M62 Overheat Fluctuation

[bimmer7410]

Good evening

Today, I had the mispleasure of seeing my temp gauge start to move pass dead center. Here are the symptoms:

Its cold as hell outside, and when the car begins to overheat, the internal heat stops blowing warm
The gauge moves to 3/4th then back to the middle. Then it goes back to 3/4th or slightly above, just below the red indicator
The fluctuation continued even after making sure the coolant level is topped off
I notice a water leak from under the car, and small traces of water around the expansion tank top


I've got the car up on jack stands in prep for going to town. I havent been able to pinpoint where the water is coming from, or why a small leak would even cause these symptoms, but I'm hoping the fount of wise car guys that is this subforum will give me a starting point :)
I know I will need a cooling overhaul but this could not have come at a worse time. Got work tomorrow and will likely have to call in. Luckily I have plenty of vacation time..but parts also take plenty of time to come in

Thanks in advance

[purplecty]

Fix the leak and bleed the cooling system. You may get away with a good bleed and a top off to get you to work and back depending on how bad the leak is...

https://www.meeknet.co.uk/E38/E38_Bleeding_The_Cooling_System.htm

[MSporting]

The fact that you are seeing coolant around the top of the tank would have me start with the bleeder screw. The o-ring on the screw may have failed, or the screw itself may have a crack in it.

From there, check the hose feeding the tank at the bleeder screw position, seeing if the connector point is bad or if the connection is simply loose.

For quick troubleshooting to start the process of elimination, I recommend you start by replacing the o-ring on the bleeder screw, you can get one that fits from any hardware/auto parts store.


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[bimmer7410]

So, an update
I drained the coolant that was in the radiator to see its condition, then topped it up with the procedures found in TIS.
The car still runs hot after about 15-20 mins at the middle temperature position.

I watched the temperature climb to around 245 degF in INPA. I noticed that after turning off the car, the temp QUICKLY dropped down to the middle level (around 230F and lower), then began to drop more slowly as expected.
This leads me to wonder if I have a sensor issue.
I am still trying to determine the place that it is leaking, as I saw no actual leaks when I fired it up yesterday to test.
I need to go ahead and knock this out before the next workweek and its taking a bit longer than I expected. Guess it wont help things to rush though

[bimmer7410]

Also, im getting code 125 in my error code list. I havent been able to directly find the description, but it seems that it deals with the coolant temp sensor

[JimLev]

If your getting no heat from the cabin heater there is still air in the system.

[bimmer7410]

If your getting no heat from the cabin heater there is still air in the system.

Roger that.
What I plan on doing now is waiting until it cools down, and running the bleed procedure again.

This time around, i was only able to get it to start overheating when I turned on the AC, which would make sense if the AUX fan was dying.

Thanks again Jim and others, i'll report back with what I find.

[Chedley]

Also, im getting code 125 in my error code list. I havent been able to directly find the description, but it seems that it deals with the coolant temp sensor

You have a defective radiator temp sensor. I'd replace it. Here is the list of all error codes for your engine (DME 7.2).

[bimmer7410]

You have a defective radiator temp sensor. I'd replace it. Here is the list of all error codes for your engine (DME 7.2).

Appreciate that. Saved for future reference.
Concerning the sensor and its possible effect on the coolant temp, theres basically no chance of this sensor causing the car to overheat, correct?
Seems like the only single components which can do that is the water pump and the thermostat. The car can even stay cool with the AUX fan only, and that fan now runs pretty much as soon as the motor is started.

[Spirit Force]

When the temp gets below freezing (32F), the car doesn't need the fan or aux fan. I've run my 540 several winters with out the viscous fan and the temp never moves past 12 o'clock. Thus the aux fan never kicks on. If you don't get heat inside it can mean only two things, either there is air in the cooling system or the water pump is failing. The impeller can separate cause intermittent overheating. The aux water pump should ensure the heat in the cabin, but it could have failed, thus failing water pump could cause no heat in the cabin. The error 125 is due to the radiator temp sensor. That will cause the aux fan to run, but it won't cause overheating. There is a separate sensor on the water pump coolant temp.

[bimmer7410]

When the temp gets below freezing (32F), the car doesn't need the fan or aux fan. I've run my 540 several winters with out the viscous fan and the temp never moves past 12 o'clock. Thus the aux fan never kicks on. If you don't get heat inside it can mean only two things, either there is air in the cooling system or the water pump is failing. The impeller can separate cause intermittent overheating. The aux water pump should ensure the heat in the cabin, but it could have failed, thus failing water pump could cause no heat in the cabin. The error 125 is due to the radiator temp sensor. That will cause the aux fan to run, but it won't cause overheating. There is a separate sensor on the water pump coolant temp.
Fortunately for me, we southern Texans rarely see 30-anything on the thermostat
I've got heat inside at idle now, which makes me believe I have succeeded in removing all air bubbles
I'm really honing in on that impeller beginning to destroy itself. I'll take it apart this week to make a final determination.
Thanks for the reply