"Dave Plowman (News)" wrote in message
...
In article ,
Trevor Wilson wrote:
**As I understand it (please note, I am not speaking from any real
authority here) no injector is perfect. Some are clogged a little. Some
are clogged a lot. Some are not clogged at all. Pumps are subject to
wear.

Yehbut EFI systems are closed loop and if the injectors are slightly out
of spec, the ECU will compensate. If they're wildly out an inaccurate OBC
will be the least of your worries...

**OK, here's where my ignorance begins to show. EFI systems are sort of
closed loop. However, under certain conditions, that may not be the case. At
startup, for instance. Under very heavy acceleration, for another. There are
probably other conditions where the system is not closed loop. So-called
'limp-home mode' would be another. I'm not well versed in automotive issues
to comment with great authority. I do understand a little, however.


As a consequence, the BEST way to measure actual fuel
consumption, is to measure the amount of fuel used, rather than
inferring how much fuel SHOULD be going into the cylinders.

That assumes you can make the perfect flow meter which will maintain its
performance.

**Indeed.


Having said all that: My present car probably uses pulse measurement
techniques and the computer appears to be respectably accurate (though
I've never actually checked it).

I have to log all fuel bought for tax purposes so I can do a pretty
accurate check of the overall MPG - and when I have, the OBC has been as
near as dammit spot on.

**As has mine.

Trevor Wilson