Cupra 300 over fueling

[Beef46]

Bit of an update. Car went back to the dealer and it looks like they may have found something. They checked the oxygen sensors and I think they said they pre-cat sensor was asking for 5% fuel and the post cat sensor was always asking for 100% fuel!

It's going back in a couple of weeks to have the sensors replaced. Fingers crossed that cures it.
I will update again once this has been done.

 

[RUM4MO]

That sounds strange as the pre cat probe is part of the fuelling control system and the post cat probe is just a monitor probe so it should not have any influence on the fuelling level - only warning for emissions being out of allowed levels.

Edit:- hopefully you have just picked them up wrong and it is the pre car probe that is forcing this over fuelling.

 

[Beef46]

That sounds strange as the pre cat probe is part of the fuelling control system and the post cat probe is just a monitor probe so it should not have any influence on the fuelling level - only warning for emissions being out of allowed levels.

Edit:- hopefully you have just picked them up wrong and it is the pre car probe that is forcing this over fuelling.

Yeah fingers crossed. I cant remember exactly which sensor they said was doing what, just that one was asking for 5% and one for 100%.

 

[mty12345]

Yeah fingers crossed. I cant remember exactly which sensor they said was doing what, just that one was asking for 5% and one for 100%.

Sounds like nonsense, only one sensor has anything to do with fuelling, as said, the second one just monitors the efficiency of the cat.

 

[Beef46]

Sounds like nonsense, only one sensor has anything to do with fuelling, as said, the second one just monitors the efficiency of the cat.

Well if it is then we'll find out soon enough and they'll have to scratch their heads some more.

 

[mty12345]

Well if it is then we'll find out soon enough and they'll have to scratch their heads some more.

In my experience a lot of dealers/garages don't have a clue when it comes to diagnostics. If it doesn't give them a fault code or it isn't covered by one of their flow charts, they'll just start throwing random parts at it and hoping it fixes it. Sounds like that's what is gonna happen here, but if they're paying then fair enough. If you were paying it'd say take it somewhere else.
I

 

[Beef46]

In my experience a lot of dealers/garages don't have a clue when it comes to diagnostics. If it doesn't give them a fault code or it isn't covered by one of their flow charts, they'll just start throwing random parts at it and hoping it fixes it. Sounds like that's what is gonna happen here, but if they're paying then fair enough. If you were paying it'd say take it somewhere else.
I

Oh believe me if it wasn't still under warranty then there is no way I'd be letting them loose on it!

 

[Beef46]

Right, its been a while so time for an update.

So, the car went back to the dealer and they replaced both lambda sensors, and as expected this had no effect whatsoever. They then had another look at it and said they checked every single parameter that they could and could not find anything wrong. As the car was tuned they recon they couldn't go deep enough. So i decided to take the car back to VGS in Swindon where it was tuned before I bought it.
However, between me booking it in and taking it, the strangest thing happened.
So on the Sunday I did a fair few miles and as what has been normal since june, the car only averaged around 24mpg. Then on the monday morning I jump in the car and do my usual 12 mile commute to work stuck behind buses and the car, out of nowhere averages 38mpg which is an absolute record for that journey!
On returning to my car at the end of the day it wouldn't bloody start (would not crank). My friends was getting his jump pack out and then it suddenly came to life. Got it home and was going to scan it for codes. However when I turned the ignition back on i got a battery warning. Checked the battery with a meter and only 9v. RAC called and it turns out the battery was the original, so no surprise that it had given up. RAC replaced the battery and all was good.
Car was continuing to be back to normal and averaging mid 30's mpg.
On the Thursday i fuelled up to go to Swindon and the theoretical range had jumped to 320 miles. On the journey there it averaged over 38mpg.
The guys at VGS gave the car a thorough going over checking all parameters, sensors and live data including fuel trims, and said everything was absolutely spot on. They could not explain the issue I have been having with fuel consumption and were just as miffed about it as me.
I didn't expect them to find anything seeing as though the car had somehow fixed itself a few days before!
I then drove over to Wales for a few days and on my return journey it averaged 40.7mpg, which i have never seen before.
I refuelled after 331 miles with an average of 34.6mpg and an indicated 65 miles of fuel left in the tank. The car has also seemingly re-learnt my driving style as after refuelling, the indicated range was saying 375 miles.
I have absolutely no idea what has happened but everything has just suddenly returned to normal - literally over night.
The only thing that has changed is the battery.
It is incredibly bizarre!

 

[James_R]

Dying battery causing immense strain on the alternator and dragging the MPG down?

 

[Mo_86]

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

Did mention new battery back then, it made a difference for me when my original efb battery died with no warning and i upgraded to agm.

 

[Beef46]

Dying battery causing immense strain on the alternator and dragging the MPG down?

I have no idea. Strange that it returned to normal just before the battery **** itself.
I popped in the dealer the other day to let them know of the change. They recon they checked the battery health when they had it and it was apparently fine.
If the cause was a dying battery, then it took a while for it to give up considering I have been having the issue since june.
And surely during investigations there would've been something showing high fuel usage.
Like I said, I am completely baffled. I know failing batteries can cause all sorts of weird faults, but I have never heard of a failing battery causing a car to use so much more fuel with no effect on anything.
Hopefully someone may see this thread and come along and reply saying they had the same problem. That would put it to bed once and for all then.

 

[nd-photo.nl]

They probably didnt check with an actual battery tester, I'm pretty sure it would have indicated something. If you just drove it and then they measure it shortly after, it will probably be in spec.

But a dedicated battery tester will show you more, for instance:

Nevertheless, this is quite the mystery haha. I also never heard of a failing battery having such impact on your mileage

 

[DaveS]

Fingers crossed that it stays fixed. A dying battery should not cause any excessive load on the electrical system/alternator. But as you have rightly said, they can cause all sorts of electronic weirdness. It is not hard to believe that it could have corrupted some values in the engine management system leading to the problems you had. At 8 years, the original battery owes you nothing. Hopefully the RAC put something decent in its place and coded it correctly.

 

[Beef46]

Fingers crossed that it stays fixed. A dying battery should not cause any excessive load on the electrical system/alternator. But as you have rightly said, they can cause all sorts of electronic weirdness. It is not hard to believe that it could have corrupted some values in the engine management system leading to the problems you had. At 8 years, the original battery owes you nothing. Hopefully the RAC put something decent in its place and coded it correctly.

Yeah they did. They replaced it with a £300 AGM battery with a 4 year warranty, and he coded it all in properly.
Glad I chose the complementary battery cover with my breakdown cover!