow warning comes on at 120º C. With the P3Cars gage I'll know way before that. Anything above 104º C on the highway or 108º C in slow driving and I'm going to know something is wrong and act appropriately. Aluminum blocks and heads can get damaged very quickly by overheating.

[JamesM3M5]

Yes, well, the head is more susceptible to warping, but the all-aluminum or aluminum/magnesium alloy motors can handle some pretty high temps even under hard load. When the pump speed starts to deviate, the throttle stops responding until it comes back. If it's slow/dead for long enough, then you get the higher temps and the yellow then red overheat warnings. But I've had at least one customer drive their N52 X3 for a very long time with a dead pump to get it all the way home then to the shop. The motor has been perfectly fine since then with a new pump and thermostat. The engine shouldn't be run like that, of course, but they have failed water pump running modes to prevent melting the engine down. Overheating full of coolant is a much better condition than overheating because of a loss of coolant.

[NoTempoLimitN54]

It was over 100º F in the Valley yesterday, and needless to say I was running my AC. Once again my coolant temps were below 85º C, and this was in moderate to slow speed driving.

I have a working theory: At high ambient temps, the DME defaults to running the water pump 100% and cooling fan. Since I'm only running 33% coolant mix plus MoCool, once my cooling system is running full blast it cools the engine to below 90º C because the coolant is more effective than the normal 50% mix with no additive.

It should be noted that my car has an oil cooler and manual transmission (so it doesn't need to cool down the transmission by transferring heat to the engine).

What do you guys think?

UPDATE: Got up to 105º F yesterday , AC on and my coolant temps below 90º C again. Once I drove home and the temps had dropped below 100º F, DME let the coolant temp rise to about 95º C. Looks like normal vehicle operation.

[NoTempoLimitN54]

Update: 4 months after the previous post, my water pump started to fail. The pump and thermostat were replaced under CPO warranty. Both showed faults according to the dealer's computer.

So it's possible that what I describe in this thread was actually caused by a failing thermostat, not by the DME's logic.

[M-Steep]

My car - 09 e93 335i - behaves the way yours did; runs cooler under higher loads at higher ambient; I've noted that behavior for almost two years now. No sign of failing pump/tstat otherwise.

I even posted about it previously; same theory as yours, and consistent with BMW documentation on cooling modes for N54.

Be interesting to hear how your car performs temp-wise after new pump/tstat.

[NoTempoLimitN54]

Hopefully it will be a while before temps get up over 100ºF where I can test the theory.

The temp control definitely got a bit wacky when the pump was on the way out. I would also hear the electric fan running full blast. Finally, I noticed the DME cut power and got a water pump code. I took it to the dealer the same day. CPO was within 6 weeks of running out.

My P3Cars gage made it much easier for me to tell what was going on.

[BMWM.D.]

Just reading your first post in this thread made me think "thermostat". When the DME sees a potential issue with the t-stat (usually activation or jamming faults), it often cranks up the water pump and fan speeds.

With no issues present, the DME will try to maintain the temperature based on the type of driving you're doing. At low load, it wants to run warm to increase fuel economy. Under heavy load, it'll open the map thermostat and increase coolant/airflow to lower the coolant temp. But running cool when cruising definitely isn't normal. That's a sure sign something is off.

[shawntech]

with -10 temps here I make 168f in 7 miles :P

[M-Steep]

Originally Posted by BMWM.D.

Just reading your first post in this thread made me think "thermostat". When the DME sees a potential issue with the t-stat (usually activation or jamming faults), it often cranks up the water pump and fan speeds.

With no issues present, the DME will try to maintain the temperature based on the type of driving you're doing. At low load, it wants to run warm to increase fuel economy. Under heavy load, it'll open the map thermostat and increase coolant/airflow to lower the coolant temp. But running cool when cruising definitely isn't normal. That's a sure sign something is off.

I think an "issue present" we are speculating about is high ambient temp, e.g., above 100F. My example was bombing through the mountains of southern AZ, pulling grades at 80 mph+, in 120F temps (yes, 120. really). and my oil temps were sitting at about 230, which is at the low normal end in my experience with my car.