Fuel consumption

[johnthomas99]

Hi all, looking for some advice....

My 05' Fab Vrs 1.9tdi (ASZ eng) appears to be using more fuel than I expected. I have carried out 2 x full to full fuel tank refuels and I have calculated it is doing 32mpg under steady, mixed driving. This appears a lot lower than I expected. It's done just over 100k and has full service history, and runs very well. Engine is completely standard as far as I'm aware. 

The only issue I have noticed is that is chucks out a lot of black smoke when under full throttle and has done for 6 years of ownership. Any thoughts or help would be appreciated. TIA 

[sepulchrave]

Replace the MAF.

[johnthomas99]

Thanks for your response Sepulchrave. Is the MAF sensor easy to do and why does it cause these issue? Thanks. 

[sepulchrave]

7 minutes ago, johnthomas99 said:

Thanks for your response Sepulchrave. Is the MAF sensor easy to do and why does it cause these issue? Thanks. 

 

Extremely easy to do and like all analogue sensors the the output gradually becomes less accurate over time until you notice how bad the fuel economy has become.

[johnthomas99]

That's brilliant, thank you. I hadn't even considered this as the cause. I'll replace this as soon as I can purchase the part. Many thanks. Appreciate your help. 

[SuperbTWM]

You might have a boost leak, black smoke is a sign of a fuel rich mixture, its worth having a really good visual of all the hoses and pipes.

 

You could try and unplug the MAF and see if that helps the fuel economy figures over a tank of fuel, if it doesn't improve things then there is probably not much wrong with the one you have.

 

Also, what sort of lengths of journeys are you doing, 32 MPG is low but if you are only doing short journeys it could be normal.

[KeithCheetham]

1 hour ago, johnthomas99 said:

"I have carried out 2 x full to full fuel tank refuels and I have calculated it is doing 32mpg under steady, mixed driving."

"6 years of ownership."

 If you have owned it for 6 years, how bad is this economy compared to what it has been over those 6 years, and is it a recent issue or a gradually appearing issue. Also, you started a thread on possible boost leak back in July with no close out - did you find a leak and did you fix it as one of the replies here suggests possible boost leak for the smoke.

[johnthomas99]

Thanks for the replies. Yes, the boost leak was my first port of call but I haven't found any issues with the pipes although I had considered replacing them just to be sure. (Expensive business) The car does shortish journeys but is driven gently and mostly dual carriageway to work and back. Consumption issue has got gradually worse, although its stating 44mpg on the dash but I'm defo only getting around 32mpg. 

Ref the Maf sensor....Can I get away with an aftermarket part or is genuine recommended? 

 

[nta16]

Please note - I am not a mechanic or expert in anything.

 

3 hours ago, johnthomas99 said:

The only issue I have noticed is that is chucks out a lot of black smoke when under full throttle and has done for 6 years of ownership.

 

54 minutes ago, johnthomas99 said:

The car does shortish journeys but is driven gently and mostly dual carriageway to work and back. Consumption issue has got gradually worse 

 

You have a diesel and going on shortish journeys driven gently and mostly dual carriageway - not good use for a diesel (or petrol) , it's a VRS so giving it some blowout runs as a VRS would be to get would help.

 

How often is the engine oil& filter changed, quality of oil, how often the air filter, ever put the "cleaner" diesel fuel in.

 

I cleaned a mate's (Citroen) 100k-mile diesel MAF (and a bit of the inside of the air intake trunking), as with anything diesel it was a bit of a black ****ty business, the air filter had been changed at the previous service.  Just doing that did improve things.  With replacing the MAF sensor just get a good one, personally I'd always go with Japanese over German brand name, unless the brand name sticker could be peeled back to reveal the part is actually Japanese.

 

 

54 minutes ago, johnthomas99 said:

although its stating 44mpg on the dash but I'm defo only getting around 32mpg. 

🤣  You really believe a VW computer about such things.  🤣  Being serious it shows variances in computer programs and the information the sensors can give so perhaps if a good quality MAF sensor isn't expensive then it might be better replaced if it's got too inaccurate.  My mate's Citroen is only a runabout not a sports model (not that his sports models get enough maintenance servicing for me liking).

 

Edited 30 December, 2022 by nta16

[sepulchrave]

3 hours ago, johnthomas99 said:

Ref the Maf sensor....Can I get away with an aftermarket part or is genuine recommended? 

 

Stick with Bosch if possible.

[cello]

Poor fuel consumption , check the brake calipers (or the discs after a very short drive). If you end up with a dirty and burnt finger when touching the discs then you have a sticking / seized brake caliper.

 

 

[johnthomas99]

Again thanks for the replies....

I realise short journeys are going to reduce the Mpg but it is mainly dual carriageway work so I guess it's not too bad. Oil and filter changed every 6 - 8 months and air/fuel filters changed around a Yr ago. 

I have gradually noticed a drop in mpg over a couple of yrs and I suppose but doing the tank to tank test has confirmed its fairly low mpg. 

I'll go for the Bosch MAF as you recommend as I think it's worth paying the extra for peice of mind, it's such a great car. 

Thanks for all your help, I'll crack on and replace the Maf and see how things are after.

[johnthomas99]

Rear brake calipers are fairly new as are the front. (312mm front brake upgrade done recently) so brake are all nice and free. Thanks. 

[nta16]

Please note - I am not a mechanic or expert in anything.

 

5 hours ago, johnthomas99 said:

but it is mainly dual carriageway work so I guess it's not too bad.

Shortish journeys driven gently and mostly dual carriageway may be good for fuel economy but it  won't be a blowout or higher temperature run.  Good and interesting A, B and unclassified roads where you can use more of the gears and rev range would be better, isn't that what the VRS badge is about.

 

Unless the factory fitted MAF sensor has with time been found not to be the best I'd match [ETA: the new sensor to factory] exactly (even if it's Bosch but I'd see if the label lifts off) I'd not buy a 'matching' or 'compatible' alternative even if it's the same label as fitted now.  If the MAF is easy to photo I'd do that and check that it's factory original and check back here for advice if necessary.

 

Good luck.

 

Edited 31 December, 2022 by nta16
ETA:

[sepulchrave]

@johnthomas99 I have absolutely no idea why everyone else here is trying to talk you out of replacing an inexpensive worn out part which is known to cause a deterioration in fuel economy on PD engines with no other symptoms.

In my considerable experience there are various analogue sensors whose accuracy drifts badly over time, the ECU will not flag a fault until these measurements fall outside accepted parameters, in the case of the MAF this generally means the sensor is reporting greater air density than reality and the ECU is therefore erroneously adding additional fuel to compensate, hence the reduction in economy.

Since you don't have a boost leak but you do have black smoke and a drop in MPG then there are very few other explanations that fit the symptoms and the raft of rampant guessing above really doesn't add much clarity.

[nta16]

Please note - I am not a mechanic or expert in anything.

 

Just in case I've caused any misunderstanding, or as often happens I've not made myself clear, I'm not suggesting an inexpensive worn out part shouldn't be changed  but just to make sure the right and best one is fitted as a replacement.  On a car of this age unless I'd owned it from new I'd not just assume the part fitted is original or best suited - but it could be one or both, I'd just check.  I've had lots of  expensive and very annoying work from inexpensive parts, and just because a part fits and works doesn't mean it's working as well as it could or should, as in the present MAF sensor.

 

And I'm certainly not against changing an inexpensive part to save hassle and getting dirtier than I need to but I'd want fit and forget, the part to be fully working as it should removing hassle not contributing hassle by giving wobbly info for the computer programs to play around. 🙂

 

Edited 31 December, 2022 by nta16

[johnthomas99]

Thank you all. Appreciate your advice. 

[johnthomas99]

Just an update on the fuel consumption issue..... I have fitted a MAF sensor (Bosch) and the smoking issue, under hard acceleration, which has always been an issue, has disappeared completely. Also the fuel economy has come back. I also seems to run a lot better. Thank you for your advice. 

[999spender]

Can anyone supply part number for a MAF for a 2003 BLT engined VRS?

TIA.