Hi guys,
Just wanted to pick peoples brains about the drop in economy that I've noticed with my passat over the last couple of months.
I noticed that the mpg figures on the trip computer were down just before the car went in for it's 60k service and cam-belt change. This has happened in the past and the mpg figures usually increase following the service (usually the same with performance). My usual figures were from high 40's up to a max of 55mpg. Most of my journey each day takes me along the motorway and I tend to drive the car pretty enthusiastically.
The figures have since dropped to high 30s to a max of about 47mpg (taking it really easy) but I've not noticed a change in performance.
I have changed the fuel filter, air filter and pollen filter for non oem parts but all are Mann products (parts were previously oem). I was also using synthetic oil (left over from when it was on a long service interval) and have since changed to semi-synthetic (fixed service interval) - correct VW spec. Could all of these changes had an affect on the economy figures? Or could it be somethging else like the cold weather?
Don't think it is the MAF as I still have the performance - but could be wrong!
What do you guys think?
Cheers
R
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Previous mpg figures are beyond my experience in the 100hp PD TDI Passat Estate I own.
I get 42mpg at motorway speeds and 44mpg overall on my hilly commute. Round town I get around 37mpg. Never had better in the 110,000 miles so far! I moaned several times to various garages without a result.
Think carefully about all the changes you've made from the standard specification and reverse them one by one.
Sorry that I can not be more positive.
I get a slight decrease in mpg in cold weather -but I put it down to winter fuel formulation, oil being thicker for longer, extra electrical loads etc.
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The reduction of economy in cold weather is also, and probably largely, due to the increased density of the air resulting from the lower temperature. To provide a given mass of fuel for a given mass of air means more fuel as the air mass is heavier. I reckon this to be worth a reduction from nearly to 60 mpg in summer to around 55 mpg in winter but I agree that there are other factors at play.
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I've got a car similar to yours and today after a 15 mile drive, quite fast, I got 42 mpg which I reckon is OK.
On a long run, again going briskly, it usually does 45-48 mpg. Once, on holiday in France, going slowly it managed 51 mpg over 150 miles. I have never been able to get 55 mpg in mine, but after having a Sierra 2.3 petrol the Passat seems very frugal, in fact it will do double the milage on a tank and goes much faster.
As I may have said in the past a couple of times, it is a fantastic car! Much better IMO than certain blue oval badged cars, but keep this quiet or there could be trouble ahead!!
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The reduction of economy in cold weather is also and probably largely due to the increased density of the air resulting from the lower temperature. To provide a given mass of fuel for a given mass of air means more fuel as the air mass is heavier. >>
But AS the engine will run more efficient the dense the air is, hence the need for an intercooler.
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The MAF is quite common for effecting fuel economy without effecting performance & quite often doesn't show up on a diagnostic check iether. There was a guy who has a VW/ Audi site and has published AFM figures for a healthy AFM & makes interesting reading. he gives very specific window's for readings but you have to understand that if the manufactures put these narrow window's in the man light would alway's be coming on. I know from dealing with customers before with 1.9s that if MPG drops for no apparent reason the AFM is the most likley cause. Normally the drop's went from 53-55 doen to 37-39.
Hope this helps
Rgds Inj Doc
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Thanks for all the responses so far. Injection doc, I was wondering whether a go at the AFM with electrical contact cleaner (?) is worth a bash? I think I've read somewhere that this is an option. Or should I keep an eye on things until they get much worse and then replace the AFM with a new one?
Cheers
R
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