Extremely Poor MPG

[GLIDN]

So after almost 3 years not owning a MK5 GTI.
I could not resist and 3 weeks ago I acquired another Mk5 GTI, only in DSG this time.

I was very happy, as the first 2 tanks of gas gave me an average of 13.8km/L on the highway and town around 10.8-11.5km/L.

For no reason of driving style I'm now averaging 5.8km/L around town and barely make 7.9km/L on the highway.
So from those fuel numbers I have gone from 580-690km per tank to 380-440kms per tank.

I have only experienced one consistent error in the whole car being overboost issue. Error message, not actual over boost as half the time car had inconsistent boost.

Since then, I replaced all the following parts to try and rectify,
Tried Factory Golf R intake as appose to FSI intake (I believe they are the same)
New airflow sensor
new front PCV, tried blocking off front PCV
Replaced both oxygen sensors
New thrust sensor on fuel pump, even tried low mileage Golf R fuel pump
New camshaft bucket
New diverter valve D and also G
New N75 (fixed overboost error message)
New spark plugs
New coil packs
Tried aftermarket intake (VWR, also made sure now fuel trim issues)

I then had the car on the Dyno today. It's running incredibly rich. Around flat line of 11ish from idle to redline

Tried both 95 and 98 octane to see if that would make a difference.
Car was at Stage 2+ when I bought the car. So none of these things have changed.

Using VCDS, shows within a few degrees of temp on cluster compared to engine sensor.
Performed an oil change and new filter as well. Using Motul 507 specific. Even tried seafoam as well.
It blew a lot of crap out the back.

Still no change however. The local VW agency just looked at the car and blamed the exhaust. Even thou it's been on the car for years. They did not appear to really know much if anything at all.

If this fuel usage issues got gradually worse over months or years I would be inclined to look into carbon build up.
However, this is only from a matter of a week or 2 at most.

If anyone has any potential ideas or possible solutions? It would be greatly appreciated.
Thanks

[bacillus]

I have only experienced one consistent error in the whole car being overboost issue. Error message, not actual over boost as half the time car had inconsistent boost.

Have you looked at the DV as this could be torn.
I think to get any further along you will have to do some live logging for boost, timing, fuel rail pressure and wastegate duty cycle at minimum.

[GLIDN]

I have only experienced one consistent error in the whole car being overboost issue. Error message, not actual over boost as half the time car had inconsistent boost.

Have you looked at the DV as this could be torn.
I think to get any further along you will have to do some live logging for boost, timing, fuel rail pressure and wastegate duty cycle at minimum.

I tried both G and D diverter valve. Neither made any difference
Also did perform boost N75 logging etc. Hence I came to the conclusion of N75.
Car is running very good. Just very poor mileage.

[bacillus]

What are your fuel trims (block 032 and you don't need to start the car to run the measuring block)?

[GLIDN]

What are your fuel trims (block 032 and you don't need to start the car to run the measuring block)?

Will check in a few hours when I get home.

[SkyJawa]

Thermostats......?

[Itguy]

Thermostats......?

That's what I wondered. How long is it taking to warm up and how long on average are your journeys in time/distance?

[GLIDN]

Thermostats......?

That's what I wondered. How long is it taking to warm up and how long on average are your journeys in time/distance?

It takes about 10-15 minutes to get up to temp. Then no fluctuation from 90, on motorway or otherwise.
I thought this as well. Hence I decided to check temp reading on gauge vs VCDS.

Also replaced the fuel filter with a new one as well.

[GLIDN]

What are your fuel trims (block 032 and you don't need to start the car to run the measuring block)?

Group 32 (in engine)
Shows Lambda Regulation
Block 1               Block 2
-2.8% (idle)        9.8% (partial)

[Human Joist]

There was a thread on here last week about warming up and it seemed to end with the conclusion that if it warms up and stays at correct temp but takes ages its the two smaller thermostats and not the main one. They stick this stopping the car warming up as quick. Apparently it should take 5 mins tops. Mine takes 10-15 and I intend to check/replace them both. Cost about £40 plus coolant which is worth doing too

[bacillus]

What are your fuel trims (block 032 and you don't need to start the car to run the measuring block)?

Group 32 (in engine)
Shows Lambda Regulation
Block 1               Block 2
-2.8% (idle)        9.8% (partial)

Long term trim could be lower but supposedly in the ok range (<10%). What intake are you using?
When logging, did you look at actual and requested lambda?
Whose map are you running?
As for your temp, you will use more fuel if you don't come up to the 90C mark asap.

[GLIDN]

What are your fuel trims (block 032 and you don't need to start the car to run the measuring block)?

Group 32 (in engine)
Shows Lambda Regulation
Block 1               Block 2
-2.8% (idle)        9.8% (partial)

Long term trim could be lower but supposedly in the ok range (<10%). What intake are you using?
When logging, did you look at actual and requested lambda?
Whose map are you running?
As for your temp, you will use more fuel if you don't come up to the 90C mark asap.

It's APR tuned. I own 2 mk5 DSG gti's and both are Stage 2+ also both on APR software.
Been for around 12 months.

Only difference in our cars is hers has the AXX engine, mine is BWA.
Like I stated earlier. I was running stock intake, then tried stock Golf R intake and now on VWR intake.
No change in figures or fuel consumption.

As for earlier regarding time taken. If car has been started from cold then only on idle. Car takes approx 10-15 minutes to warm up. If started from stone cold and driven, car is up to 90 within 3 Km's.

I have a new coolant temp sensor arriving tomorrow. So will install that.
But only 30 minutes ago. I have re-installed original front PCV and MAF.

Will do some more data logging in the few days and post up the information.

[mark c]

My off side rear caliper had seized killing my mpg for a few months until it was picked up on a routine service and MOT

[GLIDN]

Well strangely enough.

Got a chance to drive the car tonight. Managed 12.8km/l on the highway.
Still not as good as 13.9-14.4km/L. But a hell of an improvement none the less.
I have not yet swapped out the coolant temp sensor either.

I will keep everyone posted in the next few days.
But appears like I may have been supplied a faulty MAF.
Never seen a car go into crazy rich with faulty MAF. Usually just safety MAP

[bacillus]

If you think the maf is faulty, disconnect it and see if things are any better.
You'll have to disregard the CEL if you do disconnect it and drive.