FSI - poor fuel economy after major service

[sailesh]

My FSI generally gets 35 mpg around town, regardless of distance, and 45+ on steady motorway journeys, even up to 55+ at times.

In the past couple of weeks, after having a major service with genuine parts and the N80 valve changed (to a Bosch part), the economy around town has plummeted to 25 mpg at best. When pootling around (typically at 20 mph), even a gentle press of the accelerator results in the live mpg immediately dropping to 10 mpg.

Unfortunately there are no errors reported by the engine (as reported via ColourDIS), so I am none the wiser as to what may have caused this. Is there anything that a major service could have interfered with that has had such a drastic effect on economy, without triggering any engine warnings?

 

[PlasticMac]

My FSI generally gets 35 mpg around town, regardless of distance, and 45+ on steady motorway journeys, even up to 55+ at times.

In the past couple of weeks, after having a major service with genuine parts and the N80 valve changed (to a Bosch part), the economy around town has plummeted to 25 mpg at best. When pootling around (typically at 20 mph), even a gentle press of the accelerator results in the live mpg immediately dropping to 10 mpg.

Unfortunately there are no errors reported by the engine (as reported via ColourDIS), so I am none the wiser as to what may have caused this. Is there anything that a major service could have interfered with that has had such a drastic effect on economy, without triggering any engine warnings?

I've no idea of how the CDIS compares to VCDS, so I'd suggest a scan.
Check engine temperatures, before and after the radiator. G62 and G83.
Mac.

 

[Evripidis]

Hi, use vcds to check the misfire counters and the fuel trims.

They will not generate error codes but will contribute to what you are experiencing.

I am assuming that the ColourDIS project will give you error code description and not measuring block data which is what you need.

It could also be that the garage might have disconnected the battery and it will take a while for the trip computer to readjust.

 

[sailesh]

I’ll have to find someone with VCDS close to SE London..

I don’t think the battery was disconnected during service as the dashboard clock is still the right time, but could of course have been adjusted. I assume ColourDIS calculates the consumption based on live speed (wheel sensors or speedo) and fuelling (ECU), so what would put them so far out of calibration?

I won’t know the real economy unless/until I cover a full tank, but that could be a month or so yet.

For now, I’ve ruled out mechanical drag at the wheels by jacking each corner and rotating the wheels by hand. It is surprising how quickly the front wheels stop spinning (one revolution) even in neutral - I guess that’s the freewheel effect!?


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[PlasticMac]

I’ll have to find someone with VCDS close to SE London..

I don’t think the battery was disconnected during service as the dashboard clock is still the right time, but could of course have been adjusted. I assume ColourDIS calculates the consumption based on live speed (wheel sensors or speedo) and fuelling (ECU), so what would put them so far out of calibration?

I won’t know the real economy unless/until I cover a full tank, but that could be a month or so yet.

For now, I’ve ruled out mechanical drag at the wheels by jacking each corner and rotating the wheels by hand. It is surprising how quickly the front wheels stop spinning (one revolution) even in neutral - I guess that’s the freewheel effect!?


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I'd think the CDIS uses the original MPG data, from the ECU. Wouldn't think it does it's own calculations.
Are you running the standard ECU, or ProBoost?
What is the Engine, (G62), temperature?
Mac.

 

[sailesh]

I'd think the CDIS uses the original MPG data, from the ECU. Wouldn't think it does it's own calculations.
Are you running the standard ECU, or ProBoost?
What is the Engine, (G62), temperature?
Mac.

Standard ECU, no odd symptoms in the way the engine drives, and ColourDIS shows me that the coolant gets up to 110 while the dash gauge indicates a stable 90.


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[Erlingtheyounger]

Standard ECU, no odd symptoms in the way the engine drives, and ColourDIS shows me that the coolant gets up to 110 while the dash gauge indicates a stable 90.


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My 1.2 is hugely sensitive to tyre pressures when it comes to MPG. If you’ve not checked these, do that 1st. Also running CDIS which I know does take all the readings direct from the ECU

 

[sailesh]

My 1.2 is hugely sensitive to tyre pressures when it comes to MPG. If you’ve not checked these, do that 1st. Also running CDIS which I know does take all the readings direct from the ECU

That certainly has an influence at higher speeds, although the target economy (and therefore sensitivity to other variables) for the 1.2 is so much higher to start with. I’ve got a longer run planned at the weekend so will see if that hits the usual range (45-50mpg), and if not, it’s time for more investigation..


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[Daisy May]

This is interesting as I've been trying to find out what my 1.6FSI mpg is, having fuelled today I'm about the same with your pre-service 38 around town and 45 on a longer run on motorways. Now off to check my tyre pressures!

 

[sailesh]

It looks like there’s definitely something amiss, even though no errors are being generated.

A motorway and dual carriageway run at the weekend which previously averaged 52 mpg (according to DIS) only achieved 42 mpg last weekend. So instead of typical urban/extra-urban economy of low 30s and high 40s, I am instead getting low 20s and low 40s.

The main engine-related aspects that have changed since this drop in economy are:

- N80 valve changed from a brand beginning with S, to a name Bosch part;
- Spark plugs as part of major service; and
- Other major service consumables.

OEM parts were used for the service, however is Bosch the OEM brand for the N80 valve? In addition, are there are any engine calibrations that need to be reset after a major service?

Or could it be that something else has become a suspect after the service, e.g. one of the new spark plugs isn’t meeting spec, or the lambda sensor(s) is/are encouraging over fuelling?

There are no other symptoms from a driving point of view, so any other thoughts appreciated!


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[Evripidis]

You have to look at the fuel trims using vcds.

The brand is sagem and bosch parts are usually OEM spec. It is a simple solenoid so not much go wrong.

I cannot remember but have you checked for a blowing exhaust or a stuck brake caliper?

 

[sailesh]

You have to look at the fuel trims using vcds.

The brand is sagem and bosch parts are usually OEM spec. It is a simple solenoid so not much go wrong.

I cannot remember but have you checked for a blowing exhaust or a stuck brake caliper?

Thanks, it was a Sagem N80 valve that I swapped out. As you say, the part should generally either work or not.

The brakes were changed all round recently, and I haven’t found any issues when jacking the car at each corner and spinning the wheels by hand.

The exhaust is apparently heavily corroded and doesn’t sound louder than normal, but I will have it investigated when I get the fuel trims checked..

Btw, I only ever run the car on 99 octane Shell fuel, so at least that’s a constant.


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[PlasticMac]

It looks like there’s definitely something amiss, even though no errors are being generated.

A motorway and dual carriageway run at the weekend which previously averaged 52 mpg (according to DIS) only achieved 42 mpg last weekend. So instead of typical urban/extra-urban economy of low 30s and high 40s, I am instead getting low 20s and low 40s.

The main engine-related aspects that have changed since this drop in economy are:

- N80 valve changed from a brand beginning with S, to a name Bosch part;
- Spark plugs as part of major service; and
- Other major service consumables.

OEM parts were used for the service, however is Bosch the OEM brand for the N80 valve? In addition, are there are any engine calibrations that need to be reset after a major service?

Or could it be that something else has become a suspect after the service, e.g. one of the new spark plugs isn’t meeting spec, or the lambda sensor(s) is/are encouraging over fuelling?

There are no other symptoms from a driving point of view, so any other thoughts appreciated!


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Sounds like it drives much as it did before the service, but uses 15 - 20% more fuel, does that sum it up?
I'd start with a log of fuel trim, Groups 032 & 033, as suggested by @Evripidis
I'd add Group 007 too, check out what mode it's running in.
Does need to be a long drive, but must get hot, and use a good range of engine loads and engine speeds.
Watch these videos, (also courtesy of @Evripidis ), to understand more about Fuel Trim.
Mac.

 

[PlasticMac]

@sailesh
Have a look at this thread.
There are conditions, within the EVAP system, when the charcoal cannister has high resistance to flow, which prevents the engine running in Stratified Mode, so it's always in Homogeneous.
This could be your problem perhaps?
The only way to diagnose this is to log Group 007. Block 4 shows the engine mode. Look for SV, if present, then Stratified Mode is disabled.
Mac.

 

[Indi]

@sailesh
Have a look at this thread.
There are conditions, within the EVAP system, when the charcoal cannister has high resistance to flow, which prevents the engine running in Stratified Mode, so it's always in Homogeneous.
This could be your problem perhaps?
The only way to diagnose this is to log Group 007. Block 4 shows the engine mode. Look for SV, if present, then Stratified Mode is disabled.
Mac.

Stratified mode not working wouldn’t cause such a big difference I don’t think, from what pro boost owners have said it’s a 5mpg difference if not less

 

[PlasticMac]

Stratified mode not working wouldn’t cause such a big difference I don’t think, from what pro boost owners have said it’s a 5mpg difference if not less

Stratified is very lean, compared to Homogeneous, so how much it effects mpg will depend on the engine loads, and so driving style, and route.
I'd think it was worth a look. I'm not saying it is, but that it could be. I've not seen any alternative things to check. It'll only take a drive and log to find out.
Mac.

 

[PlasticMac]

Stratified is very lean, compared to Homogeneous, so how much it effects mpg will depend on the engine loads, and so driving style, and route.
I'd think it was worth a look. I'm not saying it is, but that it could be. I've not seen any alternative things to check. It'll only take a drive and log to find out.
Mac.

And the N80, the EVAP valve, was replaced during the service. Could be coincidence, of course ...
But if the N80 wasn't opening fully, that would add a bit of resistance to the flow, perhaps.

@sailesh why was the N80 replaced?
Mac.

 

[PlasticMac]

@sailesh
Any feedback?
Mac.

 

[PlasticMac]

Purely preventative - just to add, the car passed its MOT after the N80 was changed, and it then had its major service. Would the MOT lambda measurement be out of spec if the economy is out of spec?

I’m currently trying to book in for a diagnosis so hopefully can get to the bottom of this soon. Will share the resolution soon, hopefully?


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If your lowered mpg figure is because the engine cannot run is Stratified Mode, due to resistance in the EVAP circuit, I don't think there is any way, other than to log Group 007, and check for SV in Block 4, to confirm that, (or rule it out, of course).
The Lambda reading will be fine, the SV routine is specifically to keep emissions within spec, it's just that the engine is running in Homogenous Mode all the time, (instead of only at high engine loads), which will give poor economy. The whole raison d'être for Stratified operation is low consumption, low emissions. and high efficiency, at lower engine loads.
I am not suggesting there's anything wrong with the N80. There's no DTCs, so everything, (including the EVAP circuit), is working as it should. The inhibiting of Stratified Mode, (if that's happening), is a normal function, to deal with condensate in the charcoal canister, which will happen naturally, hence there's a routine in the ECU to detect it, and deal with it. Condensate in the EVAP circuit is not a fault condition.
Personally, I'd check Group 007, Block 4, before starting expensive diagnostics. If SV comes up, you'll know where to start your diagnostics. If it doesn't, your diagnostics will start elsewhere. When you have mysterious fault, ruling out possible causes is the only thing you can do. With luck you'll find the solution on the way.
You can use the free version of VCDS Lite, as that gives access to Groups 001 to 025. So apart from your time, and the fuel used, it's a very cheap bit of diagnostics!
Resistance in the EVAP circuit may not be due to condensate of course. After twenty years of petrol vapour going through it, I wouldn't be surprised if it wasn't quite as squeaky clean as it once was.
Good luck.
Mac.

 

[PlasticMac]

If you do decide to make a log of Group 007, (and I really hope you do), you'll need to make sure you get the engine, and, most importantly, both Catalytic Converters, (the CO and NOx), up to temperature.
In a previous log, with the SV symptom, it took about 12 minutes of driving, before Stratified Mode was first enabled. Over the next few minutes the ECU enabled Stratified several times, but after a few short periods in Stratified, the ECU switched to SV.
(SV probably = Schichtung Verboten).
The engine starts, and runs in Homogenous, as is normal, until Engine Temperature and Load conditions suit Stratified Mode.
In the graphic, the yellow trace shows the initial warm up in Homogenous, then the short burst of Stratified, then the rest of the drive is permanently in SV Mode, which I believe is triggered by resistance, (not a complete block), in the EVAP circuit.
Mac..


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