vRS poor mpg thread

[dancingbear84]

Hi everyone and sorry for this, i am new to my car I've only had it for a month or so but I am very confused about this.

Before I start I am not a hard driver, i used to have a 106 1.5d which was slow but got 55+ mpg. I drive a route of mostly a roads with hills etc and drive in an economic manner gear coasting down hills and never hitting WOT I don't think.

My on board computer is showing a trip average of 60 odd per journey and a total trip of 58 which i reset when i filled up last time.

The trouble is I have only done 320 miles and the tank range is showing 90 miles giving me a full tank total of 400miles or 40 mpg which given my driving style is very poor.

I know the onboard can be way out and i don't expect it to me massively economical given the big sporty engine, which is fun to drive.

I cant figure out what could be causing the discrepancy though, I've read about dodgy MAF sensors and i will check the air filter doesn't need changing. I have an obd reader and torque for my phone is there anyway i can check the maf readings? Does anyone have any other ideas as to what it could be. On both occasions I've filled up when I've got to a quarter of a tank remaining according to the needle and i have put in about 32 litres each time.

I did read on here somewhere about filling up then pushing a button i. The filler cap, what would this do and where do i look for the button?

Thanks in advance, the car is from a dealer so has a couple of months warranty left so if it is a MAF or something else i can try to get it covered under the warranty.

[Blundey]

Ive owned mine now for coming on 3 years maybe longer and can easily see a HUGE difference in the winter vs summer. In the winter im lucky to get 400miles to the tank....in the summer i get at least 450-480....they prefer the hotter climates!

Also worth nothing....driving in 6th gear at 50mph will see 70+ in the summer...

For me, its just the cold...

[Avalon]

It could be the lower ambient temps, winter fuel additive package, tyre pressures, length of commute (the PD is quite thermally efficient and takes an age to warm up compared to a petrol) or a whole host of other things.

I would log fuel in/distance for a few tanks and average the results, it could be something as simple as a short fill on your latest tank due to an airlock.

[dancingbear84]

OK so you don't think i have anything to worry about?

That would seem fine to me except for the 18mpg difference between the onboard estimate and the actual figure. I can not understand that, i accept that it is an estimate but 18mpg is a massive discrepancy. The dealer i got it from are keen for me to diagnose the issue myself. I have come to the conclusion that there is a failing 18 relay as I also have issues with certain electrics after starting so they will be looking at that hopefully this week for me. I could do it myself but hey it has a warranty so i intend to make the most of it.

[faboka vrs]

Is it mapped or have a tuning box?

[dancingbear84]

Forgive my ignorance but how would I know or find out? Is there some way I can map the torque curve using an obd reader and the torque app or some free software. There is no paperwork to document a remap, but it doesn't mean that it hasn't had one.

[Brun747]

If you've done 320 miles and have put in 32 litres then your mpg is 45.7!

[Brun747]

The button is inside the neck of the fuel filler. It's a little knob thingy which you push with the fuel nozzle which vents the expansion tank which means you can get over 50 litres in the tank if you're empty enough

Edited 11 February, 2013 by Brun747

[rover220]

The button is inside the neck of the fuel filler. It's a little know thingy which you push with the fuel nozzle which vents the expansion tank which means you can get over 50 litres in the tank if you're empty enough

how does this help poor mpg.

to the op, you need to calculate the mpg properly, not guessing from miles done plus miles to empty divided by approx tank figure. its just not accurate this way

Edited 11 February, 2013 by rover220

[Brun747]

how does this help poor mpg.

to the op, you need to calculate the mpg properly, not guessing from miles done plus miles to empty divided by approx tank figure. its just not accurate this way

The OP asked specifically about the "button" and I explained - not once suggesting that it would or could improve mpg!

The OP also posted that he filled up with 32 litres having done 320 miles hence why I posted that hd attained 45.7mpg!

[Avalon]

If you calculate mpg using the conventional method suggested above (distance traveled against fuel put in) you get a reasonably accurate figure but to allow for fill variations eg if you accidentally do a vented fill you're putting anything up to 55ltrs in so will get an artificially low mpg figure, this is why I said average it over several tanks. Anything else just isn't accurate.

[dancingbear84]

Hi rover. I use fuelly to track my MPG, which is how I know that the actual figure is 40mpg, I am using the onboard DTE figure added to my actual mileage to give me an estimate before I fill up and check on fuelly.

I've been tracking my consumption on fuelly for 2 years. I was careful when picking a car that I chose something that would be 50+ MPG on a combined cycle which makes me a bit puzzled, given that I'm not driving it hard at all, that I'm just scraping into the 40s.

My dad has a friend with a vrs who is getting close to 60.

How would I know if the engine has been remapped by a previous owner?

I'll take a look into what I get from the torque app and my cheap elm obd reader in case the MAF is returning duff values.

My main confusion is the difference between the estimated MPG on the onboard and the actual figure being so wildly different.

[Brun747]

Not sure what a dte is and am not sure why you're addin things to other things to get an estimate and god only knows what fulley is!

Use the vent - brim your tank - drive around - brim your tank - do the sums = no fannying about!

Edited 11 February, 2013 by Brun747

[Ally_bassman]

When the weather is colder I really notice a difference also, about 30 miles less actual range before filling up

[dancingbear84]

DTE distance to empty. Therefore DTE plus current mileage /10 = estimated MPG whilst between fill ups, being it has a 10 gallon (ish) tank, this gives me an idea of how I'm doing in terms of real fuel economy.

Fuelly is a site where you add your odometer reading, litres of diesel added, price per litre and it calculates and logs your MPG.

I am in effect doing exactly what you're suggesting.

[AGENT 47]

I'm getting low 40s but only on short journeys if on a run 55 plus but it ain't driven slow when taken for a long run

[dancingbear84]

I do around 18 miles each way a day so not what i consider to be short. Do you guys have such a massive difference between your on board estimate and your actual mpg figures though?

[Avalon]

DTE distance to empty. Therefore DTE plus current mileage /10 = estimated MPG whilst between fill ups, being it has a 10 gallon (ish) tank, this gives me an idea of how I'm doing in terms of real fuel economy.

Fuelly is a site where you add your odometer reading, litres of diesel added, price per litre and it calculates and logs your MPG.

I am in effect doing exactly what you're suggesting.

Ignore DTE, you're using an estimated figure divided by an inaccurate figure (45ltrs is 9.89 gallons, not 10 and a vented fill is 55ltrs or 12 gallons) to produce something that's not even remotely accurate. How long is your average trip? 18 miles is a short trip in terms of a diesel in winter, just look at how long it takes your temp gauge to hit 90c.

[dancingbear84]

Firstly thanks everyone for the input, I appreciate it.

I think that there has been some confusion through the thread as to the actual problem, so I will repeat myself, hopefully more clearly.

I bought my car a month ago and filled up until the pump clicked off which was from memory 35 litres or so. I put my odometer and fuel details into Fuelly

http://www.fuelly.com/driver/dancingbear84/fabia-vrs then on my next fill up I repeated this process and Fuelly gave me a figure of 40.7 MPG. I did another 330 miles or so and filled up again and put the odometer and fuel details into Fuelly and got 40.6 MPG.

Given that Fuelly calculates on a brim to brim method I assume that it is (fairly) accurate. I accept that there are other factors that come into play that may make a difference such as the brim level may change slightly etc. But 40.6 seems to me close enough to accurate.

My on board computer thing in the dash however shows an MPG figure for (2) which was reset on every fill up of 58.x (I can't remember the decimal point I'm afraid.) What I can not figure out is how the on board shows such a MASSIVE discrepancy between the estimate and the actual MPG. I accept again that it is not going to be accurate and that there is a margin of error, but in my opinion 18.x MPG is not a margin of error it is almost 50% incorrect.

What confuses me even more is that plugging in my OBD reader and using my Torque app on the way into work this morning showed an MPG of 38 whilst the dash display showed an MPG of 61.

Ignore what I was saying about my mental calculations of (Distance driven + Distance to Empty /10 =MPG) that is a vague calculation that I use whilst I am in between fill ups to give me a more accurate figure than the onboard computer. Which by the way on my last tank would have been almost bang on. (335+80/10) or 41.5 MPG (mental guesswork figure) 40.6 MPG actual calculated figure.

My temp hits the 90 mark after around 3 or 4 miles.

I bought the first vRS I looked at, to be honest and one of the contributing factors (aside from the fun drive) was that after a few miles the MPG figures on the onboard were so good.

Basically if the figures are going to be around 40 MPG in winter I can accept that, but I want to understand why the onboard MPG estimate is so different from the actual figure I achieve. It has been suggested that a remap could be the cause, there was no paperwork to suggest a remap, is there any other way to tell short of a trip to a dyno?

Thanks again for the input everyone.

[VRSD30]

The vRS is generally a very economical car, obviously it depends on how it's driven but even when I'm stretching the legs on mine I very rarely see a drop below 47mpg, the overall average being around the 50mpg region.

Perhaps worth looking to see if the EGR needs cleaning or even replaced.

[Blundey]

As a side note, your on options 2 of your ODO reading, change to ODO 1 and look at the current MPG for that journey?

EGR may need cleaning too...as suggested....and tyre pressure can drop you a good 10mpg in the winter...

See how the spring and summer goes, i would be suprised if you dont start seeing the figures your expecting. With regards to innacurate readings....the cars is giving you the AVERAGE MPG on that trip.

[dancingbear84]

I reset ODO 2 on fill up so the average is a tank average. ODO 1 is generally showing the same or slightly higher per trip. This morning ODO 1 was 61.x ODO 2 was 60.x

I will take a look for a guide to cleaning the EGR later, thanks for that.

I accept it is an average and it is inaccurate, but VRSD30 has a fuelly figure of 50 MPG if his ODO readings are as inaccurate as mine are it would be displaying 70 MPG on the ODO. My Dad has an Octavia and finds his onboard ODO to be almost spot on when he compares it to a brim to brim. I can't explain the massive difference, which is what I am trying to figure out. Tyre pressures are checked every fortnight.

Like I said the onboard showed 60 odd for this trip, my ECU reader and Torque showed 37 for the same trip.

[993kimbo]

I can never work out how the trip computer works.

Two readouts. Are you meant to reset every time you go for a journey?

One seems to be moving on and off all the time and at the end of the journey the other always gives the same readout.

(38 mpg btw)

[Titanium_Man]

I can never work out how the trip computer works.

Two readouts. Are you meant to reset every time you go for a journey?

One seems to be moving on and off all the time and at the end of the journey the other always gives the same readout.

(38 mpg btw)

(1) is for the last journey and will reset itself once the car has not been driven for a few hours

(2) is for overall fuel economy and will only reset if you push the button on the stalk

I'm about to start to look into my fuel economy issues I have and started looking at tyre pressures and levels... all OK. Checked and replaced boot pipe connectors and next is to install my EGR delete to see if that has an impact, then it will be cheap parts I will change first, MAP sensor, Thermostat and then onto more expensive things like the MAF sensor.

[dancingbear84]

So what sort of mpg are you getting?

Does the onboard differ as wildly as mine seems to?