Radiator fan has mind of its own. Or is it warning me that something bad may happen?

[BMWBig6]

Without warning, the car sounded like a jet engine in the middle of the drive home. Popped the hood and the radiator fan was running full blast even though ambient temp is only 56 degrees F. No warning lights or messages on the dash about temps or coolant levels. Shut it off, waited a minute, restarted the engine, and radiator fan was off (and presumably only turns on at a threshold temp?). The archives suggest this could be the sign of a failing water pump and the radiator fan cycling on max to compensate. But many of those threads mention the fan being "stuck" at the max speed even when starting a cold engine, or cycling to high speed every few minutes, and my fan hasn't cycled on (or loud enough to hear) again the few times I restarted it, or even during a 10-minute drive around the block. I'm going to check the coolant level after it cools off, but wouldn't low coolant cause its own warning?

Was this a random fluke? Or a flaky coolant temp sensor or something else? Unfortunately I don't have an error code reader (not sure if one would have been stored). I just had the valve cover gasket replaced a week ago, but that could just be a coincidence. I'm the original owner, with 66,XXX miles.

[solidsleeper]

I'm not familiar with the 28i, but if the fans come on with more frequency I'm guessing it's the water pump. The horror stories of the bmw forums make it sound like the pump can fail at any given moment, so I hope you don't have any places to be or long road trips soon. If it's not gonna significantly inconvenience you to have the water pump fail while driving, I'd say keep driving and make sure to keep an eye on temps and fan noise. If you notice the fan coming on with more frequency, it's probably the water pump.

My 2013 35i (electric water pump) had a code on the scan tool for water pump not meeting water pump targeted speed at 55k miles. At first the scan tool code would only show up after a spirited acceleration. It seemed like I could keep the fans in check and no code by being easy on the throttle. But after a few weeks every drive resulted in fan's kicking on and a code on the scan tool. I tried the auto coolant bleed procedure hoping it was just air in the cooling system, but that didn't help. The dash lights never came on, but I replaced the water pump and all is normal now.

Low coolant has it's own warning on the dash. I never got the low coolant light or any message on the nav screen diagnostics for the water pump. My BMW nav screen just said "ALL SYSTEMS OK" ( from BMW )

For a 2015 with 66,xxx miles sounds premature. If it was an N55 that still sounds premature, but "common for the forums".

I'd recommend getting your own scan tool. An OBD2 bluetooth adapter and bimmerlink (iOS/android with a nice looking UI) or Protool (by bimmergeeks, android only, more functions, but an ugly UI)

[BMWBig6]

Thanks for your input. It sounds like I won't get too much warning before the pump finally goes, so I will make an appointment (at the same shop that replaced my leaking valve cover gasket 11 days ago) and plan to drive it in the meantime (while monitoring like you suggest--my car also says "All systems OK") and maybe the shop can find some error codes or diagnose it definitively later this week (but long term will find my own code reader).

If it is the water pump (and not something else like the coolant temp sensor), then I assume it's prudent to replace the thermostat too while you're in there?

[solidsleeper]

How long was your Valve Cover leaking? When I replaced my water pump, it was coated in oil and dirt from my VC leaking, I'm assuming the oil/dirt didn't help my water pump and possibly accelerated it's premature failure. However my thermostat was not coated in any oil from the VC and looked pretty clean. I opted to change the thermostat while I was down there (2013 so already 8 yrs old), but I'd be curious how long my original thermostat would have lasted.

Is yours an xdrive or sdrive? If xdrive, I'd say replace it at the same time as the water pump (assuming labor would be more vs. sdrive if you had to replace the tstat down the road). If sdrive I don't know what I'd recommend for your tstat since you have a 2015 so it's not that old. I'd say if the tstat looks clean, you could probably re-use the original.

For science, I'd say don't replace the tstat! (if sdrive)

But if the 140$ for BMW Genuine or $70 for the Borg Werner tstat isn't a big cost to you, you might as well change it. Borg Werner is the OE mfg so they are nearly identical. I actually purchased both because the packaging on the Borg Werner I received was all beat up. I ended up replacing with the Genuine BMW tstat. I don't know if this has any significance, but although they are made by the same company and look exactly the same, the Genuine BMW tstat weighed like 9 grams more than the Borg Werner. (452 g vs 443 g) More weight equals better quality? For the water pump, I went with VDO which is also OE, but cheaper than BMW Genuine.

[BMWBig6]

The valve cover was leaking for maybe 2 weeks, primarily on the rear (and dripping on the exhaust). Mine is an sDrive. If I do decide to replace the thermostat at the same time, your info comparing the OE and BW parts will come in handy, thanks!