Thermostat soft fail?

[Boris3]

With outside temps falling here I'm pretty sure I have all the symptoms of a thermostat soft fail and just wanted some confirmation.

Symptoms: car takes a while to heat up to center on the gauge, cruising at highway speeds causes temp gauge to move off center to the left (to what would be the 75C hash mark), interior heat is weak and gets cooler as the temp gauge moves to the left. On the plus side temp gauge never moves to right of center.

No CELs, fault codes showing or pending. Should there be?

PO says radiator, expansion tank, and water pump replaced after a minor driver's front meets rear end of other car incident earlier this year. But no paperwork. Radiator, expansion tank look new. From what I can see upper hose and thermostat housing have the bmw logo and 03 date code which would be original. No leaks or noises. Confusing as to why if WP was replaced you wouldn't replace the thermostat and other bits while in there but so be it.

[DeathTrap]

Sounds like a bad or dying t-stat to me.

[PetesZ]

+1

https://www.fcpeuro.com/products/bmw...CABEgJQQvD_BwE

[Boris3]

Much appreciated!

[Boris3]

Finished a full cooling system refresh plus belts and pulleys, parts courtesy of FCPEuro. Everything is working as it should. It turns out most everything in the cooling system was original from the 11/2003 build except the expansion tank that had a 2011 tag. Thanks to all the DIYs here and on e46fanatics, and a last minute reality check email to Dane, all good.

[Oli77]

Win! and piece of mind!

[bvmackin]

Finished a full cooling system refresh plus belts and pulleys, parts courtesy of FCPEuro. Everything is working as it should. It turns out most everything in the cooling system was original from the 11/2003 build except the expansion tank that had a 2011 tag. Thanks to all the DIYs here and on e46fanatics, and a last minute reality check email to Dane, all good.

I also recently replaced a soft-failed thermostat, and have experienced a subsequent decline in gas mileage. It may be coincidental but would be interested if you experience the same.

[Boris3]

Interesting since it should have the opposite effect.

Sorry I don't have much of a baseline for gas mileage comparison. Haven't driven it much since I bought it due to wanting to sort out the cooling system first and finding the time to do it. Just did the second fill up. Rough guess of 25 on the first tank mostly highway.

[bvmackin]

Interesting since it should have the opposite effect.

Sorry I don't have much of a baseline for gas mileage comparison. Haven't driven it much since I bought it due to wanting to sort out the cooling system first and finding the time to do it. Just did the second fill up. Rough guess of 25 on the first tank mostly highway.

Agreed, I also thought mileage would improve, but honestly the thermostat had been soft-failed for at least a year and a half so I wonder if the engine management software has somehow “learned” some bad behavior. Then again, the lower MPG could be a heavier right foot on my part since repair!

[John in VA]

Then again, the lower MPG could be a heavier right foot on my part since repair!

It could also be due to the winter fuel blend - I notice a drop every year at this time.