My average MPG is normally 54 as I do a pretty consistant commute on a motorway but it is now dropping to 44 but for no particular reason. 

It happens on different journeys and at different times I've had the car (54 plate Fabia 1.2 engine) for about 18monts and it's just started happening.

Any idea what it might be?  MAF sensor, injectors?

Thanks 👍

 

Maybe you using 95 ron E10 now rather than the 95 ron E5 that was available.

 

You could try a tank of E5 again but that means Super Unleaded.  Try Tesco Momentum 99 or Sainburry Super Unleaded (97 ron)

The Extra £2.50 it will cost will at least let you see if the E10 is the reason for the worse MPG.

The colder weather will have an impact; warm up times, electrical systems, as will any changes to the fuel mixtures (Winter Fuels/E10 Swith Over). 
There is a big thread on E10 fuel economy on here, and plenty more information in the news. 

On the presumption your driving style is the same, there is no additional weight in the car.. check the basic things like tyre pressures, servicing is up to date etc.

 

 

Edited 8 November, 20214 yr by MATT0693

You have a MAP sensor rather than a MAF, but I doubt it's that. Is the car the 6-valve 54bhp variant or 12-valve 64bhp?

 

Does it seem to run any differently, perhaps at idle?

 

It's always an idea to inspect the brake servo vacuum hose for splitting on VW-group cars, they can sometimes leak air and badly affect engine running. Here's a typical failed/failing one:

 

 

 

I didn't say but the MPG can go back to 54 at different times of the day and doesnt seem linked to engine or outside temp.

It always runs and sounds a bit lumpy as it's a 3 cylinder 1.2 12v and I always use E10 although it does sometimes lose powermoving off from stopped or low speed.

2 minutes ago, nevsummer said:

I always use E10

Well, there is at least part of your answer. E10 (10% alcohols) has a lower calorific value than E5, so you burn more of it to go unit distance.

5 minutes ago, nevsummer said:

It always runs and sounds a bit lumpy as it's a 3 cylinder 1.2 12v

It shouldn't; when working well they are quite smooth. How many miles has it done? 

 

5 minutes ago, nevsummer said:

it does sometimes lose powermoving off from stopped or low speed

Find someone local with VCDS to scan the car, or invest in the necessary cheap cable and free software to do so yourself.

 

Could be your EGR valve sticking sometimes, but not often enough to trigger a fault light. VCDS might spot logged codes that could shed some light on things.

 

Inspect the servo hose (visually and by fingertip feel) anyway, it costs nothing but a few minutes of your time. 

 

Edited 8 November, 20214 yr by Wino

@nevsummer

Do you mean you always use E10 since it arrived at UK filling stations in September or you were using it before then?

8 minutes ago, roottoot said:

@nevsummer

Do you mean you always use E10 since it arrived at UK filling stations in September or you were using it before then?

I've always used E10 since september, its just that the MPG is inconsistant on similar journeys throughout the day 

& thanks for all your help by the way 😉

33 minutes ago, KenONeill said:

Well, there is at least part of your answer. E10 (10% alcohols) has a lower calorific value than E5, so you burn more of it to go unit distance.

 

3% at worst, in this case that's 52mpg instead of 54mpg.

 

OP, If you're basing this entire thread on the MFA reading then it's a waste of time, you need to brim the tank and calculate average mpg that way, the MFA isn't accurate.

I do feel like that MPG figure is rather high for a 1.2 12v. As the most I've gotten is around 50mpg all in, being light on the right foot of course. However, experiences may vary and so on.

1 hour ago, AnnoyingPentium said:

I do feel like that MPG figure is rather high for a 1.2 12v. As the most I've gotten is around 50mpg all in, being light on the right foot of course. However, experiences may vary and so on.

I'm literally the slowest car on the A14 as I'm on limited funds and I do 20,000 a year in a 16yr old car, I did manage 60mpg (honest) when the there was no petrol in the pumps and I was running on fumes but this has by far been the most economical and loveliest car I've owned,130,000 miles £550, a bargin!

5 minutes ago, nevsummer said:

I'm literally the slowest car on the A14 as I'm on limited funds and I do 20,000 a year in a 16yr old car, I did manage 60mpg (honest) when the there was no petrol in the pumps and I was running on fumes but this has by far been the most economical and loveliest car I've owned,130,000 miles £550, a bargin!

Maybe not the slowest, there's always 1.9 64hp SDIs and 1.2 54hp 6v models for you to beat. These wee cars can be quite economical compared to some of the others I've had the pleasure of driving.

16 hours ago, Wino said:

It shouldn't; when working well they are quite smooth. How many miles has it done? 

 

Find someone local with VCDS to scan the car, or invest in the necessary cheap cable and free software to do so yourself.

 

Could be your EGR valve sticking sometimes, but not often enough to trigger a fault light. VCDS might spot logged codes that could shed some light on things.

 

Inspect the servo hose (visually and by fingertip feel) anyway, it costs nothing but a few minutes of your time. 

 

You mention the VCDS diagnostics does the 'VAG K+CAN Commander' do the same thing as I can get it on Ebay for £15

I should think so. 

The software is a free download from ross-tech, so I'm always a tad suspicious (maybe unfairly) of packages that include a CD/DVD, but the key thing is a cable that goes from OBDII port K-line data to USB using an FTDI chip, which that appears to.

 

 

Inspected the servo hose yet?

 

Edited 9 November, 20214 yr by Wino

7 minutes ago, Wino said:

so I'm always a tad suspicious (maybe unfairly) of packages that include a CD/DVD

Why particularly? This used to be the normal way of distributing software, due to the need to supply the user with 10s or even 100s of MB of compiled code in order for them to install $package on their machine.

Edited 9 November, 20214 yr by KenONeill

Because VCDS Lite is a free download, so it's unnecessary. CD may include malware.

I don't know what CAN commander is, maybe that isn't freeware, but you don't need that for scans on a mk1 anyhow.

 

I've got a diagnostic cable I don't use any longer which I can post to you FOC.

 

Used to use it on my old B5 Passat, but now have a newer cable needed for the Superb.

 

Happy to post it to you if you PM me your name & address.

<whoosh>

 

Quote

Because VCDS Lite is a free download

 

I've no idea what you mean by that Ken.

Aye, same here, Pete. I feel like I've missed something.

8 hours ago, PipH said:

I've got a diagnostic cable I don't use any longer which I can post to you FOC.

 

Used to use it on my old B5 Passat, but now have a newer cable needed for the Superb.

 

Happy to post it to you if you PM me your name & address.

Thanks so much Philip

On 08/11/2021 at 15:23, Wino said:

You have a MAP sensor rather than a MAF, but I doubt it's that. Is the car the 6-valve 54bhp variant or 12-valve 64bhp?

 

Does it seem to run any differently, perhaps at idle?

 

It's always an idea to inspect the brake servo vacuum hose for splitting on VW-group cars, they can sometimes leak air and badly affect engine running. Here's a typical failed/failing one:

 

 

 

I've checked the pipe & its not split but I've just put a diagnostics cable on (thanks PipH) and it reads

 

Cote P0170 Fuel Trim; Bank 1: Malfunction
 

Any suggestions?