Potential reasons for increased fuel consumption?

wingman · 29 Aug 2019 at 11:26

Our 2014 F56 Mini runs 38mpg+ pretty much consistently and 42mpg+ on a run, but on the last couple of fill ups (doing the brim to brim test) it's dropped to 32mpg on A/B road trips, and 36mpg on a motorway run.

It's not that concerning but I'd like to know if there's any wearable items I could check/replace to make sure there's nothing else the matter with the car. After all, this was the main reason I started completing the brim test to begin with.

For what it's worth the car has been doing similar trips, carrying similar weight, using the same fuel, and driven in the same manner etc. If it was in the winter I could understand but the weather has been good, and even then, the winter mpg of this car doesn't change all that much.

mjt · 29 Aug 2019 at 11:30

Tyre pressures?

Conscript · 29 Aug 2019 at 11:36

Any unusual noises or behaviour from the brakes? Check for unusually hot discs after a drive, perhaps a caliper is getting sticky and causing drag.

jpaul · 29 Aug 2019 at 11:36

throwing in - warm temperatures lower oxygen density .. maybe turbo cars are less impacted though ... A/C climate system demands too

sovietspybob · 29 Aug 2019 at 12:10

Failing O2 sensors can cause this, sticky brakes, low tire pressures, your wheels knocked out of alignment.

Does the car drive as it used too? After a decent journey put your hand next to each brake and see if any are hot as that would be a nice easy solution.

DereksDontRun · 29 Aug 2019 at 12:12

jpaul said:
throwing in - warm temperatures lower oxygen density .. maybe turbo cars are less impacted though ... A/C climate system demands too

^ ditto, was just going to point out the recent warm days making the car less efficient

Terminal_Boy · 29 Aug 2019 at 12:12

When was the air filter last changed?

Injectors clogging?

wingman · 29 Aug 2019 at 12:17

Thanks for the suggestions all.

mjt said:
Tyre pressures?

No intentional change, but a good shout - I'll recheck them tonight.

Conscript said:
Any unusual noises or behaviour from the brakes? Check for unusually hot discs after a drive, perhaps a caliper is getting sticky and causing drag.

No unusual sounds, but I've noticed recently they can be a bit juddery on braking, not consistently.

jpaul said:
throwing in - warm temperatures lower oxygen density .. maybe turbo cars are less impacted though ... A/C climate system demands too

Definitely a great shout, we don't typically mess with the controls all that much, and the AC unit it on all year round anyway, but I guess the full AC button has been used a few more times than typical. As the weather starts to cool it'll be interesting to see the next fill up.

sovietspybob said:
Failing O2 sensors can cause this, sticky brakes, low tire pressures, your wheels knocked out of alignment.

Does the car drive as it used too? After a decent journey put your hand next to each brake and see if any are hot as that would be a nice easy solution.

Car had an alignment about a month back so I don't think it's that, but the rest I'll look out for. Had a pair of new tyres on the rear but they're the exact same make and model that were on there before - not sure if that can change anything.

Drive is still identical as far as I'm concerned, certainly nothing vastly different by any means, not that I've picked up on at least.

Edit: the missus has mentioned that the engine can feel like it's about to stall sometimes, especially when cold. Another clue perhaps?

wingman · 29 Aug 2019 at 12:19

Terminal_Boy said:
When was the air filter last changed?

Injectors clogging?

Probably at the last service, if they bothered to change it that is!

Terminal_Boy · 29 Aug 2019 at 12:22

Do you know anyone with a scan tool that you could borrow to see if the car’s throwing any error codes?

Mercenary Keyboard Warrior · 29 Aug 2019 at 12:31

jpaul said:
throwing in - warm temperatures lower oxygen density .. maybe turbo cars are less impacted though ... A/C climate system demands too

Oddly mine (AUDI TFSI) seems to go up during very hot weather. I wondered if its actually retarding slightly and dropping boost, doesnt feel any different but then I would probably not notice even 5% less in all reality.

Lashout_UK · 29 Aug 2019 at 12:36

Warm air is thinner, so drag is reduced – and engines get up to temperature quicker. All helps save a bit of fuel, in some instances.

alec · 29 Aug 2019 at 13:03

wingman said:
AC unit it on all year round anyway

Not that having AC on all the time doesn't mean it is active all the time. Just like a fridge, it is "on" all the time, but compressor only turns on occasionally as temperatures fall below threshold

wingman · 29 Aug 2019 at 13:09

alec said:
t having AC on all the time doesn't mean it is active all the time. Just like a fridge, it is "on" all the time, but compressor only turns on occasionally as temperatures fall below threshold

Good point!

JBeck · 29 Aug 2019 at 13:13

MAF or injector issue will both cause poor MPG.

Mercenary Keyboard Warrior · 29 Aug 2019 at 13:35

Lashout_UK said:
Warm air is thinner, so drag is reduced – and engines get up to temperature quicker. All helps save a bit of fuel, in some instances.

Just made me think of an example of something that raised fuel consumption considerably for a mate, a faulty engine thermostat, it was stuck open and the engine struggled to get up to temp properly in his mainly town driving.

Also one I have had although you would tend to know, a hole in front of exhaust, the drop in back pressure seemed to cause a decrease in power and hence was compensated with more right foot. (was an NA car I had at the time), I didnt test for increased HP up top due to lower back pressure as it sounded like it was going to fall off!