Poor fuel consumption X5 diesel

[andrewwynn]

Wife's F10 535 has shutters.

Today i did an experiment with my N55 E70;

Start and idle in a car port 10-15 min. before drive to town ≈ 20 min. mountain roads so about 400' up and 300' down on the trip there, opposite on return.

DME was requesting 109C from the engine but it never got over 82. It was about 5F=-15 c ambient.

Per usual i had the heat off until the engine hit 70c but that was within a mile since it warmed for 12-15 min. (only was ECT 42 i think).

So i covered 2/3 of the radiator with bubble wrap for the return trip. Engine temp was at 102c within 10 min.

(Normal to avg about 100 due to the hills vs. flat roads where it'll be 106-108)

In the case of N55, i don't believe the engine is proper running below maybe 85c so i encourage it to have the ability to get the temps at least close to operating temps.

If the newer cars know to shut off the airflow to the radiator, seems pretty common sense to do the same albeit manually with the identical motor. (Wife's F10, my E70 both have N55)

I wondered why her car warmed up so much faster even if the heater is on. It does run the coolant pump slower but i think that's because I have more internal volume and have digital climate back seat.

If i had a diesel i would do the same as i do with gas especially because they are so efficient they just don't heat up when ambient is low.

The normal coolant temperature range for the BMW M57 engine is typically between 80°C to 112°C (176°F to 234°F), with the thermostat generally opening around 105°C (221°F) during normal operation.

If the tstat opens 105 i would be wanting to see numbers between 90-100 by ten minutes into a drive even if ambient is -15c.

Some people like to keep there engines cold there's a whole train of thought that heat kills and keep it cold as possible. I'm ok with the DME temp management though like the oil being too low psi, i would be ok with the top temp being 105 v 109.


–awr–

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[BB_cuda]

Winter fuel has lower energy content. Common to see lower mileage in winter months. Coolant temp running lower can also contribute as well.

[josiahg52]

The M57 thermostat is nominally ~88–89 °C, and in normal ambient conditions the engine will typically stabilize in the high-80s to low-90s °C range.

In my case (−15 °C ambient), after ~25 minutes and ~9 miles of mixed driving, coolant is ~75 °C, then rises to ~80 °C shortly after sustained 55 mph. When ambient is near freezing, it readily reaches ~85 °C+.

More importantly, the overall thermal behavior is coherent:
- DPF regenerations occur normally and raise coolant into the low-90s °C regardless of ambient temperature
- Cabin heat is available quickly even in deep cold
- DPF delta-P scales normally with load
- Fuel consumption remains typical for conditions
- No under-temperature faults

That tells me the engine and aftertreatment temperature strategy is functioning as designed. Partially blocking the radiator might raise steady-state coolant temperature slightly, but on a modern emissions-controlled diesel it also alters charge-air, oil, EGT, fan, and regeneration behavior. It becomes a moving target depending on ambient temperature, vehicle speed, and whether a regeneration is active.
Since the system is already meeting its operating objectives, I don’t see a clear benefit in manually overriding airflow.