Odd temperature issues

[Zchild]

In my new to me 99 E39 540i since replacing the thermostat, aux fan the car temperature gauge reaches the mid-point temperature setting for the first three miles in the morning commute to work. After the three miles every morning like clock-work the temperature gauge moves to half-way between the mid-point and the three-quarter point for a brief moment and then back to the mid-point. At about the thirty mile point in my commute it repeats this action moving from the mid-point to half-way between mid-point and three-quarter point for a few seconds and back to center. There doesn't appear to be any huge coolant loss or leaks that are apparent. It appears more electronic because it happens in the same exact location everyday like clock-work. Any and all suggests are welcome, I'm going to begin with a leak/pressure test. The only other thing I can think of is maybe I had the wrong thermostat installed... Thanks!

[Zchild]

A little more research revealed another layer to this issue; up to 08/98 the replacement was 11531437526 which was replaced by 11531436386 for all cars after 08/98. I suppose I need to figure out which one was used in the replacement...

[schatzy62]

In my new to me 99 E39 540i since replacing the thermostat, aux fan the car temperature gauge reaches the mid-point temperature setting for the first three miles in the morning commute to work. After the three miles every morning like clock-work the temperature gauge moves to half-way between the mid-point and the three-quarter point for a brief moment and then back to the mid-point. At about the thirty mile point in my commute it repeats this action moving from the mid-point to half-way between mid-point and three-quarter point for a few seconds and back to center. There doesn't appear to be any huge coolant loss or leaks that are apparent. It appears more electronic because it happens in the same exact location everyday like clock-work. Any and all suggests are welcome, I'm going to begin with a leak/pressure test. The only other thing I can think of is maybe I had the wrong thermostat installed... Thanks!

One thing I would do is take a different path to work and see if it is still at the same mileage points. Maybe there is something like extra slow/fast traffic in that specific location. It really should not jump like that for slow or fast traffic but maybe there is a points where the aux fan is not kicking in quickly enough so you see a quick spike.

[Mach540]

Look up "unlocking the cluster" or "OBC test" to view a digital read out of your coolant temp. It's test #7. That will verify if your engine temp is "actually " jumping around.

[6spd540]

Might be something as simple as not being bled well. When I had coolant leaks I'd get similar gauge behavior caused by air pockets (bleed and top off would correct).

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[Dave Sal]

Agree with Mach540. The temperature gauge is buffered, so you have no idea what the temps really are unless you unlock test 7 on the OBD.

[Zchild]

I've bled it and topped it off correctly and further did a bleed and top off this morning to verify the issue isn't being imagined. The vehicle exhibited the same behavior and didn't lose any coolant. Maybe unlocking "Test 7" might shed some light on the issue. Additionally I occasionally travel and alternate route to work and this issue hasn't been observed on the alternate route. Hence my befuddlement... lol

[edjack]

Bad dual temp sensor?

[Mach540]

Bad dual temp sensor?

Possibly this, too.

When you do get test #7 up, you can see both temps given by the dual temp sensor.
With the key in position #2 (engine running position) it gives one temp.
With the key in position #1 (ACC position) it gives the other temp.

I found this out recently when my temps were reading abnormally low. I just unplugged the sensor and replugged, and the problem went away. Both temps should be within 1 possibly 2 degrees C.

[Zchild]

It turned out to be a hair-line crack in the radiator which was causing all of the issues. Changed out the radiator and the temperature is happy once again, thanks for all of the input.