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shindigger

Car Serviced. 25% Hit To Mpg On On Board Computer

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Had my 09 1.6 petrol Focus serviced locally before xmas. Just done a 150 mile round trip to Devon.

Car normally reports 45-47 on a run. Now 33ish for duration of said trip.

Car had new plugs, front pads and oil and filter done.

The commonest causes i can see online are the following:

Wrong oil?

Overfilled oil?

Car is currently on the flat outside my house and in an hour i will check the levels.

The pads don't appear to be binding.

Could it be cheap or incorrectly adjusted plugs? Car goes ok, (doesn't pink etc) but certainly no better than it did before, and original plugs had been on there for 30k. First time ive gotten in a car after a plug change and felt no increased responsiveness. Was a tad surprised by that.

Im not a happy bunny.

Is there anyway to verify oil viscosity?

My car uses 5W30.

TIA.

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You may find they have unplugged some kind of sensor either deliberately, as part of doing the job, and not reconnected properly or disrupted accidentally. MAF is a common one, after a service, that will hit MPG negatively and may not light the EML immediately.

Getting it plugged in to a diagnostics computer is the best idea to avoid faffing with all sorts of red herrings.

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Interesting. I had my car serviced in September. In October I drove across Spain on a trip I do regularly. The mpg was around 8 - 10 less than before. I put it down to Volvo adjusting the onboard computer to reflect reality following the Volkswagen scandal. Any thoughts?

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Interesting. I had my car serviced in September. In October I drove across Spain on a trip I do regularly. The mpg was around 8 - 10 less than before. I put it down to Volvo adjusting the onboard computer to reflect reality following the Volkswagen scandal. Any thoughts?

Thats the first thing I thought of after reading this thread..all computers reset to reality?

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Check tyre pressures.

Also I don't go by MPG (my car doesn't have one), I just use the mile counter. I can usually get 350miles out of a tank.

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Oil level - you can check. Wrong viscosity wouldn't make that much difference.

Binding brakes: drive car for a few miles, respectable speed, don't hammer the brakes. Coast to a halt in a lay by. If any of the wheels are hot then potentially they are binding. (Did they fiddle with the handbrake?).

Plugs - if the car goes ok, then they are probably fine. Missing at idle and the like points to something wrong.

If all that fails, borrow a fault code reader and check for fault codes.

Beyond that, you're into sticking a wideband o2 sensor ahead of the catalyst and seeing what is happening.

For that sort of drop I'd be suspicious of plugs, coil packs and the like, but it would be running like crap at idle.

What is the actual MPG? Ignore the dash readout, what are you getting for a tank? (I always reset the trip when I fill up). They might have dome some sort of update to the ECU - dealers do all sorts of weird shit to modern cars, and the dash readout may be more/less accurate.

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I wouldn't think they've done any sort of deliberate upgrade but you can get changes after resets of various stored data and service resets. I wouldn't expect them to be enormous and should go back to usual after a reasonable period of driving.

Any significant deterioration in MPG and I would be looking at reasons why the vehicle may be overfueling like the car thinking the engine is cooler than it really is and chucking too much fuel in or oxygen sensor problems making it fuel incorrecty, that type of thing.

Edit to add: By saying locally assumed this isn't main dealer, if it is main dealer the possibility of some sort of update when plugged into the computer is much more likely.

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Thats the first thing I thought of after reading this thread..all computers reset to reality?

My computer tallies to my actual as I always get a similar number of miles per tank so I've checked the calculation and it's correct; surprising really as it's VWAG.

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Veery intersting replies. Thanks. Hadnt occurred to me that they would have dialled in some reality to the situation.

I should have mentioned, i went to see the mechanic when i got back today, and car is going in on the diagnostic machine on thursday.

I have only ever used it as a reference point and would never swear to the accuracy of the OBC .

Of course he tried to suggest that i should do it on a more mathematical basis and i dont disagree with that,but the mere fact that there is now such a big discrepancy can't go overlooked.

Tyres are all fine. I check them ever 2/3 weeks and they were new all round 6 months ago.

Im a tad narked that the car doesnt show any improvement from the new plugs too.

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I know you said tyres were changed months ago, but new tyres will drop mpg temporarily.

Thanks but this seems to be a very black and white, pre and post service situation.

God knows what they've done.

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There's either something very wrong with the way the computer is calculating the economy, due to some messing with the computer, or very wrong with the car, due to some messing with its engine.

It's fair enough for them to say the dash display may not be 100% accurate but suggesting a 1.6 Focus is doing 33mpg isn't really an acceptable margin of error.

If it is the car I would be surprised if it was something like tyres or brakes at that sort of percentage MPG drop I think there would be some very visible signs.

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Just checked dip stick by torch light. ooer etc etc. So not ideal.

It does look a tad (3-4mms) over the top mark. Car has been stood for about 45 minutes since moving it to level ground.

So i'd guess theres still more on the way back to the sump. Could be it.

The main dealer actually did over fill on its first service 6 years ago, so there maybe some actual spec confusion going on here.

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There's either something very wrong with the way the computer is calculating the economy, due to some messing with the computer, or very wrong with the car, due to some messing with its engine.

It's fair enough for them to say the dash display may not be 100% accurate but suggesting a 1.6 Focus is doing 33mpg isn't really an acceptable margin of error.

If it is the car I would be surprised if it was something like tyres or brakes at that sort of percentage MPG drop I think there would be some very visible signs.

Apart from it not showing any improved responiveness as i'd hoped, its going much the same as before it went in.

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You may find they have unplugged some kind of sensor either deliberately, as part of doing the job, and not reconnected properly or disrupted accidentally. MAF is a common one, after a service, that will hit MPG negatively and may not light the EML immediately.

Getting it plugged in to a diagnostics computer is the best idea to avoid faffing with all sorts of red herrings.

This MAF ^^^ is where i would be checking first

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After a service?

Not directly after a service. I think you'll need to give it time to settle again to see if this a one off or a consistent change. Even down to a poor tank of petrol.

In general though I'd disagree with whole "car is better after a service meme" if the car is otherwise running correctly.

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I honestly don't think it's the oil level you'd need a lot more overfill than that to cause problems the worst being the crank rotation effectively being impeded by the overfilled oil in the sump which I really don't see happening without it a lot further up the dipstick.

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I honestly don't think it's the oil level you'd need a lot more overfill than that to cause problems the worst being the crank rotation effectively being impeded by the overfilled oil in the sump which I really don't see happening without it a lot further up the dipstick.

Open the oil cap and if it's filled to the top you've got problems :lol:

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Not directly after a service. I think you'll need to give it time to settle again to see if this a one off or a consistent change. Even down to a poor tank of petrol.

In general though I'd disagree with whole "car is better after a service meme" if the car is otherwise running correctly.

Yes. I'm hoping a bit more of a run will normalize things.

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Not directly after a service. I think you'll need to give it time to settle again to see if this a one off or a consistent change. Even down to a poor tank of petrol.

In general though I'd disagree with whole "car is better after a service meme" if the car is otherwise running correctly.

Depends. New air filter, for example, if the old one was clogged, should be noticeable. Varies a lot some vehicles the air filter will look nearly new at the recommended service interval and others completely suffocated.

New plugs, leads, coil packs etc. should yield a noticeable improvement assuming you're not just throwing parts at it trying to fix poor performance and everything else is as it should be.

A favourite is to replace plugs and leads assuming they're the culprit for sluggish performance only for it to be a MAF sensor that's seen better days. A lot of sensors on modern vehicles ought to be seen as mileage based service consumables really and it's the fact they are malfunctioning means anything else you do, new parts wise, appears to make no difference to the vehicle running.

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I honestly don't think it's the oil level you'd need a lot more overfill than that to cause problems the worst being the crank rotation effectively being impeded by the overfilled oil in the sump which I really don't see happening without it a lot further up the dipstick.

Righto. Cheers.

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