[eaclark]

Hi,

I have a 2012 Kia Pro Ceed 1.6 CRDI diesel (D4FB engine). The car is just shy of 100,000 miles and has been serviced regularly. This year I installed a new air filter and fuel filter when servicing.

I log all of my fuel and noticed a bit of a general decline in recent months, not massive but noticeable. Here is a chart of rolling average MPG of the last 5 fill-ups: chart

Additionally, I notice frequently on the instant MPG display that the car is getting lower MPG when on flat motorway using cruise control whilst previously this would always be greater than the limit of the display at ~60mpg.

The car otherwise runs okay, tyres are inflated appropriately, I did get 4 new tyres a year ago but not sure how much difference they could make. The tyres have the same C fuel rating. There is not excessive amounts of stuff in the boot etc.

The roads are not to different, generally reasonable runs on motorway or other large roads with plenty of cruising at 60-70mph.

Having had a read around I understand oxygen sensors going bad can be a common cause of reduced economy. I have a cheap OBD reader and downloaded some free software but looking for some advice on interpretation.

Here is a chart of the voltage and equivalency ratio of the O2 sensor in the exhaust manifold. Whilst cruising along on flat motorway the voltage tended to hover around 0.6V, which I understand is slightly rich?

Any advice on interpreting this data and other things to consider that may contribute to worsening economy?

[Lorian]

I'd only suggest two things

My D4FB loved millers diesel power ecomax. I'd run a bottle through a few tankfulls.

And jack it up and check none of the brakes are binding. (chock it as appropriate).

[CraigB]

Seasonal weather changes can affect economy a bit too, depends on how much extra you’re using.

Cooler weather will use more fuel, headwinds and wet roads don’t help either.

[Dazzler]

My Hybrid hates cold weather and wet roads. The cold makes the internal combustion engine run more to get it up to temp  but the wet roads probably has an even more adverse effect,  I assume due to increasing the rolling resistance of the tyres. ( making them work harder to rotate) 

[eaclark]

Thanks for your suggestions guys.

I've been slowly using Miller's, about 5 tanks worth so far but went for awhile not using it, double dosed a bit at the start. Didn't really notice any difference!

Regarding fuel economy and weather - can see that trend in my chart but definitely felt the economy was worse than previously even through this summer and the data seems to show that too so I think something else at play. I did get new tyres around a year ago so slightly suspicious that might contribute.

I've redone all my brakes over the last year including ensuring all the slide pins move freely and don't get any pull under hard braking so best as I can tell no imbalance to suggest binding.  I also rotate my tires regularly so have the car off the ground messing with the wheels and they all turn freely when reinstalling.

I've picked up some intake cleaner and waiting on a cheap borosocpe to look at the intake valves on suggestion from somewhere else. Also monitoring distance between DPF regens (so annoying there is not official indicator!) having read a saga on here about someone's i30 doing an active regen every 100 miles or so.

Bought a new lambda sensor as well, reasonable cost and found a few places suggesting 100,000m interval so figured it wouldn't hurt, pretty harsh environment it has to function in!

I'll keep you posted.

[AlanHo]

My hybrid - like Dazz's - also suffers from poor economy in cold weather.  In the summer it averages 64mpg and it is now hovering around 58 mpg and it isn't really cold yet. The other factor is we are currently only doing short journeys - covid restrictions ban us from our usual travels on days away and holidays.

[cocosamba]

Did the new o2 sensor improve your mpg?