406 - C5 2.0. Hdi fuel consumption

[PeterN]

Hi Folks

I have mentioned this before but not elicited your opinion. I have a 406 2.0.Hdi estate which gives an indicated 52mpg, I have confirmed this is pretty accurate by brim to brim tests, I also have a C5 with the same engine which also indicates about 52mpg but brim to brim tests indicate that its only doing around 42mpg.

Firstly any idea when its driven in the same way and area why the consumption should be so poor, secondly why the read out should be about 25% high. The C5 starts and performs fine, in fact its marginally better than the 406, the only exceptional thing I have noticed is that the speedo reading is spot on by the sat nav and I have never had one like that before, they have always been fast as is the 406. I appreciate that will bias the figures a little but 25%?

Peter

[lozz]

Is the c5 a heavier car..

[PeterN]

Yes but only by 13kg !

[lozz]

Perhaps a remap might give it some extra 'mpg's..

[scotty73]

Yes but only by 13kg !

Is it really?

I ordered a egr blanking plate for mine on Wednesday... My first step in the search for better mpg.

What i find odd is the two readings i got on one of them i done 70 miles of towing and managed only 2mpg less.

[gumby6371]

Have you had planet on the car?

My possibly over simplified thought is that the ECU uses the MAF, MAP and injector values along with the road speed and gear ratio to calculate the MPG figure you see on the display. The only fixed reference point (unless your Doggy) would be the gear ratio so any of the others could be giving a false reading and throwing the MPG figure out.

[PeterN]

No, I have Lexia but it doesn't work. The strange thing is that everything works as it should, no lights, no smoke unless you thrash it, the performance is pretty well the same as the 406, it just uses about 25% more fuel. I think the 406 is higher geared if the rev counter is to be believed although the wheel sizes are the same but I wouldn't have thought it would have made that much difference.

Driven gently the 406 will do well into the upper 60s, the C5 is indicating about the same mpg for the journey conditions as the 406 but not doing it.

Peter

[benczuk]

Strange thought, but have a look to see if anyone has stuck a 'tuning box' on it? (will be plugged into the fuel rail) as that could cause more fuel to be injected than the ECU is demanding causing the discrepancy?

[Doggy]

+1 a tuning box could account for it.

[scotty73]

Hi Folks

I have mentioned this before but not elicited your opinion. I have a 406 2.0.Hdi estate which gives an indicated 52mpg, I have confirmed this is pretty accurate by brim to brim tests, I also have a C5 with the same engine which also indicates about 52mpg but brim to brim tests indicate that its only doing around 42mpg.

Firstly any idea when its driven in the same way and area why the consumption should be so poor, secondly why the read out should be about 25% high. The C5 starts and performs fine, in fact its marginally better than the 406, the only exceptional thing I have noticed is that the speedo reading is spot on by the sat nav and I have never had one like that before, they have always been fast as is the 406. I appreciate that will bias the figures a little but 25%?

Peter

Done my third brim up today these are the figures so far.
1: 39mpg for 150 miles (75 towing) the rest around the valleys.

2: 42mpg for 190 miles (90 mile trip) the rest valleys driving

3: 41.76mpg all valleys driving.

Now looking at those figures got me thinking.. Most of my driving is valleys with the odd 400 mile trip (once every 3 months or so)
If I can get around 42mpg around here I'm actually saving because the 406 would do 38 to 40mpg around here and less in winter... I also saw Doggy quote a 5mpg increase after he blanked his egr so if i can get some of that i'll be happy.

[scotty73]

I fitted the egr plate yesterday and after a 10 mile drive the good old anti pollution fault flashed up... Got it cleared today and it's stayed away so far but what are the odds of it coming back?

[Doggy]

You can count on it.

But hopefully not anytime soon.

[scotty73]

You can count on it.

But hopefully not anytime soon.

It did... The day after is there any way it can be properly cleared?

[Doggy]

Depends on what it's complaining about - did you get the full fault description?

[scotty73]

Depends on what it's complaining about - did you get the full fault description?

No mate he just cleared it, it happened within an hour of blanking the egr so i put it down to that.