So we normally see idle to lower engine revs temps of about 94-98°C. Driving at higher engine speeds, the DME sensing the increased load will force the thermostat opening a bit sooner. (Lower)

Keep in mind that the fan is NOT operated on engine temperature, rather it is dependent upon RADIATOR temperature.

 

I never go over 3000 revs, so the stat is opening at the latest possible time😊.

I've looked at the wiring diagrams and they are pretty basic - from what I can gather, the fan is triggered by a voltage governed by the resistance at the temp sensor in the bottom hose. If I unplug this bottom sensor and key to position II, would the fan trigger if I heat the sensor up?

 

Read this:

• Electric Fan Guide

 

There's a lot of misunderstanding about the thermostat, engine temperature, radiator temperature and the fans; be it only electric (manual transmission) or both mechanical & auxiliary fans. (automatic transmission)

The purpose of the coolant thermostat is to keep the engine hot, NOT cool it down. The engine consumes less fuel and produces less tailpipe pollutants when the engine is up to full operating temperature. Most normally this is somewhere in the 93-99°C range. The job of the thermostat is to maintain that delta.

Once it gets beyond that, if the thermostat is operating correctly it opens and an exchange of lower temperature coolant is sent from the radiator into the engine. At the same time the hotter coolant in the engine is sent out to the radiator (really a heat exchanger unit, so keep that in mind) for a period of time to cool down. At the same time the now lower temperature coolant that was in the radiator is staring to warm up, now that the thermostat has closed.

Most usually in the time it takes the engine coolant to reach the upper threshold of opening the thermostat for the next exchange to happen, the coolant that is inside the radiator (heat exchanger, remember?) has had a chance to cool sufficiently.

This constant exchange happens seamlessly as long as some basic parameters are met:
The thermostat operates at the desired lower & upper thresholds.
The water pump is properly circulating coolant.
The radiator doesn't have the fins all clogged up with dirt, bugs, road dirt, leaves and road trash?
The inside of the radiator tubes are clean and don't have a heavy scale built up on them?
The mechanical fan clutch is working properly?
The electric puller fan (primary on manuals and back up on automatics) works.
The cooling system is bereft of trapped air.

Lets say that you have a failed thermostat that is stuck shut. Not the usual failure; and it does NOT open. Your engine coolant temperatures will go very far beyond the normal opening delta of ~ 98-99°C. However the coolant inside the radiator is very cool. Your electric fan or the mechanical one is NOT going to save the engine.
This is because (lets leave the A/C part out of this for now) the primary driver of the electric fan is the sensor located in the lower radiator hose.
So in this example, the engine temperature is heading up and going over 100°C however the radiator is very cold. That electric fan will not come on.
Also if you have your fan/fan clutch installed (my strong recommendation) since the radiator is cool, so is the air passing through it. The bi-metallic strip will not lock up the fan clutch.

Only if the cooling system components I listed above are working correctly, will the fans & the radiator function correctly.

 

Lets say that you have a failed thermostat that is stuck shut. Not the usual failure; and it does NOT open. Your engine coolant temperatures will go very far beyond the normal opening delta of ~ 98-99°C. However the coolant inside the radiator is very cool. Your electric fan or the mechanical one is NOT going to save the engine.

I will make sure I check the radiator when I arrive at work tomorrow, just in case my new stat is locked shut.

 

I can only answer the question no 2...
My fan will spin when it reach 96 C~97 C and the temperature will stay around 95 C~96 C during idle with all accessories off.

 

3. Is there a way to check whether or not it's working in response to the temp sensor? For example, pulling the sensor out of the bottom hose and pointing a heat gun at it with the ignition on?
Bear in mind, I haven't got access to ISTA or INPA or any of that tech - just me and my spanners.

Thanks.

I've made this test years ago, for a different/weirdest behaviour on my Z3 but it worked.
I wired a temporary 2-wire cable from the radiator fan connector into the car,with a little lamp bulb XD
When you see the lamp on, park the car and see if the fan spins. Rude, but easy

 

Does the fan come on at 97 C, along with the stat? If not, when should I expect it to kick in?

Regardless the temp the dash hidden menu showing, the fan only turned on whenever the lower hose sensor reports 85C and above.

To test the elec fan, turn on the AC and the fan should spin. If not it is bad.

 

I have ordered the INPA cable; might get the PA Soft one as well, if I want to play about with the cluster😁.

In the meantime, I logged this on my way home from work (from the cluster temp sensor):

This looks right to my eyes, but I'd be happy to see a second opinion; I don't think I can expose the outlet temps with this scanner, but the warm-up time and levelling off seem reasonable.

I don't think the fan came on, by the way!

 

In the meantime, I logged this on my way home from work (from the cluster temp sensor):

The temp looks dangerously close to 100C. You didn't write if the test was done with engine idling or driving at what speed. For my E46 cars, they normally are at 93 to 96C.

 

Looks correct to me.
Keep in mind, that the DME will hold onto the warmer temperature (not heat up the heater probe in the thermostat) for a little longer on the first cycling of the thermostat from a cold start. In the above the 22°C is cold, when compared to the normal delta of 93-98°C of the engine operating temperature.

We see it all the time on test driving after a cooling system replacement. The first cycle of the thermostat, takes just a little longer that the subsequent openings. We have attributed such to getting the emissions in check sooner.

Gets a little scary on those M62 engines and the 105°C thermostats. Takes forever until it opens....

 

I'm not used to engines that run this warm, to be honest... I'm coming from Fords that like 90😀.

If I can get INPA going I will poke the fan to make sure it's working (might pull the outlet sensor out and tickle it with heat gun to see if the fan responds), and do a run home from work with the outlet temp sensor displayed; I don't think INPA can do graphs, which is a shame, but I've seen something called Scan Master on Jason (50skid)'s Youtube channel that can.

 

If I can get INPA going

This guide makes it easy.

• How to Install BMW Standard Tools

I don't think INPA can do graphs

Correct, but a 3rd party program called TestO can log data values that INPA only displays live, then you can import that log into Excel and graph as desired.

 

Managed to get INPA going - against every one of my expectations - and gave the fan a nudge at 25%. It came on, so we know that the 12 V supply and the PWM control work.

If I put the ignition on, pull the outlet temp sensor and hit it with a heat gun (up to at least 85, of course), will the fan switch on? Or does the engine have to be running?

 

For what it's worth, putting a resistor in parallel with the outlet temp sensor (or at its input to the DME) would trick the fan into coming on a few degrees earlier - might cover the "lag" between the sensors better. Let's say your outlet temp sensor reads 250 Ohms at 85 C, and 300 Ohms at 80 C. A resistor of

1/(1/250-1/300) = 1.5k Ohms

across the sensor would make the system think that 80 C is 85 C, and the fan would come on. I would get the specific values for my sensor and use a trimmer (at the DME input under the engine bay cover!) to get things more precise.

The only side-effect is that the fan would stay on for longer, because the system would also think it's running too hot at the normal fan switch-off temperature - it might have to get down to 65 C before the system thinks it's at 75 C, for example (and it would think that 20 C was 45 C, though I don't know if this would matter).

 

For what it's worth, putting a resistor in parallel with the outlet temp sensor (or at its input to the DME) would trick the fan into coming on a few degrees earlier

I think you don't need to do this but find out why the fan not running at 99C coolant temp.

 

Out of interested, does anybody know which program was used in the E46 Fan Guide to see/set the on/off temps and PWM curve? I can't find its name anywhere in the guide.

 

The freeware INPA listed on the forum.

 

Is there a way to check whether or not it's working in response to the temp sensor?

Pull the sensor connector (not the sensor) off the sensor and it should force the fan to run with engine running.

I've just changed my thermostat and bled the system per BMW instructions

What exactly the steps you used in the bleeding?

 

What exactly the steps you used in the bleeding?

Cabin temp highest, fans - all of them - lowest, pour coolant in until bubble-free coolant comes out of the bleed screw; seal up, remove coolant from the tank until it's between MIN and MAX.

 

Just found an error 125 in INPA - something to do with fan startup. No idea how old it is, so I have deleted all the errors there and I will see whether or not it comes back.

 

Necro, for anybody that might read - just watched an MS43 ECU tuning video that showed a target coolant temp of 102 C degrees at low rpm/airflow.

That would explain why mine went up to 99 C without the fan coming on, as I never go over 3,000!