01 BMW X5 temp. 1/4 when heater is active

[Boopunn]

Only when I turn the heater on does my temp gauge go down to quarter at idle .
When I am on the highway it is about 40%. Any other time of the year it is centered and perfect until I use the heater.

The complete cooling system was replaced 40k ago when rebuilt the 3.0 engine from bottom up. Radiator,hoses,sensors, thermostat and water pump. I also changed the heater core solenoids that open to let the water flow for the heater.

It had the older type thermostat that you can not remove the thermo heater and last year I replaced that one with the newer type that the heater will twist out of the Thermostat housing. Still no change.

I have backflushed the system to make sure there were no blockages in the heater core.

Could it possibly be the heater control board in the dash?

Does the temp signal go through the heater control board for the climate control before it gets to the dash?
Thanks for any advice.

[Zulu95]

Get an OBD II based temperature gauge or any decent scanner and monitor your engine temperatures directly. Ignore the idiot gauge in the dash. I have a Scangauge II I run all the time to do just that. I use the cover of the in-dash CD player to keep it in place.
If your vehicle runs cool I would suspect the thermostat and or the fan clutch.

[andrewwynn]

If the thermo indicator (not really a gauge it only has three feedback values) ever is not at "noon" there is something broken.

Almost always if the needle shows lower than noon ever, the thermostat is stuck open. Use the hidden menu test 7 I believe to watch the core temp and it should stay about 90-93c. If it doesn't your tstat is stuck open.

[oldskewel]

And if you want to get the gauge working like an actual gauge, it's really not that difficult. And compared to all the other work you did on the engine, etc., it's trivial. Good info is always helpful when solving a tough problem, so in this case, accurate knowledge of temperature may help you solve whatever is going on. OBDII will give it to you as well, but programming the gauge may be more convenient to always have it right there.

The problem and solution have been discussed in a few places, but when I decided to do it, I found that there are a lot of details left out, and things that vary between exact car models. My car is an '01 like yours, and it has the high cluster - so there is no warning light on the gauge, if it overheats, there would be a gong sound and a warning message on the display.

I did not want to brick anything, so I went carefully, and it all worked.

Details of what I did, including buying and installing the programming tool (eBay $14) on my Windows10 laptop (nothing ancient or special needed) are summarized in the following post, with the rest of that great thread providing more info.
https://xoutpost.com/1163366-post50.html

Full thread:
https://xoutpost.com/bmw-sav-forums/...hermostat.html

[Boopunn]

Thanks for the replies.

I have a maxisys 901bt tablet scanner that confirms the temp is running low when the heater is active. I have had 3 thermostats (1 old type and 2 new models) and I have had the problem with all 3, so i dont think it is the thermostat.
It may be the clutch fan possibly though. It is original. Easy enough to change.

[andrewwynn]

If the clutch fan locks it will longer to warm up but you'll still get to full temp but also, it will make a ton of noise and possibly send a fan blade through your hood.

If the tstat heater is coming on too much, that is a different problem you need to figure out why. Make sure all your temp sensors are reading within reasonable bounds.

Eg when cold they should all read the same. When hot, oil and exit should be similar, inlet should be a lot colder than outlet etc.

[cn90]

Depending on the brand, this may indicate tstat failure.