Cupra 300 over fueling

[Beef46]

Hi everyone. First post here so be gentle!

So i have owned a 2018 Leon Cupra 300 ST 4 drive since January and I absolutely love it.
However recently the car has developed a significant drink problem!

Normally when I fuel up, I get a theoretical range of 385 miles, and i usually refuel at around 330 miles with the trip computer giving an average of around 30-32mpg. When i took the car to Yorkshire a few days after I bought it, in January it averaged 38mpg which was great.
A few weeks ago a drove over to the Netherlands for the Dutch Motogp and was interested to see how it would do on fuel. Well, from start to finish it was absolutely terrible with it struggling to average around 25-27mpg.
And likewise since ive been back and doing my normal driving it is still really bad. So much so that when I fuel up now, it only gives me a theoretical range of 295 miles. The last time I refuelled was after 232 miles which is absolutely terrible. Around 24 mpg seems to be the average now! That's worse than my old Focus ST that had the 2.5 five pot, which is renound for being thirsty!

The car has no symptoms at all. It drives and performs normally which is really odd.
Today it went back to the main dealer as its still under warranty. Well to say they were useless was an understatement. They did no fault diagnosis whatsoever other than plug a laptop in and scanned it for codes - which there wasnt any, so then proceeded to say there was nothing wrong, implying I was lying!

Anyway, without waffling on too much, have any of you knowledgeable lot ever experienced such over fuelling with no symptoms and if so what was the cause?

From the research I've done I'm thinking its got to be either injectors, fuel pump, pcv valve, cam sensor, maf sensor, ecu???

As i said its still under warranty so I am fighting with the dealer to get them to do some good old fashioned fault finding which they are pushing back on.
Think im going to have a bit of a fight on my hands! Bloody hate main dealers!!

I really hope someone on here has experienced something similar and can offer some possible solutions.

 

[James_R]

If the car was over fuelling to the point it was affecting the emissions and fuel economy, then there would be accompanying fault codes to back the symptoms up.

Have you considered non engine related issues being the cause? Such as low tyre pressures, sticking brakes, etc?

With the car being covered by an extended warranty plan (basically an insurance policy that covers sudden component failure), you are generally responsible for covering the diagnostic fee until the cause is located and approved by the warranty provider.

 

[mty12345]

If it was 'overfueling' then there would be a host of fault codes.

Low MPG could be caused by weight/load, poor tyre pressures, high speed, stop/start driving, inclines etc etc.

 

[Beef46]

If it was 'overfueling' then there would be a host of fault codes.

Low MPG could be caused by weight/load, poor tyre pressures, high speed, stop/start driving, inclines etc etc.

This is why it's so odd that there are no warning lights or fault codes.
Tyre pressures are checked regularly and my driving style hasnt changed.

When I went to the Netherlands it was literally a 1200 mile trip travelling on motorways at 70-75 mph. From Calais right up to the Netherlands and back there were literally no inclines - it was pretty well completey flat, so the car was under no load at all.

Since ive been back ive just been doing my usual driving consisting of my commute to work, a bit of town driving and some A road / motorway driving.

Nothing has changed apart from the fact the car is using considerably more fuel. For example, my commute to work involves me being sat behind multiple buses heading out to Hinkley Point C. Its the same every day. When I getbto work the car generally says ive averaged 35-37 mpg for that trip. Now - for exactly the same trip the most ive seen is 27 mpg. It really is bizarre and a real head scratcher, hence why I've come here in the hope that someone has experience of this.

 

[DaveS]

The AC will be working harder now, that can knock a few mpg off. When I have travelled to the Netherlands in my FR, I do get very good mpg, because of the flat roads and lower speeds. Maybe 5 more than UK driving.

 

[TVK]

How many miles does the car have?
Sounds like carbon build up in our intake.

Carbon build up

 

[Beef46]

How many miles does the car have?
Sounds like carbon build up in our intake.

Carbon build up

Car has done 94k.
The change in fuel consumption was very much a step change and not gradual at all.

 

[nd-photo.nl]

What are your fuel trims looking like? If its overfuelling, you should see it at least by looking at the fuel trims. Long term fuel trims should stay within the -5 / +5 range. If its going to 10-15 range, something is wrong...

 

[cuthound]

What octane fuel are you using?

I generally get 35-40 mpg out of my ST300 4drive, depending on the type of journey using Tesco Momentum 99, but once I had to use 95 RON fuel and the fuel consumption dropped to the high 20's.

 

[Beef46]

What octane fuel are you using?

I generally get 35-40 mpg out of my ST300 4drive, depending on the type of journey using Tesco Momentum 99, but once I had to use 95 RON fuel and the fuel consumption dropped to the high 20's.

I have used 95 RON fuel the whole time I've had it. However, as an experiment I'm now going to use super unleaded for the foreseeable to see if anything changes. I know it'll probably take a few tank fulls for the car to learn im using different fuel. In all honesty though, ive tried these fuels before in other cars and never noticed a difference.

Also, yesterday while on the motorway I did a little experiment. The motorway was flat and quiet so I set the cruise to 64mph and looked at the instantaneous fuel consumption, and where I would expect to see it in the mid to high 30's, it wouldn't go above 28.

 

[Beef46]

What are your fuel trims looking like? If its overfuelling, you should see it at least by looking at the fuel trims. Long term fuel trims should stay within the -5 / +5 range. If its going to 10-15 range, something is wrong...

Hi and thank you for the advice. If I can get something to read live data I'll have a look. Otherwise I'll recommend that is something the dealer checks when its back in for further diagnostics.

 

[Beef46]

One other thing ive thought of is could it be a problem with the haldex and the car is stuck in 4wd?
I dont think this is an issue though as the haldex has been maintaimed throughout the cars life, and was serviced by the main dealer when I bought it. However, you never know through?

 

[Beef46]

What are your fuel trims looking like? If its overfuelling, you should see it at least by looking at the fuel trims. Long term fuel trims should stay within the -5 / +5 range. If its going to 10-15 range, something is wrong...

So I've just updated Carly so I can look at data and scan for codes etc.
Scanned for codes and none found. No surprise there as the dealer couldn't find anything codes.

Just took it for a drive (A road and motorway), and monitored the short term and long term fuel trims, and i think they are absolutely fine. Long term doesn't deviate much from 0.781%, and the short term is very steady as well, occasionally deviating a couple of % either way with a spike of around -18 to -25% for a split second when I lift of the throttle.
So the mystery continues

 

[nd-photo.nl]

Did you add a blowoff spacer by any chance? Or fitted another DV which vents to atmosphere (partially)?

If not the fuel trims look absolutely fine

 

[Beef46]

Did you add a blowoff spacer by any chance? Or fitted another DV which vents to atmosphere (partially)?

If not the fuel trims look absolutely fine

Nope none of that nonsense.
Its just got a Milltek exhaust system with sports cat on it and it was mapped by VGS Performance in Swindon. All of which was done by the previous owner.
I have been in touch with VGS and they were really helpful on the phone. They said they'd be more than willing to have a look at it, although they have never had an issue with one of their maps. Thats not to say that mine could be the first though.

I think for me to travel all the way to Swindon for them to interrogate the map would be a last resort, plus its still under warranty from the Skoda dealership where I bought it from, so dont think they'd appreciate me letting others getting involved just yet.

 

[nd-photo.nl]

Ah ok, crystal clear

If your tool supports it, you can always safely reset the fuel trims. It will take about 50-100km to really settle. But if its really overfueling, you would smell that or see it in the fuel trims.

I tested a blowoff spacer and little did I know it started overfueling. I didn't notice it until a while and did not expect a small mod like the spacer would impact fueling that much. I was running about +12-17% after removing it and resetting the fuel trims, all went back to normal...

Some extra info:

Fuel Trim Info - Ross-Tech Wiki

 

[Beef46]

Ah ok, crystal clear

If your tool supports it, you can always safely reset the fuel trims. It will take about 50-100km to really settle. But if its really overfueling, you would smell that or see it in the fuel trims.

I tested a blowoff spacer and little did I know it started overfueling. I didn't notice it until a while and did not expect a small mod like the spacer would impact fueling that much. I was running about +12-17% after removing it and resetting the fuel trims, all went back to normal...

Some extra info:

Fuel Trim Info - Ross-Tech Wiki

Thanks for that. Just had a look online and I cant reset fuel trims with Carly unfortunately. Would've been good to try it.

 

[Mo_86]

Maybe a new battery and pcv valve could make a difference?

 

[James_R]

I wouldn't count on getting much, if any repairs carried out on the powertrain under extended warranty if it's been remapped.

 

[mty12345]

I have used 95 RON fuel the whole time I've had it.

Not related to the MPG, but if it's stage one mapped then it should be run on 99 octane anyway.