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post #16 of 19 (permalink) Old 28th March 2007
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Originally Posted by hidnmine View Post
I don't agree that the ecu has to know precisely how much fuel is flowing to set the fuel air ratio correctly. So long as it has a rough idea, the precise adjustment is done based on the Lambda sensor output (i.e. Lambda sensor says lean mixture; lengthen injector on time etc). The mpg computer must do its sums based on the standard calibration values of the fuel pressure, temperature etc. Any inaccuracies in these sensors will inevitably give an mpg error.


So that's what the lambda sensor does.
Is there one of these per cylinder? if not how does the ECU know which injector on time to lengthen?
My point is that the injectors need to be calibrated or atleast matched (to a degree). Can one injector be replaced or must you alway replace them together to ensure they are still matched?

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post #17 of 19 (permalink) Old 28th March 2007
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Hi Eeps - as far as I know mass produced cars don't have a Lambda sensor for each cylinder and usually rely on one or possibly two to measure the exhaust gases. And yes ideally all injectors should be replaced at the same time - just like you should renew all the spark plugs and coils similtaneously.

P.S. Does anyone actually know what data the on-board computer or the ECU gathers to make its calculations or are we just guessing.
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post #18 of 19 (permalink) Old 29th March 2007 Thread Starter
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I'm beginning to think it's tied in to the average speed calculation.

Technical data will specify a return of 48mpg based on a steady 56mph for example, so I'm guessing it translates the average speed into consumption from a reference table, and nothing more - this would explain it's innacuracy but it would also give a truer average consumption as well as it's not just about the distance you've covered, it's about how quickly you've covered it - I don't think it can gauge fuel consumption this accurately which is why the figures don't tally.

Tomorrow I will have a new theory, and eventually we will stumble across the answer. Sadly we'll probably be completely unaware of the fact, or at the point of not particularly giving a monkeys cuss any longer, either way, it's fun to speculate.

1 rep point to anyone who's theory involves naked women and natural yoghurt.
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post #19 of 19 (permalink) Old 29th March 2007
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I think that if you get a naked woman to rub natural yoghurt over the dashboard it will revert to what you expect to since my Scenic went in for a recall to do with updating the ecu I only seem to be averaging 40.1mpg...dunno if that has anything to do with it. By the way it was only serviced less than a week before the recall and the air and pollen filters were changed (supposedly)
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