Fuel consumption conundrum

[jimgironde]

My passat 140 tdi 57 plate has a massive problem with fuel consumption.

I have had the car since new and averaged 45mpg measured pump to pump and records kept.

Had the fuel injectors replaced last September and economy got slightly worse to 42mpg.

Over the last couple of weeks this has crashed to circa 35mpg???

Car has been serviced religiously by the main dealer at each10k and 2 months ago had its 50k service. Air filter was changed at 40k. The car, as always, runs sweet as a nut with no apparent problems, no warning lights etc. For the last couple of years I have added Millers at fill ups. No smell of diesel or signs of leakage. Non DPF model.

When the injectors were fitted the replacement wiring loom failed within 48 hours and replaced again. At the same time I had 2 new front tyres fitted and wonder whether that accounted for the drop from 45 to 42 mpg.The replacements were same spec Bridgstones as the ones that came off. I wouldn’t have thought so.

But this latest increase in consumption is too big to ignore, therefore 2 questions.

Firstly, what factors could be causing this. Is there any self diagnosis that is easy to do.

Secondly, assuming no fault codes, what approach will a VW dealer take apart from presenting me with a sizeable bill?
I have no probs paying to put something right but the thought of paying for investigative work that bears no fruit does worry me:(

[Teflon]

Apologies if this suggestion is a bit of an insult, but sometimes the easy ones are the problem. Tyre pressures? Or have you switched to running around with a full tank after the recent goverment stuff up over the fuel strike? Under inflation or extra weight are killers for mpg.

[jimgironde]

thanks for the suggestion Teflon. Tyre pressures are fine and my tank filling habits haven't changed.

[rapport25]

Teflon is right.

As I do a lot of mototway miles My tyres are over inflated by around 2psi all round and filled with nitrogen to help mpg and wear.

Dont forget outside temp can make a huge differance to mpg.

Hths Rappy.

[jimgironde]

My 140tdi is the estate version and I always run 38/43 front/rear (do a few wine runs to France!!!!). I would expect the consumption not to increase by 20% now the weather has got a bit warmer?

[::SPG::]

jimgironde, I had my injectors replaced a few weeks back under the warranty and for the first few days I thought I was getting lower MPG (I always drive with the average MPG up) on the same drive to and from work. I will watch my overall average which has been around 47MPG (I have been taking it steady), if that starts to drop with no change in driving patterns I'll worry. I find it hard to diagnose in a short space of time, if I get stuck behind a lorry I get 53, then I have a hot lap home I get 35 . . but over the longer period it all averages out. I suggest not trying to watch too closely every day and see what happens to the overall average.

[zollaf]

it could also be a amm playing up, or a cts, so i would get it read for codes and live data checked by someone experienced to see if there is any underlying fault. should only cost an hours time for an initial check over.

[jimgironde]

it could also be a amm playing up, or a cts, so i would get it read for codes and live data checked by someone experienced to see if there is any underlying fault. should only cost an hours time for an initial check over.

but I thought there are things that can go wrong with no fault code. One I know of is the EGR valve.
I had a 407 when it first came out and had terrible problems with consumption. I took it into the dealer quite a few times and they would hook it up to their system and tell me there was nothing wrong. It kept getting worse until the valve failed and had to be replaced which cured the problem.
It is these sorts of items I am looking for prompting on and any easy way of eliminating them.

[zollaf]

there are things that can fail and not log a code, the amm being one of them, but an experienced mechanic can then look at live date readings and see what everything is doing. a cts can throw a wobbly and tell the ecu the engine is only at 50 degrees when it is actually at 90. that wont log a code but will cause extra fuel to be used. the amm can fail and show a false reading. again no code will be there but if its stuck on 4kg/min, (as landrover td5's do ) the experienced person will spot this. the person that doesn't know how to interpret live readings and just reads fault codes is not going to find all the faults .

[martin1810]

The B6 passat is surpisingly self aware. It can spot failing CTS, AMM and EGR. The readings from them are checked against stored maps for specific conditions. If the value doesn't agree with the stored data you get a fault code logged. Not all faults put the fault light on though. As zollaf says, the best thing is a live data read checked by someone who knows the systems.