Fuel consumption 2.0 136 auto 407

[moizeau]

I've been noticing the 407 is gradually using more fuel. A couple of years ago on a long trip to the UK and back at 70-75 mph on motorways it gave me 53 mpg. Just done a 450 mile run at the same speeds yesterday, again on the motorway and it's doing 42 mpg. Last time it went to the MOT the tester said the emissions were unstable but it passed. I've checked for binding brakes, seem fine. There is a little disc noise on the rears when spinning by hand but I had just released the handbrake. I connected Diag up and the injector correction values were 94, 105, 108 and 91 at idle on a cold engine. Air flow ref was 594 and measured was 490.

Do any of these figures tell any stories and is there anything else I can look at?

It's due a regen and that will happen tomorrow on a 140 mile run on A roads and motorway (497 kms since the last one). I'll also reset a trip to see what it tells me.

Cheers, any pointers gratefully received.

[GiveMeABreak]

Rear wheel bearings? I guess these will have some impact on economy overall if starting wear... but other things to consider could be the air inlet manifolds clogging up / EGR too for example.

Elis had a look on his 2008 C5:

viewtopic.php?p=584453#p584453

and here

viewtopic.php?p=583334#p583334

[moizeau]

Thanks Marc, The wheel bearings are fine. The dosers should be reasonably clean, I did those a couple of years ago, the inlet manifold will be a different story I fear. The cars just about to hit 200000 miles and it will have never been removed. Egr same story I guess. The car has been meticulously serviced from 60000 miles with the oil changed at 1/2 the service intervals. Might go for a bottle of injector cleaner first, Facom have one one Prime. If that doesn't improve things then possibly a leakdown test only because those two are easier and quicker. If that doesn't show anything it's rubber gloves time. I'm guessing the air flow ref and measured should be closer than that so quite possibly intake or MAF.
All suggestions welcome.

[MattBLancs]

In theory, clogging inlet tracts should not have a bit detrimental effect on economy, more so on maximum power (which needs maximum airflow)

Obviously, taken to extremes, an inlet so very clogged as to give a huge power drop (and hence a car needing foot to the floor ALOT of the time) would then have a bearing on economy.

Likewise, I would not expect much economy drop from wheel bearings (as I'd they were to be absorbing a significant proportion of power from the engine, the only route to "shed" this absorbed power is heat dissipation - and the bearings are small, enclosed and not designed for shedding heat (unlike brake discs which it's their role in life )

Cleaning inlet is a grim job, if it's an alloy inlet then I prescribe fire!

[MattBLancs]

Like so!

Simply add fire!

[moizeau]

Fuel consumption wasn't as bad as I'd thought. I obviously hadn't reset the trip in a long time so it was averaging over many 000's of miles including short journeys and towing a trailer several times with a tonne of sand in it. Reset it for the 140 mile round trip and it gave me just over 48. Still down on the 53 so will get some injector cleaner in it and have another look.