So I have a 2008 550i. I have an oil leak and the coolant needed to be topped off. Not sure where the leak is coming from.
I added maybe like 5 oz of the mix but I don't think that would be enough missing to do make the car trip out.

the whole car was acting super weird Saturday.
From time to time second gear wouldn't do shit
It would rev up but it didn't really push the car and the exhaust tone sounded suppressed. Like if it was clogged

I went to the store and when I got there I got the "engine hot - drive moderately to cool the engine" message as I was pulling into the parking lot and when I left, I got it again and the car felt like it had no power. It would feel really super sluggish and it would rev higher than usual to shift. I had to go into manual shift mode but even then it felt sluggish. After the message went away, it all went back to normal with the occasional 2nd gear feeling slow.

I drove the car all day Monday and I didn't get the message anymore but I did notice that when I would be idling at a stop light, the air from the AC would come out hot. I was able to make the cooling issue happen again on Monday night while it was connected to a scanner. I was monitoring temperatures and the fan would kick on at about 95 degrees Celsius and the temp wouldn't go higher than 97. After taking it out for a spin, I parked the car again and let it idle and the temp went all the way up to 120 and the fan never kicked on. That's when I got the engine hot message again and shut off the car.

Today, I turned the car on and checked all fuses and everything works but I don't know how to check relays and don't know what else to troubleshoot. I turned the AC on and off many times and all the times the fan kicked on and wouldn't heat up.

Any idea on what could be causing this?
My main thoughts are fan or ECU going out, both of which are expensive fixes that I'd like to avoid.

 

How many miles are on the car and how many have you put on it since ownership? Personally think you could be having poor coolant flow from a worn water pump or faulty thermostat? Sounds like the car is going into a soft limp mode to protect itself. Stop driving it or you'll regret it....take it to an Indy and check the cooling system. It's not too complicated on the V8s being a mechanical pump and not that expensive.

 

Miles on the car are 99800. Nearing that 100k mark. I've put about 17k miles since I bought it. I took it to a sort of Indy shop. (Not certified but only works on bmws and has the software necessary to check the car.) He scanned it and everything seemed to be fine, but he wasn't getting a reading from the thermostat temp sensor. We changed out the lower radiator hose temp sensor and also replaced the auxiliary water pump under the air box. After those changes, it seemed to have fixed it but the problem came back today. What I've noticed is that the car will only act up after about 45 minutes of driving. During short trips, the car will idle and it will cool and the fan will turn in normally. However there's that occasional time when the fan won't turn on. Would a faulty thermostat or water pump still cause this?

 

A faulty temp sensor could make that happen. Also make sure you battery is charged. If you have not changed it check it out

 

The temp sensor was changed out (the one on the lower radiator hose) and the battery hasn't been changed but I have never had any battery issues. Never had the car die on me or had to get it jump started or anything. Tbh, it looks pretty new.

 

I'm probably off base, but i still think there is a coolant flow issue. I think the water pump and/or the thermostat is intermittent. When I had a water pump failure about 4 months ago, my fan ran at full speed which signaled a problem. I didn't get any warning on the dash but I think that was because I didn't give the car a chance to overheat...I shut it down immediately. It's gotta be either the thermostat and/or the water pump. You can try to feel the temp of the hose running from one side of the thermostat and see if it is hot when the car is running at normal temp...if it's not scalding hot, it's a flow problem. I can't remember which direction is the flow...whichever direction the water pump pulley is turning when running is the direction of flow. I think the flow is from right of the thermostat to the left but someone correct me, might be opposite. Either way, you're looking for lower than normal hose temp on the output side of the tstat, which could indicate no flow. Just something to check.

 

I'll give them a test to make sure. I kind of did think it could be the thermostat. When the mechanic connected his computer with the BMW software, he ran tests on the fan and other stuff and it all worked fine. The thing we noticed off a bit was that he wasn't getting a reading on the thermostat temp sensor. It didn't read as a fault either though so we discarded it as a possibility. I'm gonna see about changing them anyways as a preventive measure even if they prove to not be the problem.

 

So you changed the water pump and not the tstat.

 

Main water pump was not changed. Auxiliary one was changed. That's the one by the air box on the side of the engine.
The one mounted to the engine has not been touched as it wasn't displaying symptoms of being bad. (Leaking or loud noises)

 

Hi, you have listed pretty much what I have going on. I have the 06 v8 and didn't think these had a secondary pump. Did changing out the aux pump solve your coolant loss and heading issue?

Is the pump under the air box.