Another MPG thread

[Mark_DF05]

I know there's been plenty of these over the years and it is what it is, if you're bothered about saving fuel you've bought the wrong car. However over the past 12 months I've noticed that my mpg has dropped quite a bit and I'm curious to try find out why as if there's something wrong I'd like to get it sorted. On long motorway journeys there's very little difference to before, often returning around 32mpg yet on mixed journeys where I hit traffic it seems to affect it more and often struggle to get over 20mpg.

I'm aware there's numerous things that have an impact on it so I may never find out but I wondered if anyone had any ideas?

[pudding]

20mpg worked out at the pump, or going by what the MFA is telling you?

I ignore the live mpg readings on position 1 and only refer to position 2, which is the long term average.  If that one is in the low 20s, then you have a problem, but you haven't said if it's modified or standard.  More boost = more fuel used.

The usual suspects for excessive fuel consumption are the main thermostat, primary lambda sensor, MAF sensor and the cam position sensor.

The last 2 don't usually throw a fault code except where there is a loss of connection, or a short circuit (Short to B+ or ground), completely dead or give a whacky reading.

Have you scanned it for faults?

[lukemk5gti]

Temperature is a huge factor.

Mine drops by up to 5mpg in Winter. That could also be due to the fact that the exact same commute in Winter is easier in Summer due to schools being off and better driving conditions.

[ZigzGTI]

From looking into previous discussions about this I've seen the list of 'What could be the issue' go on for pages. 
i 've noticed the MPG also drop but i suspect having the heater/AC on is having its effect plus the heated seats along with the winter cold resulting in the increased fuel consumption.

So here's what i plan on doing to mine:
-Get Vag-com diagnostic to see if there is any sensor failure etc.
-Service is due; so to get that done
-Make sure tyres are well inflated and not worn (the wet road surface and less grip would have an impact especially over a 12 month period i suspect).

After that i guess its a matter of having some extensive checks and replacements or.......... change in vehicle.

[Mark_DF05]

@Pudding That’s from the MFA on short term, The long-term reading is showing around 26. Doing the same journeys previously was 30. Worked it out at the pump and 26 was correct. Oh, forgot to mention that it’s totally standard.

Will get it scanned this week and see if there’s any issues

[pudding]

That is pretty low then, but not mega horrendous.  It does depend on the journeys and how long you sit in traffic for etc etc and how long it's taken to build up that long term average.

For comparison, my Stage 2+ ED30 is showing 29.8mpg on the long term average (and has for over 2 years now) which also matches the pump calcs.  That's based on a daily 30 mile commute with warming up in bumper-bumper traffic at both ends, country roads and dual-carriageways, so a wide mixture of throttle positions and road speeds.  It also includes weekend shopping jaunts, trips to the dump etc etc.

Pretty happy with that economy for the power and fun it gives      If I babied the car everywhere, I'm sure mid 30s is easily possible but it's not how I drive 

If no fault codes and the thermostat is working properly, my money would be on a defective MAF sensor.

[Mark_DF05]

Yeah agree it's not horrendous which is why it's not really concerned me up until now.

My daily commute sounds almost identical to yours albeit a bit further (50 miles) and had a very similar long term average before.

I'll get those bits checked as a precautionary measure and if there aren't any issues I'll forget about it!

[pudding]

The MAF and cam sensor have both played a big part in the power delivery, smoothness, consistency and economy of my car.  Neither were fault coded.   The Cam sensor was covered in engine swarf and the MAF sensor to all intent and purposes looked absolutely fine, but it wasn't because shoving a new one in completely changed it's character for the better!

It seems to me the ECU just ignores sensors it's not particularly happy with and runs the 'rough running' map, which is a sub-optimal map to keep the engine running as seamlessly to the driver as possible.

Dunno, it's worth a shot replacing them......but at £120+VAT for the MAF and £35 for the cam sensor......it's not a cheap punt.

I don't normally recommend firing the parts cannon at a car, but in this case both of those sensors showed no (electrical) faults and improved things no end.

You can of course go over everything and look for leaks, split DVs and what not to outrule that stuff as well.

[r5gtt]

It's very rare for a MAF to fail so I'd look at things like fuel pricing and weather conditions as colder weather equals excess fuel consumption but I'm no expert. Make sure stat is at working temp 90 degrees constantly and doesn't drop.

[pudding]

They don't fail as such, as in the ECU fault coding it (that is rare, I agree), but they do deteriorate.   Contaminants such as oil and dirt gradually bugger them up.    They are also very sensitive, complex and difficult to diagnose without an oscilloscope, and even then, a healthy looking waveform still doesn't always translate into a punchy and smooth engine.   They've always been like that with VWs.  You could try a batch of 4 or 5 MAFs and 1 or 2 of them drive better than the others.

[neilw]

Hope you get it sorted, I can't offer any further advice as to what it might be.

I had a look at my long-term average its just over 31

[r5gtt]

They don't fail as such, as in the ECU fault coding it (that is rare, I agree), but they do deteriorate.   Contaminants such as oil and dirt gradually bugger them up.    They are also very sensitive, complex and difficult to diagnose without an oscilloscope, and even then, a healthy looking waveform still doesn't always translate into a punchy and smooth engine.   They've always been like that with VWs.  You could try a batch of 4 or 5 MAFs and 1 or 2 of them drive better than the others.

Very true mate, dirt or dust particles can form inside the MAF and you wouldn't even know 

When my 1.8T maf failed I didn't get an error code either as VAG do suck lol but did feel car running a bit lumpy  with lack of power imo so I unplugged it and drove around the block and found it ran a lot better so bought a Bosch one and car was all good again. I bought a spare for my tfsi off fb but it didn't change anything on my car so it lives in the boot with my many other bits 

[Mark_DF05]

Cheers for the replies so far guys, very useful info.

I dont have vcds but have an OBD2 scanner to use with my phone so scanned the car this morning. It brought up 2 faults:

P000A - Camshaft A (Bank 1 Intake): Positioner Slow Response
&
P0016 - Bank 1: Camshaft Position Sensor (G40) / Engine Speed Sensor (G28): Incorrect Correlation

I’ve done a bit of research this morning and I think the first code(P000A) may require a new n205 valve (Part No: 06F109257C).

The P0016 code, returned more worrying results Possibly requiring the timing chain, adjuster and tensioner to be replaced although I’m pretty sure I’d know about it if this was the case so this could be a faulty G40 sensor (Part No: 06C905163B)

Can anyone shed any further light on these codes?

@Pudding are either of these sensors what you mentioned in your previous post?

I've reset the codes for now and will scan again this evening after my commute home from work. It's probably useful to know what car I'm driving too since I never mentioned it in my original post. It's an early Edition 30 (56) DSG, no eml and seems to run fine bar the fuel economy.

[pudding]

Those errors mean:

1) The VVT unit is not keeping up with the angles requested by the ECU. Either because of N205, worn out VVT adjuster, or a chain which is too slack.
2) The ECU can no longer syncronise the crank to cam angle.  It doesn't necessarily mean your sensors are faulty.

That explains the horrible mpg because if the ECU can't correlate the crank angles, it will fall back to a default 'rough running' map, which is added fuel and reduced ignition timing for safety.  I would expect the car to feel less responsive than usual too?

Before lighting the fuse on the parts cannon, it would useful to monitor measuring blocks 91 - 93 in VCDS to track the VVT movement and cam position.

We need to see 93 in the 0 - 4KW range, and block 91 actual and requested should follow each other closely.

It could all just be a worn chain, in which case N205 and G40 aren't required.

[Mark_DF05]

Ah right, thanks! To be honest I've not felt much of a difference driving it 

How would I go about monitoring the measuring blocks - if I find someone local with vcds would they be able to do it or is it a job for a specialist? I don't think my cheap scanner is capable of that

I'm a little out of my depth here, I've never had to use vcds for fault logging so apologies for all the questions.