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2004 325i
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Discussion Starter #1
I’m about done with this car.

Basically, the car has been running really cool now, it hasn’t gone above 78c, unless I stop and then the temp rises to about 90, which is normal from my understanding. I’m pretty sure that if the thermostat failed, it would throw a code, but all I have is lean codes which i’m still working on. It almost sounds like somethings cooling it off too fast, but the fan clutch seems normal and the radiator fan is working as it should be. Could the thermostat still fail without throwing a code?
 

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1999 BMW 328i
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Mine does the same thing. Mine doesn't really go up at idle either. The needle never makes it passed the first white line.

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E30M3 Race F10 535 R1150Rt M Coupe
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I’m about done with this car.

Basically, the car has been running really cool now, it hasn’t gone above 78c, unless I stop and then the temp rises to about 90, which is normal from my understanding. I’m pretty sure that if the thermostat failed, it would throw a code, but all I have is lean codes which i’m still working on. It almost sounds like somethings cooling it off too fast, but the fan clutch seems normal and the radiator fan is working as it should be. Could the thermostat still fail without throwing a code?
Could the thermostat still fail without throwing a code?

It can, a lot depends on your driving cycles. A failing thermostat has lower priority for setting the Ck. Eng. light.
Mode $06 gives deeper insight.
 

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It almost sounds like somethings cooling it off too fast, but the fan clutch seems normal and the radiator fan is working as it should be. Could the thermostat still fail without throwing a code?
78C is very low.
1) how do you know the elec fan is working properly? Does it run at 78C trying to cool the cold coolant further, and how often the fan turned on? I should not run at this temperature I think.
I say the thermostat is questionable, unless the the bad fan ran all the time.
 

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Most likely causes are a bad thermostat (stuck open) or an air pocket in the system.
 

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2004 325i
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Discussion Starter #7
Okay, here's some more information. At startup, it goes up to 95c in about 15 minutes, and it stayed there for about 10 more minutes of driving, and then it fell down quick to 78-80c, and the stayed there for about 10 minutes before it went back up to 95, and then 10 minutes later it dropped down to 80c again right as I pulled into my driveway. Still, no codes, and mode $06 checks out as MrMCar told me to do so. Also, the aux fan doesn't kick in until 90c, which is normal, and the mech. fan is rotating at its normal speed and thankfully doesn't sound like a diesel engine. So my final verdict so far is that the thermostat is beginning to fail, and I have no idea how old it is, but the guy who had the car before me did the radiator, coolant tank, and maybe some hoses in 2017, so I'm assuming the thermostat was replaced along with the WP and other hoses.
 

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2004 330Ci 85k miles
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The M54 thermostat has two modes of operation; thermo-mechanical and electrical. It mainly operates as a typical mechanical thermostat, but under certain situations (higher temperature, higher load, etc.) DME electrically opens the thermostat wider for increased flow.

See the full write-up here.

Wiring diagram here.
 

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Air pocket at coolant temperature sensor in head.
I doubt this is the case because the sensor head and the threaded body is one piece of brass which is screwed to the head that was hot regardless of the air pocket. The majority of the coolant is at the same temperature as the head, and the air pocket cannot change the big thermal mass of the head to change the sensor reading.
 

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Okay, here's some more information. At startup, it goes up to 95c in about 15 minutes, and it stayed there for about 10 more minutes of driving, and then it fell down quick to 78-80c, and the stayed there for about 10 minutes before it went back up to 95, and then 10 minutes later it dropped down to 80c again right as I pulled into my driveway. Still, no codes, and mode $06 checks out as MrMCar told me to do so. Also, the aux fan doesn't kick in until 90c, which is normal, and the mech. fan is rotating at its normal speed and thankfully doesn't sound like a diesel engine. So my final verdict so far is that the thermostat is beginning to fail, and I have no idea how old it is, but the guy who had the car before me did the radiator, coolant tank, and maybe some hoses in 2017, so I'm assuming the thermostat was replaced along with the WP and other hoses.
The sudden big delta temperature like this indicates the DME turned on the Tstat electric heater based on how hard car was driving. What is your drive style?
 

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2004 325i
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Discussion Starter #12
The sudden big delta temperature like this indicates the DME turned on the Tstat electric heater based on how hard car was driving. What is your drive style?
Well, the first drop was when I was cruising at 70 on the highway, and the second drop when I was cruising on the main roads, and then the last drop was when I gunned it from a light just cause. But I've gunned it in the past and there wasn't any big drop or rise like this before.
 

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2004 330Ci 85k miles
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I doubt this is the case because the sensor head and the threaded body is one piece of brass which is screwed to the head that was hot regardless of the air pocket. The majority of the coolant is at the same temperature as the head, and the air pocket cannot change the big thermal mass of the head to change the sensor reading.
I agree, was just providing explanation behind @LeMansteve’s comment.
 

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So my final verdict so far is that the thermostat is beginning to fail
+1 for this. I had similar symptoms when my thermostat was stuck open. Oil temp needle would tend to get closer to the middle when I was stopped or in stop & go traffic. It would get colder while I was cruising.
I replaced the thermostat and now the needle sits dead center after warming up. 🤷‍♂️

edit: I also wasn't throwing codes related to this. Only lean codes, which I'm still working on, lol
 

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+1 for this. I had similar symptoms when my thermostat was stuck open.
But I would say you didn't have the temperature suddenly going up again then down then up as the OP's case. A normal wax motor Tstat should not behave like this, unless the heater was turned on by the DME.

OP, buy and replace the Tstat and see what happen. If not keep it as spare and I would worry about the DME going a little crazy doing this.
 

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2004 325i automagic
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pineapple_tree,

Pull the t-stat electrical connector and safely secure it away from the fan. This will eliminate the DME control and test whether the t-stat is mechanically functioning properly. If temp goes to 97/98C and stays there, then something is causing the DME to activate the tstat heater and over cool the car. Could be a bad temp sensor elsewhere feeding bad data to the dme causing it to react.

Btw, how old is the t-stat?
(nevermind, saw the info above)
 

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Most likely causes are a bad thermostat (stuck open) or an air pocket in the system.
If the thermostat was stuck open the engine would actually overheat at high rpm, because the coolant would be flowing trough the radiator too fast to be cooled down. So it would be like, too cold in low rpm, too hot in high rpm. But it can be faulty in a different way. Also the electrical issue with dme sounds very possible, either way a new thermostat could fix the issue
 

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If the engine temp drops when driving, your thermostat is bad. This is a common thing with these cars. Definitely not the most reliable thermostat design. Typical of bmws from the era. Don't overthink this. The thermostat is the problem 99% of the time. And it's easy to get to.

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E30M3 Race F10 535 R1150Rt M Coupe
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Many good thoughts here, some true others are true but caveats apply.

Air in the system? I very much doubt it. 99.9% any air = overheating possibilities.

Yes the fan works albeit when called for, however as the thermostat cycles if the coolant in the radiator is NOT hot enough the fan is not invoked.

Fully stuck open thermostat would result in the coolant being overall too cool. The primary job of the thermostat is not to keep the engine cool. Rather it's job is to see the engine stay within a targeted delta. When the coolant gets to the upper limit of the delta, it opens and exchanges hot coolant for cool then closes again. Only to start the process of seeing the engine come back to norms. In general the thermostat operation tries it's best to keep the coolant circulating in the 92°C-98°C range. The assumption is made that your cooling system is in good order and all the rest of the stuff of being able to exchange coolant when it's hot out of the engine and bring in cooler.
This is accomplished with no air in the system, a properly operating thermostat and water pump, along with a properly operating heat exchanger; the radiator.

A fully open thermostat could witness an overheating condition however some caveats apply.
The ambient temperature must be rather high. The engine load must be of fairly hard order (as in at the track) and higher RPM's (heat placed into the engine via friction AND long periods of WOT, producing a lot of heat via long combustion events) could witness the coolant reach dangerous levels.

In summation for some a paradigm shift is needed when thinking about the primary job of the thermostat:
It's job is to keep the engine hot and within a delta. Most of the other parts of the cooling system are there to deal with the now too hot coolant and reducing the temperature of the released coolant simultaneously as the thermostat utilizes the exchanged coolant to maintain it's delta.
 

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If the thermostat was stuck open the engine would actually overheat at high rpm, because the coolant would be flowing trough the radiator too fast to be cooled down.
It must flows at speed of light for this to be true!
No way Jose.
 
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