406 Hdi fuel consumption

Unread post by **Peter.N.** » 23 May 2011, 21:23

We have just returned from our anual or sometimes semi anual trip to the far north of Scotland only this time we did it in our 406 estate, for the last 15 years it was in an XM, this is the first really long journey we have done in the 406 so I was interested to see the mpg figures.

The car was loaded with myself and wife and our luggage and our German Shepherd and her luggage, over the total distance of 1,853 miles we averaged 60.2 mpg, that was driving at a fairly steady 60 mph with the occaisional excursion to 70 mph down hills. We managed over 1000 miles on a tankful and I suspect the figures were actually better than that as the speedometer packed up several times and was not recording the miles travelled, when we refilled on the way home for 135.9p per liter at the new Asda store in Langley Mill, the 'mileage remaining' indicator showed 1212 miles, it had showed around 1200 on the previous refill.

I tried the effect of coasting down hills - purely as an experiment and it actually uses mor fuel! In gear with foot off throttle it counts to 999.9 then the display goes blank so obviously is not using enough fuel to register, knock it out of gear and it reads about 400 mpg, the ammount needed to keep the engine ticking over. This of course is not a conclusive test though because being in gear slows you down so the car may have gone further and faster out of gear, especially with the engine off - not recomennded though, you still need to steer and brake.

We came across the A66 from Penrith to Scotch corner with a following gale and apparantly used no fuel

the remaining miles were 199 when we left Carlisle and the same when we arrived at Scotch corner, the instant read out was showing 70-100 mpg nearly all the way along the road with the occaisional dip to 50 going up hill.

Pleased as I was by the figures, we used to get 50-55 mpg on the same journey from the XM estate, which is bigger and heavier the the 406 which goes to show what a brilliant IDI engine the 2.1 is.

Peter

dieselnutjob · Unread post by **dieselnutjob** » 23 May 2011, 21:59

with diesels you should never coast in neutral

in neutral the engine will use a little bit of fuel to maintain 900rpm or whatever

in gear the weight/momentum of the car is pushing the engine. If your foot is off the accelerator and the revs are above normal idle the engine will actually inject no fuel at all. You are using the momentum of the car to turn your AC, power steering pumps and alternator and not fuel.

I once drove up a big mountain and down the other side (I went over the top instead of through the St Bernard Tunnel, beautiful drive). This was in my 305 diesel. On the way down the engine actually went totally cold and the heater just blew cold air, because during the one hour decent no fuel was injected at all.

Unread post by **Peter.N.** » 24 May 2011, 00:08

That's what I would expect all the while the engine is turning at above tickover speed, funnily enough I was just thinking whether you could go down a long enough hill to make the engine so cold it wouldn't start when you did put your foot down, although if you are doing enough revs it would anyway.

Peter

dieselnutjob · Unread post by **dieselnutjob** » 24 May 2011, 15:34

the engine would be higher than 900 rpm which is way higher than the speed that a starter motor can acheive. Also I suspect it won't go completely cold as there must be some heat generated by compressing air.

After all if you are going down a hill in gear then you are engine braking. That means that the energy is (at least partially) being absorbed by the engine/transmission rather than being turned into heat by the brakes. I suspect that it just results in the energy being converted into heat by the engine instead, so no it won't go completely cold.

Unread post by **Peter.N.** » 24 May 2011, 17:06

True.

cox377 · Unread post by **cox377** » 01 Jun 2011, 14:21

What a fasinating read! I've always thought rolling would save a few miles for years, a friend once told me it did no save fuel and I thought he was talking ****. The way you've explained it seems to have made perfect sense! Saying that, 1200 out of a tank is amazing!

Unread post by **Peter.N.** » 01 Jun 2011, 14:42

Its got a lot to do with the way you drive, on the same journey I have been getting up to 55mpg from my 2.1 XM estate which is about 1000 miles on a tankful and that is bigger and heavier car with an old but very good IDI engine.

Peter

jgra1 · Unread post by **jgra1** » 01 Jun 2011, 15:15

good stuff Peter.. I have been contemplating that sort of journey to N scotland over summer, and averaging 60 in a 406 sounds pretty good

.. I dont think anything i drive could match that.. (notwithstanding the WVO aspect in the Xant)...

certainly the VFR**************[VIN obfuscated, can be read by forum staff] wouldn't

steelcityuk · Unread post by **steelcityuk** » 02 Jun 2011, 11:43

Hi Peter,

I was surprised what coasting achieved when judged correctly. I have a Scangage II connected up to the A Class which gives me all sorts of info depending on how you've configured it - it really shouldn't work according to the age of the car but it does so MB must have made them EOBD compliant long before they needed to. Anyhow using this gauge it's possible to see how many BHP the EMU thinks the engine is producing along with MPG figures, coolant temperature, boost, etc. Sure enough coasting uses more fuel than engine braking but on a steady down hill slope the engine braking will slow down the car to such an extent that some throttle will be needed to keep up to a reasonable speed whereas freewheeling results in a much longer period at very reduced fuel useage. On a steep slope I use the appropriate gear and sometimes the Air Con to control the speed.

It's good to see you've come around to the charms of the HDi. Strange though, you didn't like the idea of them but I did. Now you have one and I don't.

Hope you had a nice holiday.

Steve.

Unread post by **Peter.N.** » 02 Jun 2011, 12:24

Hi Steve

Yes I did make that point in my original post, on a steep hill you lose nothing by coasting because you would be having to brake which is wasting fuel anyway, on a gradual slope engine braking robs you of a considerable ammount of energy so whether the fuel used on tickover would be more or less than you save by having it out of gear is debatable but I suspect it would be less. In the olden days before power assisted brakes and steering (which I well remember) you could get away with coasting - except that the brakes may not have been good enough to stop you

It was no easy decision to go common rail, the potential problems I have read about plus the difficulty in repairing them has put me off for a long time, but comments that have bee made about the early 8 valve Hdi engine especially by Paul citroenxm who knows what he is doing, caused me to do some research but the thing that really convinced me was the fact that there are quite a number of 406's using this engine that are being advertised for sale with in excess of 300,000 miles on them, that puts them in the same reliability/durability league as the XM 2.1.

I am still of the opinion though that many common rail engines are not as durable, particulrly those that produce a lot of power, the higher the power that is extracted from them, the less reliable and more expensive they seem to be. I have read about a considerable number of turbo failures for instance, I have never had a turbo fail on a diesel in over 20 years and about half a million miles of running Citroen/Peugeot diesels, but they seem quite commonplace with modern engines in fact everything on them seems to wear out more quickly.

Peter

dieselnutjob · Unread post by **dieselnutjob** » 02 Jun 2011, 13:38

our 806 HDI has over 200,000 miles on it now
The engine uses less than 1 litre oil between 12,000 mile services
I mostly drive it flat out and occasionally tow a caravan with it.
The only problem I've had with the engine was a knocky injector at 120,000 miles but second hand injectors are now quite cheap for these.
It's also had a new intercooler and worn out two DMFs, a power steering pump and had a new rack (though probably it only needed the track rod end).

I would say that the original 2.0 8 valve HDI is one of the toughest engines ever made.
They are very nearly valve safe and don't seem to wear out head gaskets like the XUD did.

The early 16V ones (like my 607) seem to like breaking their cambelt/intercam chain which is a real shame because other than that they seem to be a good engine.