MPG problem

[Husbandofstinky]

Went to a funeral yesterday (some distance) decided to take the Seat (as we were running late and were mostly A roads). Reset the average mpg and came back with 56.7mpg on a 170 mile run. This included plenty of overtakes and bimbling along at 70-75mph.

Forgot to mention Chris, my test with the mpg was without the climate on. Its on 95% of the time but just over the last two weeks or so its spent most of its time off. Why?

 

[the_fbi]

I got 42MPG @ 30MPH in 4th on a flat road. Revs about 1200RPM.

Got stuck on the M40 this morning.

4th gear, 30 mph, very slight uphill incline, 112-118mpg, 1150rpm.

This was with cruise on over about 1/2 mile.

 

[ChrisGTL]

Got stuck on the M40 this morning.

4th gear, 30 mph, very slight uphill incline, 112-118mpg, 1150rpm.

This was with cruise on over about 1/2 mile.

Blimey!

What fuel do you use?

 

[the_fbi]

Blimey!

What fuel do you use?

Just Sainsbury's Diesel.

Its only an Ibiza though, so lighter than yours.

 

[ChrisGTL]

Its only an Ibiza though, so lighter than yours.

Surely not 70MPG lighter though

Maybe I just drive hard without really realising......

 

[Fergal84]

Did the mpg test on the way home and in 4th @ 30 it was between 60-70 mpg, if not a bit more the 2nd time i tried it, just using tesco fuel

 

[Husbandofstinky]

Did the mpg test on the way home and in 4th @ 30 it was between 60-70 mpg, if not a bit more the 2nd time i tried it, just using tesco fuel

Well that sort of backs my test then. Good.

 

[depresion]

You got any before and after MAF readings I could look at?

Unfortunately not, but having run it a bit since it's defiantly fixed the problem MPG wise as my 30 mile commute averaged 63.5mpg on the way out yesterday and 54.3mpg on the way back (I came back a different way and it's always much worse MPG on that DC, probably because I make good time and there are quite a few roundabouts to stop at then get back up to err, 70.)

 

[the_fbi]

Mine's improved a bit, not back to where it was though.

Motorway cruising is fine, was up into the mid 60's coming back from Wales on Friday (130 miles of motorway) but the mpg gets killed at junctions/roundabouts.

I'm probably imagining it, but I think the turbo is noisier than normal, so thinking it may be a boost leak somewhere.

New MAF will be here next week, so I'll what that does.

I tried to get an OT-1 log as I've got one from a journey to work when the car was fine, be nice to compare (tps, maf, load, rpm, etc) but the OT-1 was being a pain so I gave up on that.

 

[the_fbi]

What unit is MAF measured in? I can pick pretty much anything, but none of them gave a figure around 200-600.

MAP appears to be reading nothing (well maxed at 255), which is a slight worry. The ECU's publishing a MAP sensor as being present, I presume VAG-COM can normally get a value from it?

Note: I've realised the MPH and RPM are only reading half speed, apart from at idle where the rpm is OK.

 

[ChrisGTL]

Unfortunately not, but having run it a bit since it's defiantly fixed the problem MPG wise as my 30 mile commute averaged 63.5mpg on the way out yesterday and 54.3mpg on the way back (I came back a different way and it's always much worse MPG on that DC, probably because I make good time and there are quite a few roundabouts to stop at then get back up to err, 70.)

Might have to treat mysen to a new one. Did you get yours from GSF?

 

[the_fbi]

New MAF hasn't sorted the issue for me, at least not in the first 110 miles

Going to have to have a good look at all the boost pipes sometime.

 

[ChrisGTL]

New MAF hasn't sorted the issue for me, at least not in the first 110 miles

Going to have to have a good look at all the boost pipes sometime.

Bugger!

I was keeping my fingers crossed that your new MAF may have fixed yours.

 

[muddyboots]

With the engine idling the ECU was getting readings of air flow that were 70% higher than it had expected, so I guess it was over fuelling as well.

Another snippet I forgot to mention before.

AFAIK, the amount of fuel injected is limited by a number of factors. From memory they are airflow (MAF reading), driver's request (ie right pedal position) and boost pressure (MAP reading). Whichever of these factors calls for the lowest amount of fuel is the one that "wins". There's a measuring block in vag-com that measures all of these. which means you can log them and always see what the limiting fuelling factor is. I suspect there are other factors too, but those three are the main ones in the vag-com block together.

Anyway, even if the MAF is reading higher than requested, if the driver's request is zero because the foot is off the pedal, then it's the driver's request that wins - it won't inject more fuel at idle because of the apparently high MAF reading.

Just to illustrate the point further, I vaguely recall that Cheshire Cat on here ditched his MAF, and replaced it with a diode, which has the effect of registering maximum airflow permanently. The reason this can still work is because the driver's request and boost pressure still act as the limiting factors for fuelling.

So if a MAF is over-reading, it's unlikely to cause a problem with over-fuelling. However, if it under-reads, then it will cause a problem by restricting fuelling.

 

[ChrisGTL]

There's a measuring block in vag-com that measures all of these.

Don't suppose you know which block it is do you?

 

[the_fbi]

Don't suppose you know which block it is do you?

I'd like to know too, as my OT-1 is getting 255 back from the MAP, which either means the OT-1 is confused or the ECU isn't getting a MAP signal.

 

[muddyboots]

Sorry I can't rememer which block it is.

And I've a feeling that I it might be RPM may be one of the limiting factors in that IQ limitation measuring block, not the MAP reading.
Although, if you have vag-com, you cam log MAP readings anyway.

 

[ChrisGTL]

http://forums.tdiclub.com/showpost.php?p=2060890&postcount=12

Might be useful to check..

 

[ChrisGTL]

Also taken from TDIClub.

'You might try these for starters:

Group 3, MAF from idle through full throttle in 3rd gear. Watch for differences in demand and actual. Some differences at low loads can be explained by EGR operation.

Group 11, MAP, from idle through full throttle in 3rd gear. Watch for differences in specified and actual, smooth response with short spikes for overshoot.

Group 7, Temperatures, from cold ambient (overnight) through fully warm – 15 mile drive. Watch for all sensors near ambient at startup, fuel should slowly climb to about 70 deg C, coolant should climb to 87.5 deg C. These sensors and the thermostat and thermostatic Tee are often the sources of poor mileage and performance.

Group 13 at idle. The closer everything is to zero, the better. One that is really different from the others *may* indicate a problem.

Basic settings with the car running, group 003, do the test, verify your EGR is working and your MAF – you should see somewhere around 170 to 200 at minimum air flow, with the maximum about twice the minimum during the test.

Go through the output tests, too. I start with the engine running, then continue with it off as it dies during the intake flap test.'

 

[MOZ_LC_TDI]

Hi guy's,
Did the 30mph in 4th test. It is really difficult to get a consistant reading as slight incline / decline has a dramatic effect, but over quite a number of attempts my average is about 83MPG ( for a very flat stretch) but varies between 65 and 98 dependent on flatness. ( I live in yorkshire so it is quite hilly). Using shell Extra. 1150 to 1200rpm ish.

I still have the fast idle problem ( see other thread) so i don't know if this is affecting things but that sounds about right for me.

My overall average trip MPG has never got into the 60's and rarely gets above 50.

The best i have had was last weekend with some motorway driving averaging 54MPG over a 65 mile run at ( 70 to 75MPH) on the way there and 47MPG ( 75 - 80 MPH) on the way back.
Most of the time over 40 mile round trip to work and back ( town and country driving) I generally get mid 40's, high 40's if stuck behind tractors etc.

May be my fast idle problem is affecting my MPG but I suspect the readings are not that accurate and vary from car to car. I get 500 miles out of an unvented tank regularly which i'm pretty happy with, withmy MPG showing 42. Or maybe I drive with too much of a heavy right boot

I suppose the hilly terrain I live in won't help, as I suspect on average I use more fuel going up the large hills, and stop starting on them, than I save when going down the other side so I suspect it doesn't evenly average out as it would if I lived in a flat area of the country.

Thats my contribution, for what its worth.

Regards.

MOZ