Fuel economy problems...any suggestions appreciated

[Tigger]

Hi All,

I have a strange question.

I have NEVER been the type of guy that worries about fuel prices, or shops around for cheap stations etc...
Also, I have never really cared about the mpg of my car...

BUT over the last few months I have noticed something odd:

(I have had the car for three years, and it is running really well. Properly serviced, and used as a daily driver.)

I would fill up, and the distance to empty would ALWAYS show 660kms....like pretty much always.

BUT in the last few months it has shown 550kms....then, 500kms...then 480kms after filling up yesterday!!!

Now I don't baby the car, but I don't thrash it either. I drive normally everyday.

It doesn't feel like its running rough...it is running perfectly well in fact.

Any thoughts?

[wfdTamar]

Gunked up throttle body or one of the sensors? Have you checked for error codes?

[Antiplastix]

I d check what the actual consumption is, not what the computer tells you it is.

[Gerrycan]

One of the reasons I record my fuel consumption in Fuelly.com is to confirm the engine is still performing ok.
Sure it varies between fills but nothing I cannot account for through factors like type of motoring, season, who has driven it etc.

Has your actual consumption deteriorated and reflecting the predictions?

What sort of distance have you done, perhaps it is carbon build up on the inlet valves?

[Tigger]

Thanks for the replies and suggestions.

I have checked the actual figures, not just the cars computer, and it is indeed using more fuel than usual from the past...
I will plug it into a computer and check for error codes.

If there was a blockage or build up etc, wouldn't that affect the way it runs or idles etc?

[Martin]

It can be an efficiency issue rather than incorrect running

If you haven't cleaned the inlet yet that could be it - you've done enough k's to need it
A give away is leaving the car for 3 or 4 days unused, if it starts up and runs rough for a short time that would indicate a lot of carbon build up in the inlet

[afterdarker]

I habitually turn the trip meter to zero every time I fill up the tank- a good habit to get into in my opinion - I have to do it on my motorbike which doesn't have a fuel gauge, only a fuel warning light. Resetting the trip meter each tank gives you a good 'seat of the pants' feel for fuel consumption and you know pretty well immediately if something is amiss.

[brad]

Go for a drive - 20-30m at a minimum (constant) 4000rpm. You've got to keep the revs (and heat) up above 4k). If the economy improves for a few weeks after that then it's probably inlet gunk.

You might try a chemical clean after that but generally the inlet removal & manual clean is the best solution.

It won't help the inlets but try a bottle of Wynns Complete Fuel System Cleaner - it might just help the injectors a fraction.

[notflyingscot]

Go for a drive - 20-30m at a minimum (constant) 4000rpm. You've got to keep the revs (and heat) up above 4k). If the economy improves for a few weeks after that then it's probably inlet gunk.

You might try a chemical clean after that but generally the inlet removal & manual clean is the best solution.

It won't help the inlets but try a bottle of Wynns Complete Fuel System Cleaner - it might just help the injectors a fraction.

Are there no 21st century equivalents to the 1950's Redex which I still recall my father and his contemporaries either adding to the tank or squirting into the carburettors? Was a pretty instant partial de coke as I recall. Never quite the same as lifting the head and cleaning manually so suppose in many ways nothing has changed. Maybe these 'old' solutions have no place with modern EFI and ECU controlled engines?

[Gerrycan]

Are there no 21st century equivalents to the 1950's Redex which I still recall my father and his contemporaries either adding to the tank or squirting into the carburettors? Was a pretty instant partial de coke as I recall. Never quite the same as lifting the head and cleaning manually so suppose in many ways nothing has changed. Maybe these 'old' solutions have no place with modern EFI and ECU controlled engines?

Anything you add to the fuel tank gets injected with the petrol directly into the cylinder (direct injection) bypassing the inlet manifold and valve stem where the carbon accumulates. All the petrol mk2 models were DI as are the mk3 except for the 1.8 and vRS which have both direct and port injection systems. The latter system has at least a chance of 'wetting' the inlet valve stem and doing something to remove the carbon build up there which is the main issue.

There are chemical (including the Subaru ) products that you spray into the inlet manifold after warming up the engine leaving it a while and then revving the loosened gunk and material through the exhaust. Worries me a bit what it would do to the catalyst on the way through
I remember GTR27 related a story where he used something similar on his totally gunked up motorbike and expelling a whole load of black soot and chemicals onto his driveway. Ran better after though.