MPG problem

[ChrisGTL]

but it's at light throttle (yes I know it doesn't have one but you know what I'm saying) that the readings are lower than I expect

This is what makes me think i've got the same problem as you, when I first got the car light throttle used to read high 40's low 50's MPG. Now on light throttle I get low to mid 30's.

Just don't understand it. Setting off from a roundabout for example shows me single figure MPG's - but I kinda expect that when pulling away.

 

[Husbandofstinky]

 

[westoncraig]

wow low 30's thats bad! i only get that when popping to the shop at the end of the road!
trip says im averaging 41 mpg over 10 tanks

 

[ibiza lad]

I've got a 08 plate Ibiza FR TDI. I've now done nearly 7000 miles in mine. First thing to note is the trip computer MPG is way off reality on my car.

I've average about 46mpg over the 7000 miles. That is measuring all of the fuel I have put in the car. [Yes I know I am sad, I keep a log book of fuel].
The long distance MPG reads about 56mpg. So that is way off.

The trip average [1] is also way off. As an experiment I did a 200 mile motorway trip with the cruise control set to about 55 mph, in sixth gear. So I was doing about 1700rpm give or take. The trip computer suggested a magical 75mpg, in reality I’d used about 15 litres of fuel which works out at about 60mpg.

So okay my first point is don't believe the trip computer, not at least on my car. My average of 46mpg seems a little low. Okay I do occasionally drive enthusiastically!
But the 60mpg figure is really disappointing; I would have expected something closer to 70mpg on the TDI driving like that.

 

[ChrisGTL]

Yup lol

MAF readings are spot on, car goes like stink (for a TDI), really happy with how the engine feels/drives.

Just baffled as to the almost overnight reduction in MPG. one thing I have noted though is when I make my car do the wooooooooooosh noise it really makes my MPG drop quickly.

Next time your bored and driving on a flat road with no loaded boot get to 30MPH and keep it constant, in 4th gear. Report what your MPG says (make a note of the revs too) - will be interesting to see the difference (if any)

Don't cheat though, a nice flat straight road.

 

[Husbandofstinky]

Next time your bored and driving on a flat road with no loaded boot get to 30MPH and keep it constant, in 4th gear. Report what your MPG says (make a note of the revs too) - will be interesting to see the difference (if any)

Don't cheat though, a nice flat straight road.

Will do . Plenty of nice flat straight roads round here.

Obviously at full operating temperature (which can be a bit of bugger as the car only warms up fully toward the end of the journey - 5 miles). Could go long way home for the benefit of research.

 

[ChrisGTL]

Will do . Plenty of nice flat straight roads round here.

Obviously at full operating temperature (which can be a bit of bugger as the car only warms up fully toward the end of the journey - 5 miles). Could go long way home for the benefit of research.

Thats what we like to see

I shall get my engine nice and warm on the way home - via some nice roads......then do the 30MPH thing.

 

[oldman]

I don't think if you stayed under 2k revs all the time it'll do the engine any good let alone clog up the turbo. I don't and I assume that the other members on here don't either. It'll get 90% normal driving and possibly 10% spirited.

Unfortunately the Leon is not exactly a 'performance' car more of a reasonably quick hatch.

If you're getting under 40 all the time then there is either a problem or you're crucifying your motor.

ditto my driving style really
also isn't the leon about 20tonnes heavier than a ibiza, which i thought may make a difference, slight though it may be

 

[depresion]

Got the car onto a Bosch FSA 740 today, no errors but the nominal air flow was 265 the actual was 440 (blip the accelerator and it went back to normal for a few mins) so went to TPS, £70 and some scraped knuckles later a new MAF sensor in place and all looks to be fixed. Will need to run a few tanks trough to be 100% but even after letting it idle for 20 min the readings were much better. I just need to readjust the tension on the front strut brace now as that had to come off.

 

[the_fbi]

MAF was where my thinking is too. Need to check my part number tonight then see how much one can be picked up for, I notice there are some ~£25 new ones on eBay but....

That said, MAF being screwed should either give loads of black smoke, or a slow car. Neither in my case

Guess I can check the airflow via OT1 or VAG-COM.

Whats yours reading now after the new MAF?

 

[ChrisGTL]

the nominal air flow was 265 the actual was 440 (blip the accelerator and it went back to normal for a few mins)

Can you explain further?

a new MAF sensor in place and all looks to be fixed. Will need to run a few tanks trough to be 100% but even after letting it idle for 20 min the readings were much better.

What do you mean by this? Your MPG readings before the new MAF went down when on idle? My AVG MPG goes down about .4 to .7 every 4-6 seconds when on idle, I'm sure it didnt used to go down so quickly.

Maybe my MAF is fubar too, I have VAG-COM so can check (if I knew what I was checking for lol)

 

[ChrisGTL]

MAF was where my thinking is too. Need to check my part number tonight then see how much one can be picked up for, I notice there are some ~£25 new ones on eBay but....

That said, MAF being screwed should either give loads of black smoke, or a slow car. Neither in my case

Guess I can check the airflow via OT1 or VAG-COM.

Whats yours reading now after the new MAF?

Yeh, I dont have any black smoke (apart from the normal for a diesel) and certainly not slow either.

 

[depresion]

Can you explain further?

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. Pressing the accelerator causes the ECU to expect more air and the air flow increases when I did this the two readouts became quite close and remained that way after the revs dropped back to idle for a little while then the "actual" figure jumped back up to around 440. (So it's probable that it wasn't the actual flow just that's what it was being told the flow was)

What do you mean by this?

Well for now I only have the fact the air flow readings that were steady around the 265 reading that they should be at idle.

As the fault seemed to be only at low loads and low engine speeds and the MAF readings were correct at higher RPMs (the 740 is a bit big to take with me for a run) I can't be sure it's fixed but I wasn't getting black smoke (well no more than I ever got after the chip and decat) or a noticeable performance drop.

 

[ChrisGTL]

Just had a look at my latest MAF readings.....

At idle (903RPM) my ECU requested 335mg/R but the typical actual is 436mg/R.

Thats 30% more than needed.


At motorway speed (3318RPM) the requested is 850mg/R and the actual is 1156mg/R.

Thats 36% more than needed.

Anyone know if this seems right?

 

[Husbandofstinky]

Reporting findings sir:

At 30mph in fourth gear readings were 65mpg and revs were approximately 1200RPM.

Its quite difficult to get it all spot on i.e. 30mph, flat road and constant pressure on the accelerator but I think that my figures are as reasonable as I can get them. Do you want me to provide figures with three attempts and an average if it helps?

 

[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. Pressing the accelerator causes the ECU to expect more air and the air flow increases when I did this the two readouts became quite close and remained that way after the revs dropped back to idle for a little while then the "actual" figure jumped back up to around 440. (So it's probable that it wasn't the actual flow just that's what it was being told the flow was)



Well for now I only have the fact the air flow readings that were steady around the 265 reading that they should be at idle.

As the fault seemed to be only at low loads and low engine speeds and the MAF readings were correct at higher RPMs (the 740 is a bit big to take with me for a run) I can't be sure it's fixed but I wasn't getting black smoke (well no more than I ever got after the chip and decat) or a noticeable performance drop.

Remember that at idle and low loads, the EGR will affect how much intake air actually goes past the MAF - when the EGR is operating, the engine will take some of it's air from the exhaust through the EGR valve, meaning that less will pass the MAF.
As an experiment you could always try temporarily disabling your EGR by blocking the vac hose and see what effect that has when idling/blipping etc.

Also got a vague recollection of reading that blipping the throttle will have some temporary effect on the EGR operation whilst idling.

Personally I wouldn't read too much into differences between requested/actual MAF readings at idle and low load. The important thing to check is what happens under full load over 3000 rpm, ie full throttle run in 4th gear right up through the rev range.

 

[ChrisGTL]

Reporting findings sir:

At 30mph in fourth gear readings were 65mpg and revs were approximately 1200RPM.

Its quite difficult to get it all spot on i.e. 30mph, flat road and constant pressure on the accelerator but I think that my figures are as reasonable as I can get them. Do you want me to provide figures with three attempts and an average if it helps?

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

WIERD!

 

[depresion]

Personally I wouldn't read too much into differences between requested/actual MAF readings at idle and low load. The important thing to check is what happens under full load over 3000 rpm, ie full throttle run in 4th gear right up through the rev range.

I would when a replacement MAF gives the correct readings.

 

[ChrisGTL]

I would when a replacement MAF gives the correct readings.

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

 

[the_fbi]

Interestingly enough, since I filled up again last night, I'm getting a lot closer to my normal MPG.

Probably only 4-5mpg off what it should be. So think it was probably a bad tank of fuel I had, which is now washing through. Hopefully it'll keep on improving back to where it was.