Fuel Measurement

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upanddown22
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Fuel Measurement

Postby upanddown22 » Wed Oct 21, 2009 2:10 pm

Done a search for this, could be blind but couldn't see anything that covered it.

I've been monitoring my car over a few tanks of diesel to see how accurate the computer is. It seems to be completely rubbish! Did a 550ish mile round trip to the peak district at the weekend computer said i'd used 8.5 gallons but when I went to fill up I managed to put in 10.4 gallons!!! That's a hell of a difference! And I've had a similar thing on two refills previously. It's slightly worrying as the car thought I could still get another 250 miles out of the tank!

How come it's so far out? What is it that measures how much fuel the engine is using?
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Gav
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Re: Fuel Measurement

Postby Gav » Thu Oct 22, 2009 11:49 am

I've regularly seen errors between the mpg on the trip computer compared to a spreadsheet of the fuel put in and milage driven. Usually within 3mpg either way. I think that the trip computer takes it fuel usage from the ECU (but it might not, I'm not invloved in the motor industry).

I know on my facelifted version I needed a fuel return replaced under warranty as it was leaking at about 20 months old. One longer run when it was at its worst I got 50mpg on my spreadsheet, and ~58mpg on the trip computer. I wonder if the computer thought unburnt fuel was being returned to the tank, when it was actually dripping onto the starter motor and road below. So possibly you have the start of a leak - after a run (especially if you acclerate and then lift right off, so the engine has excess fuel that it needs to return) you might get a slight diesel smell, and of course have a look for signs that any diesel is leaking out.
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upanddown22
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Re: Fuel Measurement

Postby upanddown22 » Thu Oct 22, 2009 3:58 pm

Cheers Gav, seems like a sensible suggestion. I'm taking it in to get serviced next Tuesday so I'll mention this as a possibility.

Any ideas where the ECU gets it's reading from? I think that would be my next place to check.
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Gav
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Re: Fuel Measurement

Postby Gav » Fri Oct 23, 2009 11:38 am

upanddown22 wrote:Any ideas where the ECU gets it's reading from? I think that would be my next place to check.


My guess is that the ECU knows how much fuel has been used, as it has controlled the ammount of fuel being pumped into the common rail. So just keeping a total of the volume I suppose. I could be wrong as I've never worked on this kind of thing, but am an embedded software developer, so hopefully would understand what the ECU actually does...
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upanddown22
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Re: Fuel Measurement

Postby upanddown22 » Thu Oct 29, 2009 9:50 am

Car went in for a service on Tuesday, the guy couldn't find anything wrong with the fuel system which is quite disappointing in a way! No leaks or anything.

Guess it must just be due to errors in the readings, it's a big difference though at 25% more fuel being used than the ECU reckons. Oh well! I'm still getting 55mpg so not all bad!
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Gav
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Re: Fuel Measurement

Postby Gav » Thu Oct 29, 2009 12:34 pm

upanddown22 wrote:Guess it must just be due to errors in the readings, it's a big difference though at 25% more fuel being used than the ECU reckons. Oh well! I'm still getting 55mpg so not all bad!


I'm guessing that you're already using good fuel (non-supermarket) that contains detergents to keep everything clean (and/or an additive like Millers), good oil, and have tyres properly inflated. Earlier this year I needed new tyres to get through the MOT while I was unemployed, so got cheaper tyres (Goodyear Excellence) rather than refitting Michelin Energy's - I'm now paying for that with worse consumption (although I now have a much shorter urban commute) which is noticably worse on the motorway. Next time I'll be going back to Michelin.
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upanddown22
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Re: Fuel Measurement

Postby upanddown22 » Fri Oct 30, 2009 9:49 am

Didn't realise the supermarket fuel made such a difference so will switch to Shell and see how I get on. Tyres are Pirelles (sp?) but not sure which, will possibly look at changing them once they wear out.

Even so all this things shouldn't affect what the computer is actually reading in terms of fuel used?
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Gav
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Re: Fuel Measurement

Postby Gav » Fri Oct 30, 2009 12:41 pm

I wonder if an injector or whatever was blocked slightly then the ECU may mis-calculate how much fuel to pump in???

The only time I tried supermarket diesel I had noticably worse efficiency, although a single tank is a very small sample to go on. I always use Shell and every so often use V-power, but don't notice much change over the regular diesel.
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upanddown22
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Posts: 36
Joined: Thu Oct 01, 2009 11:06 am
Currently Drives:: Megane 1.5DCi Dynamique 2003

Re: Fuel Measurement

Postby upanddown22 » Fri Oct 30, 2009 1:01 pm

Quite possibly, hopefully the service may have cleared it out. I've got a 200 mile run tomorrow so will see if the results are any closer!

tbh though the fuel economy isn't that bad, got 63mpg out on a 200mile run a couple of weeks ago, but the computer was reading 71mpg!!! Guess I just want to be able to rely on the computer a bit more rather than get the mpg up much.
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nickbell69
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Re: Fuel Measurement

Postby nickbell69 » Sun Nov 01, 2009 3:32 pm

upanddown22 wrote:Didn't realise the supermarket fuel made such a difference so will switch to Shell and see how I get on. Tyres are Pirelles (sp?) but not sure which, will possibly look at changing them once they wear out.

Even so all this things shouldn't affect what the computer is actually reading in terms of fuel used?


I'd argue about supermarket fuel, i drive approx 26k a year and fill up at supermarkets 95% of time, have tried others shell etc and never found any difference in mpg whatsoever. Car is on 110k of which i've done 100k of those and never a problem with the engine.


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