auxiliary fan automatic? starting temperature
 

At what temperature will the auxiliary fan on a 2001 X5 3.0i come on, if there is no mechanical fan, the replacement aftermarket electric puller fan is unplugged, and the HVAC is switched-off? I'm assuming that the DME has a specific temperature programmed. Or not?

I'm going to be using carb cleaner to test for vacuum leaks, and I'm wondering if the auxiliary fan can be trusted to keep the engine from overheating if I take too long a time underhood.

And I'll have some cardboard blocking the air blast from the aux fan, placed between radiator and engine, if the aux fan might come on too early, giving me added time to spray the hoses.

Meanwhile, I'll have a phone with Torque Pro app displaying coolant temperature, engine revs, and fuel trims in hand while spraying the lines, but am wondering how much leeway/safety margin I'll have if I get distracted.

I find it takes on the order of ten minutes at idle to get to where the cooling fan kicks in. If you put the cardboard behind the radiator you don't have to worry about disabling the fan, it will kick on to cool the engine if needed.

Pros use propane or butane to check for leaks use torque to monitor O₂ sensors which will go rich when they sense unburned fuel.

using what I've used before
 

Thanks for the info. But will the engine go past 220F degrees (my normal upper range temperature that I've always tried to maintain), or even get to my 240F shut-down temp that I never exceed, even in emergency situations?

I'm cautious about over-heating the X5, so I don't want to chance it, even for a moment. That's why I installed a 3000 cfm fan in it, as the first mod I'd do, before I drove it over 10 miles from home. I've set my electric fan to maintain 190F +/- on the X5, and that was true for the other T-stat settings on my other vehicles, if I modified them from factory settings. Race car & towing vehicles usually got 160F T-stats, and their electric fans came on at that temperature, just to be safe (mostly used in hot weather in Texas).

Does the fan automatically turn on when you turn on the AC?

Aux fan still works fine
 

Yes, it does the 10-second delay start-up, then spools down and restarts when the temperature rises. At least that's what I observed this past summer when I installed the electric main fan replacement. I haven't tested the Aux fan since then.

It seems to work as it should, because with the A/C on, the coolant temperature (that I monitor on a Torque Pro display) rises fairly slowly until the aftermarket fan kicks-in at around 175F.

My two small General Motors cars, with Ecotec 4-cylinder engines, have electric fans from the factory, that start up automatically when the A/C is switched on (or when the coolant reaches 217F, if the A/C isn't being used). I like the BMW system better, because the Aux fan gives you an indication if it's still functional, before it gets too hot (and if your/my main electric fan is not working).

Here is a post with some measurements about how the temp sensor in the lower radiator hose relates to the ECU turning on the aux fan.

https://xoutpost.com/1210848-post45.html

For some reason, I made it a little research project to separate fact from folk lore regarding turning the fan on (while debugging my fan that would not spin; eventually fixed when I replaced the fan), and started trying to measure everything rather than believing youtube or the intertubes.

In that post there, I measured (my car is a 2001 3.0i) what the DME (as reported by Foxwell) thought the lower radiator hose temp was as I swapped in different resistors (so the actual sensor was unplugged from the connector).

I did confirm that the 100 Ohm resistor simulated a 241*F temperature, which triggered the fan to come on. So from that, you can conclude that - no surprise - the fan will be turned on when the ECU thinks the lower radiator hose is at 241*F, regardless of other factors.

With a failed fan, I could tell it was turned on by measuring the signal the ECU sends to the fan - it varies the amplitude of a ~1Hz square wave.

In my case, I was just looking to confirm that it would trigger, and get an idea of the resistance. I was not looking to find the exact trigger point.

So I'll suggest that it's a pretty easy experiment to do - either with a potentiometer (in the range of 100-200 Ohms should do it) or a bunch of individual resistors - set whatever operating conditions you want (AC on/off, engine idling, whatever) and see what triggers the fan. Once you know that resistance, if you don't have a Foxwell, just let me know, and I'll measure it on mine for you.

You may find a variable speed output as the temperature ramps up. May be tough to tell exactly by looking at the fan, but since I was looking at the control signal, I was able to see this vary precisely.

I have a few other posts on this site with details about the fan testing I did. My main goal was to confirm the fan was actually bad before replacing it, and then due to all the inaccurate folklore, I became curious about how things actually worked.

EDIT - also, re-reading your first post here as well as the following two posts below this one, some basics ...

The thing I looked at here was how the ECU triggers the AUX fan based on the lower radiator hose temp sensor. That sensor is not the one used for the instrument panel gauge, and possibly other engine cooling control functions. For example, I believe the cylinder head temp sensor is on the left side of the head, between cylinders 4+5, and is used by the ECU to control the engine cooling thermostat (sending current to open the thermostat wider than it would by coolant temp alone).

On @AndrewWynn's following post about rigging something up to adjust the setpoint, if you can confirm what temp the fan turns on at, and what the resistance of the sensor is at that point, and what the resistance would be at the temp you want the fan to turn on at, you could permanently add a new resistor in parallel with the sensor - that would make the ECU think the lower radiator hose is hotter than it really is, turning on the aux fan at a lower actual temperature. But if the coolant thermostat is still doing its thing, this might not be a great way to manage engine temp.

If you just want to turn the aux fan on during vacuum leak debugging, there are easier ways. Off the top of my head, if you turn on AC and unplug the AC line pressure sensor, that will do it within seconds. I'm sure I've posted details on this somewhere here.

You should be able to install a voltage bridge on the output sensor to tweak that set point so it's just ratio'd and you can have the fan come on at a lower temp.

Fans don't really get in the way of a carb cleaner leak hunt. I suggest that you're overthinking this.

Also, once you pull the manifold, you need to replace *everything*. Even if it's not leaking now, there's a good chance you'll make new leaks in the fragile hoses/gaskets that are there. Precision isn't really important. All you need to do is confirm that there is a vacuum leak somewhere in there.