MK5 GTI Poor MPG

[chrism3]

Hi all,

Been hunting down poor MPG in my manual Mk5 GTi. Have already fixed the following:

- Main thermostat (I only have 1 in my car) changed. Was sitting at 70deg now fixed and sits at 90deg.
- Replaced both rear brake calipers which were binding

Im now seeing 32mpg on a 100 mile run sitting at 70mph, very light right foot, mostly cruise control. I believe this is still a little low as my mates TT with the same engine is getting close to 40 on an easy run.

What should check next? I have cleaned my airflow sensor a few times but could this be reading out and causing overfueling? Anything else to check?

Cheers

[GVK]

Do you have VCDS, could check fuel trims etc?

http://wiki.ross-tech.com/wiki/index.php/Fuel_Trim_Info

This page shows what groups etc to look at http://www.goapr.co.uk/support/datalogging.php (Lambda etc)

[chrism3]

Unfortunately not.

Can I test the MAF by just unplugging it and going for a longish drive at 70 and seeing if MPG improves?

[GVK]

You could do but I would expect it put a fault in and put your engine light on.

When you've cleaned it before was it very oily?

If the cost of VCDS is a problem and you have an Android phone you could try OBDeleven, I have it and it's a great little tool that can check the live data. Find it on eBay.

Saw this handy tool for VAG stuff, that's been out about a year or so now called OBDeleven, it's a dongle that plugs into your OBD socket and uses Android device for the free app. from Google play. https://obdeleven.com/en/  £66 from Lithuania on eBay for the Pro version, you can get a cheaper one that has less features.

#obdeleven by GVKmotorsport, on Flickr

No IOS support as yet.

If you have something to check live data you can easily check the MAF

Here's a good sanity check for the status of your MAF. Do a full-throttle run all the way to redline in a single gear (second works fine). Group 002 usually shows air mass in g/s (in many modern ECU's, consult your Factory Repair Manual for the specific group for your particular vehicle). Your peak airflow should be roughly 0.80 times your horsepower if you are close to sea level. So, if you have a stock 150 hp 1.8T, expect around 120 g/s. If you see significantly less than that, you MAF may be on the way out. Also note that airflow will be markedly different at higher altitudes due to reduced ambient air pressure, especially with naturally aspirated engines that do not have forced induction to overcome that deficiency. This still works if you are chipped, but "race" programs may make more power through timing, rather than airflow. Therefore, take all readings with a grain of salt.

You should see about 160 g/s on a 200ps engine.

[gtipirelli]

32 mpg is not too bad

I would only see about 35 in mine

Check tyre pressures, was it into the wind, was the road very wet, etc etc

Is there much stuff in your boot weighing it down a bit

Just some easy to fix things maybe

[chrism3]

Was perfect conditions at the time, perfect pressures, new brakes all round, 2 people etc.

I will try the app thanks!

[ROH ECHT]

If you do get a look at LTFT..."Multiplicative/off idle" is the one you want to focus on. If losing mpg due to an engine issue, this will be a positive addition of fuel. Anything over +5% will reflect a noticeable drop in average mpg on your MFD. Addition of fuel is usually a result of a small vacuum leak, poor Bank 1 Sensor 1; O2, or a failing MAF. Another way to quickly test your MAF is check the mass air data at redline in 3rd gear. Mass air=Known HP(0.80). For example; If my mass air data shows 170g/s=210 HP(0.80). If I know my HP is 350...350/0.80=280g/s as what I should see for mass air data.  I know what my data should be when the MAF is new because I noted its data then...280g/s at 2nd gear redline_275g/s at 3rd gear redline_270g/s at end of 4th gear.

[TheBALDpuma]

I agree with gtipirelli, 32mpg is not bad at all.  On a 250 mile motorway trip to see my parents I will only average about 34/35 if I'm lucky and that's due to there being very very long sections of 50mph on the M1 which nudge the average up a fair bit, sat at 70mpg I'd get low 30s like you.

The TT is a slipperier shape than a golf, he might drive like miss daisy, it could have been a very hot day with a tail wind the day he got close to 40 mpg.

[chrism3]

Too late, im on a mission now ;)

Going to test MAF and report back

[Mark_DF05]

Don't think 32mpg is bad at all either.

Was getting low to mid 20's when my thermostat was knackered.