Overheat immediately after start up

[JustAWhisper]

New hot weather brings new overheating issues.

Today, my car started overheating immediately after start up. I was running errands and ambient temps were around 110 (yay Vegas), and each time I got in the car the temp needle spiked briefly right after start up. The first two times it came down immediately without the needle even going past the 2/3 mark. The third time I got stuck in a traffic jam trying to get out of the Walmart parking lot and the temp needle went all the way to 2/3 and stayed there orr longer than I was comfortable with so I immediately drove home. Fortunately it never went past 2/3 and it came down as soon as I got out of the traffic jam. The temperature rose briefly again at a long stoplight, but was fine when the car was in motion.

At home I checked the clutch fan (still resists the paper bag trick very strongly) and the electric fan sounds like it's going full bore. Water pump is brand new. Radiator and thermostat were replaced within the last few years.

I thinking my thermostat has taken a dump and is not opening all of the way. But it is an odd failure as normally they fail open and this one is only a few years old. No other water leaks that I can see. Everything in the cooling system except the expansion tank is less than 4 years old.

Thoughts?

[Lukem3]

Probably a coolant sensor.

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[bigjae46]

Might be a bad sensor or water pump.


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[COVID-19]

After start up it’s usually bc the water has been sitting in place and not moving. I had a similar problem when in traffic and it end up being the fan clutch even after doing the paper test.

[FBloggs]

Try bleeding the coolant circuit again if the water pump has just been changed.
Also see what codes are being thrown. A bad sensor should show up.

[sapote]

But it is an odd failure as normally they fail open and this one is only a few years old.

Grab the radiator lower hose to feel how hot it is at this condition. It should be painful to hold for 1 minute.

Tstat: if the wax leaked out of the motor then it failed to open; if the rod corroded (mineral deposited) then it failed to close.

[JustAWhisper]

Thanks for the responses. I don't think its the sensor but I'll see if any codes are popping up. That's interesting that replacing a fan clutch that wasn't fully failed fixed the problem for someone else, as the funny thing about extreme Vegas heat is it causes cooling problems to show up before they're really problems, so that might be a good place to start. My fan clutch is original, so it's probably worth trying that if I can't find anything else more obvious.

Thanks all.

[heinzboehmer]

Maybe bad o rings causing a loss in pressure? 100% just guessing, but I think I remember reading about someone having a similar problem and that being the fix.

What hoses did you buy? I've heard some say the rein hoses can have problematic o rings.

[jet_dogg]

Thanks for the responses. I don't think its the sensor but I'll see if any codes are popping up. That's interesting that replacing a fan clutch that wasn't fully failed fixed the problem for someone else, as the funny thing about extreme Vegas heat is it causes cooling problems to show up before they're really problems, so that might be a good place to start. My fan clutch is original, so it's probably worth trying that if I can't find anything else more obvious.

Thanks all.

These are the kinds of things which lead me to preemptively change out everything before issues have a chance to arise.

[sapote]

I'm not sure if a little "weak" clutch fan would cause the overheat issue given the AUX fan is working normally. At full speed the AUX fan alone has the capacity to remove the heat, but I am not suggesting to delete the clutch fan (this would shorten the AUX fan life).

[Arith2]

Sounds like you have some air in the system. It could be something else too however, another bleed costs no money. Start there. It'll give you a chance to see what the clutch fan is doing as well. 3 to 4 years is enough time for the thermostat to fail. It seems ridiculous but it's a BMW.

When you bleed the car. Rev it to 2000 rpm like 10 times. It creates waves essentially. This will help get out an bubbles.

[Casa de Mesa]

Agree with Arith2 - start with a bleed. It's free and it does sound like air pockets. If you turn the heater on, can you hear air in the heater core?

[Obioban]

What brand water pump?

[Slideways]

The cooling system will bleed itself. I would take a look at the temp sensor under the manifold as that is the sensor controlling the coolant gauge. It is also a lot less messy to replace that than a t-stat and costs less. I had an OE sensor that started giving wild temp readings after 20k of use and threw no codes.

[Casa de Mesa]

I know the S54 self bleeds, but if it's a brand new pump, and the heat has never been run (it's hot now), stranger things have happened. It *sounds* like air in the system, but could be a bad pump or sensor. Get the nose up on ramps if you can, run the heat and perform the bleed procedure. It's free and you can at least rule it out. Then move into the harder stuff