# Resetting ECU after switching to high octane fuel?



## peter-ss (Sep 3, 2008)

I have decided to give higher octane fuel a go and have now filled up twice with Tesco's Momentum petrol.

After a few hundred miles of driving the car, in the same manner as usual, I am getting exactly the same calculated and displayed fuel economy of 26mpg.

I understand that the car has to adapt to the higher octane rating, which may take some time, as the ECU will have made adjustments to cope with the lower octane rating of fuel that I've used in the past.

My question is this: Does anyone know how to reset the ECU so that it can relearn its settings again? I've read that if you clear the ECU DTC's with Vag-Com, even though there aren't any, it may do this but haven't found anything conclusive.

Has anyone else done this or know of another way other than disconnecting the battery.


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## TT-driver (Sep 14, 2010)

Interesting question. I'd think that the ECU constantly tries to optimise the engine. And as it has a knock sensor it should be able to adjust timing just so constantly. Perhaps adjusting can be triggered faster by using the engine in more various ways than you might do otherwise. So have the engine explore the map so to speak (throttle position, engine load and RPM).

But does the v6 benefit from high octane or was it designed for 95 only?


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## ChadW (May 2, 2003)

According to the manual and sticker inside the fuel cap the V6 prefers to eat 98 RON but can also run on 95 RON fuels.

My question then is if I fill up with Total Excellium which is 97 RON then would the engine just stick to 95 RON timings or would it go to 97?

Would it even think it has 98 RON Fuel in and run at the level anyhow?


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## Hoggy (May 8, 2002)

Hi, Advancing the Ign, which the ECU will have to do to get peak performance, when changing to higher octane, takes longer than retarding the Ign, because the knock sensor "pinking" will do it alot quicker, almost instantly. Pulling the ECU fuse or disconnecting the battery for 15 minutes will speed it up, but I don't know the ECU fuse on the Mk2, someone will.
Why don't you want to disconnect battery, Peter? 
Hoggy.


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## TT-driver (Sep 14, 2010)

Don't you loose any long term parameter adjustments by making the ECU powerless? I could imagine the computer takes into account pollution on the MAF sensor and the like.


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## TT-driver (Sep 14, 2010)

ChadW said:


> According to the manual and sticker inside the fuel cap the V6 prefers to eat 98 RON but can also run on 95 RON fuels.
> 
> My question then is if I fill up with Total Excellium which is 97 RON then would the engine just stick to 95 RON timings or would it go to 97?
> 
> Would it even think it has 98 RON Fuel in and run at the level anyhow?


Again an interesting one. My experience so far on the 2.0 seems to be that the car runs slightly better on RON 97 (v-power) but that could also be due to additional cleaning additives. Then because of traffic situations I ended up using RON 95 Shell petrol a couple of tanks. No real difference in behaviour or fuel consumption. Now I'm running my third tank on BP Ultimate, RON 98 and the car seems to run better and more economical. So RON 97 may not be mapped at all. Further investigation is required. How many maps can the ECU hold?

And I discovered a RON 100 fuel pump 40 km from home. Need to test that one too.

Once I have my scangauge installed I can monitor ignition timing while driving. That should reveal some info.


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## MarcF-TT (Jun 14, 2011)

Sorry I can't offer any help but I'd really like to see if using 99 RON increases the MPG from 26mpg (which is very similar to what I'm getting on 95 RON fuel).


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## peter-ss (Sep 3, 2008)

Hoggy said:


> Why don't you want to disconnect battery, Peter?
> Hoggy.


There's no particular reason to be honest, I just wondered if there was another way.

I've just nipped out and reset the ECU DTC's, as I'd read about, but get the feeling that it's not done anything.

If there's no change I'll disconnect the battery; last time I did that on the Mk2 I got about fifteen different warning lights on the dash!


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## TT-driver (Sep 14, 2010)

peter-ss said:


> last time I did that on the Mk2 I got about fifteen different warning lights on the dash!


Was the low octane warning one of them?


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## martinbanshee (May 24, 2010)

I always use v-power because the car's designed for it & the car runs fine. 
On one occasion I needed fuel and the best I could get was standard BP fuel so I tried a tank of that to see the difference. 
There is a small but noticeable difference between them. 
The wife also complained that the car wasn't as smooth to drive and that it didn't seem to pull away as cleanly so the car definitely prefers higher octane fuel.


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## peter-ss (Sep 3, 2008)

Well, after a few tanks full there's still no difference in mpg.

I have disconnected the battery for twenty minutes today and filled up again so we'll see what happens.

I'm not sure if the ECU is fully reset as the mpg info and clock cleared but the date and trip distance were remembered. The car did feel a bit different at first though so I'm assuming that it had cleared any adaptations.


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## LEO-RS (Apr 13, 2009)

peter-ss said:


> Well, after a few tanks full there's still no difference in mpg.
> 
> I have disconnected the battery for twenty minutes today and filled up again so we'll see what happens.
> 
> I'm not sure if the ECU is fully reset as the mpg info and clock cleared but the date and trip distance were remembered. The car did feel a bit different at first though so I'm assuming that it had cleared any adaptations.


Peter, bare in mind that winter mpg is less than summer mpg so comparing figures in November to figures in July is pointless. You'll use 10% more fuel this time of year (Although November has been pretty mild so far)


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## talk-torque (Apr 30, 2008)

As I understand it, the ECU is constantly adjusting to allow for driving style, ambient conditions and fuel spec. Resetting it will just return it to factory default settings, from which it will have to recover to current parameters. If you just give it a while, it will adjust to the better fuel. Not promising any improvement, just suggesting you allow the system to do it's best. 

I managed to convince myself that Shell Optimax or Tesco Momentum gave better performance/economy, and never use anything else.


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## Gaf (Oct 26, 2011)

I've also made the switch to Tesco 99, after quarter of a tank I can feel the difference, but still waiting for the better fuel economy!

May be a while to wait now as the TT's been toed away to the garage! Grrr.


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## Toshiba (Jul 8, 2004)

Damn, that picture is scarey!


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## TootRS (Apr 21, 2009)

Why should there be better fuel economy from higher octane fuel?

The higher octane allows timing to be advanced in order to increase power AFAIK.


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## Gaf (Oct 26, 2011)

Oh Damm.

You mean the advertising for Tesco has got to me?

I must be feeling ill. Next i'll be believing that a further increase on fuel duty will help the world by cutting our Co2 emissions, as everyone will all of a sudden start using public transport!


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## talk-torque (Apr 30, 2008)

Performance/Economy it's your choice! :twisted:


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## TortToise (Aug 22, 2009)

Can't say I noticed any difference between 98RON and 95RON when I decided to save some cash by going to the cheaper stuff (I initially religiously put 'Super Unledaded' in).

That's on a standard 2.0TFSi (197PS). Mapped cars, TTSes or RSes might be different though.


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## TootRS (Apr 21, 2009)

TortToise said:


> Can't say I noticed any difference between 98RON and 95RON when I decided to save some cash by going to the cheaper stuff (I initially religiously put 'Super Unledaded' in).
> 
> That's on a standard 2.0TFSi (197PS). Mapped cars, TTSes or RSes might be different though.


Main difference is reduction in power as the ECU pulls back timing to compensate for lower octane fuel. On a mapped car it is particularly important to only run on higher octane fuel if the map has been set on that basis otherwise you run the risk of pinking.


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## peter-ss (Sep 3, 2008)

Mitchy said:


> Peter, bare in mind that winter mpg is less than summer mpg so comparing figures in November to figures in July is pointless. You'll use 10% more fuel this time of year (Although November has been pretty mild so far)


I thought that engines were more efficient in the winter due to colder charge air?


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## peter-ss (Sep 3, 2008)

toot3954 said:


> Why should there be better fuel economy from higher octane fuel?
> 
> The higher octane allows timing to be advanced in order to increase power AFAIK.


The way I understand it the engine's supposed to run better (with more power) on higher octane fuel so I thought that if driven the same then the mpg would be better.

Tesco certainly advertise massive increases in mpg using Momentum.


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## CWJ (Aug 24, 2010)

peter-ss said:


> toot3954 said:
> 
> 
> > Why should there be better fuel economy from higher octane fuel?
> ...


As has been mentioned, the higher octane means the timing can be advanced without detonation resulting in more power under full load. At lesser throttle settings there will be little difference. I can't really understand how this would translate into better MPG.


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## powerplay (Feb 8, 2008)

CWJ said:


> peter-ss said:
> 
> 
> > toot3954 said:
> ...


Well to me this seems perfectly logical.

You might think about it like so:

Given you drive a set route from A to B in an economical driving style, accelerating gently, changing up quickly, anticipating, engine braking, keeping engine revs low etc etc, you could measure the amount of petrol used and devise a figure for miles-per-gallon.

If you then refuelled and repeated the same journey under exactly the same conditions but this time drove aggressively, accelerating hard, higher revs etc, basically demanding more power from the engine, you could make the same measurement and deduce your miles per gallon to be less. In other words, to output more power in a given time, given all other variables remain the same, you need to use more fuel.

So far a no brainer, presumably you agree.

You could reduce this down further to work out the exact amount of power p yielded by unit amount of fuel f.

Now change one of the variables - the octane of the fuel. Everyone is largely in agreement it is possible to produce more power if the engine can take advantage of this change.

Given you then make the same journey a 3rd time with the variables changed but require exactly the same amount of power from the enfgine, it is mathematically correct that the engine will require less fuel to do so, and this will be directly proportional to the amount of extra power produced by the increase in octane rating.

So basically if amount of fuel f now yields power+1 then power+0 has to be f-1.


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## TT-driver (Sep 14, 2010)

peter-ss said:


> Mitchy said:
> 
> 
> > Peter, bare in mind that winter mpg is less than summer mpg so comparing figures in November to figures in July is pointless. You'll use 10% more fuel this time of year (Although November has been pretty mild so far)
> ...


Wintertime means colder starts, resulting in extra fuel to be burned before the engine reaches operating temperature. Plus you need your lights, heaters, wipers etc. All require additional fuel.

Last but not least, in the Netherlands, there is a substantial difference between summer and winter fuel. Winter fuel contains lighter fractions of crude oil for easier evaporation. (so that engines still start when it's really cold) From September till April, the fuel is crap.

To my experience engines run best on a cool sightly damp summer evening.

I came across another suggestion on octane just an hour ago. Over here, fuel contains up to 5% ethanol. Apparently BP ultimate RON 98 contains no ethanol. (again in the Netherlands). According to the www Audi engines can handle up to 10% ethanol (E10 fuel, Germany). Now the suggestion is to add up to 10% E85 biofuel (85% ethanol, 15% petrol). That would bring the RON number up to 100. Then the engine does not adjust it's timing any more as a result of pinging. 
Perhaps not good for MPG, but indeed good for performance.

In VAGCOM, group 20 shows the amount of adjustment the engine does to compensate for less than optimal RON. (I have no experience with the VAGCOM though)


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## CWJ (Aug 24, 2010)

powerplay said:


> Well to me this seems perfectly logical.
> 
> You might think about it like so:
> 
> ...


The problem with your argument is that an engine only makes more power on higher octane fuel under full throttle conditions because as mentioned earlier the ECU doesn't have to pull back the timing but at lesser throttle settings this is not the case. Octane ratings make no difference to the preset air/fuel ratio (AFR) determined by the ECU unless your car is continuously pinking.

I have used both 95 and 99 in my car in similar conditions and have seen no change in my MPG even after 10 tanks of each. My major MPG gain came from a custom remap whereby I went from 32 to 34MPG.


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## peter-ss (Sep 3, 2008)

If that's the case then i might as well use the cheaper fuel.

My is very rarely driven quickly; only the other night I pulled over to let a fast moving Vauxhall Corsa pass. 

I will save my 5p/litre.


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## powerplay (Feb 8, 2008)

CWJ said:


> The problem with your argument is that an engine only makes more power on higher octane fuel under full throttle conditions because as mentioned earlier the ECU doesn't have to pull back the timing but at lesser throttle settings this is not the case. Octane ratings make no difference to the preset air/fuel ratio (AFR) determined by the ECU unless your car is continuously pinking.
> 
> I have used both 95 and 99 in my car in similar conditions and have seen no change in my MPG even after 10 tanks of each. My major MPG gain came from a custom remap whereby I went from 32 to 34MPG.


Okay granted it's not that clear cut or simple and you probably do have to be pushing full throttle to see the benefit.

I have used both ordinary 95 and Tesco 99 and have most definitely noticed better economy on the 99, given the same driving style. And not just a slight increase, a lot - maybe 30 miles extra from a tank.


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## CWJ (Aug 24, 2010)

peter-ss said:


> If that's the case then i might as well use the cheaper fuel.
> 
> My is very rarely driven quickly; only the other night I pulled over to let a fast moving Vauxhall Corsa pass.
> 
> I will save my 5p/litre.


Personally I think it is only worthwhile using higher octane fuel in turbo cars and more so in remapped cars using high boost applications running the risk of detonation. In a normally aspirated car not being driven hard such as yours you are probably going to see minimal to no benefit in terms of power and definitely no improvement in MPG.


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## CWJ (Aug 24, 2010)

powerplay said:


> CWJ said:
> 
> 
> > The problem with your argument is that an engine only makes more power on higher octane fuel under full throttle conditions because as mentioned earlier the ECU doesn't have to pull back the timing but at lesser throttle settings this is not the case. Octane ratings make no difference to the preset air/fuel ratio (AFR) determined by the ECU unless your car is continuously pinking.
> ...


Mate read around the topic on the net. It is a myth that higher octane fuel in itself (ie without other additives) makes any difference to MPGs. 99 octane simply needs more energy to ignite it making it an ideal fuel in high compression settings but yields the same amount of energy per part as 95 octane.


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## Hoggy (May 8, 2002)

Hi, If an engine is designed to run using 98 Ron fuel, especially a turbo engine, then to use 95 Ron, the Ign will have to be retarded to prevent knock "pinking" & then there will be a loss of performance, so get the same amount of power from that engine using 95 Ron fuel, the engine will have to be worked harder so will use more fuel. 
The reason why using 98 Ron makes the engine more efficient & can give more miles per gallon.
Audi state this engine is designed to use 98 Ron fuel, 95 can be used but there will be a loss of performance.
Hoggy.


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## TT-driver (Sep 14, 2010)

CWJ said:


> Mate read around the topic on the net. It is a myth that higher octane fuel in itself (ie without other additives) makes any difference to MPGs. 99 octane simply needs more energy to ignite it making it an ideal fuel in high compression settings but yields the same amount of energy per part as 95 octane.


Well as the chemical properties are lightly different between the various RON fuels, the amount of energy per litre is slightly different too. But not enough to explain any difference in MPG.

It might be though, that under given circumstances the engine can run in a kind of lean burn mode that requires advancing the timing of ignition in order to have the fuel mixture burning at the right time. This mode is more fuel efficient. With RON 98 it is safer to advance the timing, hence the engine management might switch over to lean burn more frequently. And this is where the benefit comes in. Lean burn modes often occur in periods where the load on the engine is low. Lifting your gas foot ever so slightly may trigger it. The car will loose a mile per hour every so many seconds, but the instant fuel consumption drops impressively. Much more than to be expected for the slight loss in speed. In my previous car (Civic Hybrid  ), I had a tool to monitor the ignition timing. You could see the engine management advancing timing a lot under these circumstances. I'm sure the TT can do the same. It's an ideal situation during the standard NEDC test where the official MPG figures are being measured. The more lean burn, the better MPG and CO2 figures. The more sell-able a car is.

Another thing: as RON 98 will not ignite so easily, the compression can be higher which equals to better efficiency. This means turbo pressure can stay higher too. No need for letting go of any pressure due to pinking.


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## jaybyme (Mar 10, 2010)

I would always give my cars a good thrashing,5 or 6 full power runs to let the ECU adjust timing to the higher octane fuel.
We definitely had more power after a good few runs on a dyno,and maybe slightly better mpg on long runs.
My TT RS is only running 102 octane, and the fuel economy is brilliant over long runs considering how I drive on the Autobahns


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## TT-driver (Sep 14, 2010)

jaybyme said:


> I would always give my cars a good thrashing,5 or 6 full power runs to let the ECU adjust timing to the higher octane fuel.
> We definitely had more power after a good few runs on a dyno,and maybe slightly better mpg on long runs.
> My TT RS is only running 102 octane, and the fuel economy is brilliant over long runs considering how I drive on the Autobahns


Just stay off the brake. Helps tremendously on fuel economy, especially on the Autobahn: full throttle till the next group of slow cars, off from the throttle on time, many free miles, once the road is clear, full throttle again. That's how I try to drive in Germany when I want to play. (yeah I'm from the 'Nur Links' country :lol: )


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