406oc.co.uk

My 406 is excellent on fuel doing a genuine 60+mpg on a run, confirmed by brim to brim fill ups - I do drive very gently though.

I also have a C5 with the same engine (if I can mention that here) which is showing similar mpg on the display but having now checked it several times by brimming the tank I find that it is actually doing about 10mpg less. I put the more frequent fill ups down to a slightly smaller fuel tank until I checked.

Two questions, first why is it using so much fuel and secondly why is the display telling me porky's?

Any suggestions?

Thanks. Peter

Not 100% sure on how the ECU calculates the fuel useage but clearly it doesn't measure the injected volume (ask Red Bull racing )

Presumably it calculates how much air/fuel should be used for a given power output but if the fuel or air flow are not as expected MPG's will be lower.
Injectors, fuel filter, air filter, MAF and MAP can all have an effect and fuel leakage from injector seals or leak off pipes for example, but 10MPG is a big hit so I'd assume you'd clearly see the spillage if that were the case.

Can you run a global test using citroen's 'planet'? it could flag dodgy MAF or MAP and check injector correction values.

the ecu calculates mpg by calculating the injected amount and something else so if the injector has a blocked jet or there is a soot build up then it may block the air from the cylinder so give it an italian tune up with some shell nitro or other premium diesel and see if it changes

Thanks for your comments. Unfortunately my Lexia has stopped talking to the laptop so someone is looking at it for me as I can't make head or tail of it (and I used to be a TV engineer) but I will check when I get it back.

I meant to check the air filter but forgot, I have noticed that it smokes a bit if you push it hard, which I very rarely do. I have pushed it hard one or twice going up hill but with no noticeable difference so far.

I think the Lexia is the way forward but in the meantime I will check the filter, I don't think I have changed it since I had it but on the other hand I may have done - the memory is not what it was.

Peter

an Italian tuneup involves getting the engine as close to the red line as you can and still be at a legal speed for 6-12miles that will burn the soot and sh*t out of the cylinders and off the back of the valves

Your Lexia would be a great help, especially as you have two examples to compare.
I don't have much practical experience monitoring live data on a Citroen via Lexia, but you can definitely see the measured air flow, injection quantity and individual injector correction values on a 406 using PP2000. I imagine the Citroen equivalent should be the same.

The other possible advantage you have is the ability to swap sensors etc between the two, which could certainly reduce the cost of diagnosis by substitution.

If you can give any more details of what's happening and not happening with your Lexia, maybe one of us can help you get it going again.

My 2.0 computer typically UNDER estimates by 9%. It will say something like 39 when the truth is 42.

How you're getting 60 baffles me.

Well I drive like an old man & keep records (like an old man!) and can say that my HDi90 averages 50mpg (tank to tank refills) over the last 3 years or so, most of these miles being on longer journeys.

The computer tends to say around 46mpg and generally maxes out at around 54mpg.

Incidentally I need a new front tyre, this Pirelli 9000 has proved great value having covered 36'000 miles at old man speeds of course!

Pretty good for a front tyre I think

must be all my burn outs and street racing outside Mc.Donalds.

I'm lovin' it.

I have been checking the economy of my 2002 2.0Hdi Estate by brim to brim method, and have so far returned
47MPG, I drive smoothly but not obsessive about it...... Coming from a 4x4 I am well happy with this

That all sounds pretty reasonable, my average consumption for the 406 round here is in the low 50s but we live in a rural area and rarely make journeys of less than 15 miles, I seldom exceed 2000 rpm or 60 mph. My holiday journeys to the north of Scotland always show in excess of 60 mpg, the year before last when it was very hot coming home it showed 67.8mpg over 650 miles, that was confirmed by getting over 1000 miles on a tankful. Brilliant engines.

Peter

Just an update on this thread. I tried giving the C5 a good thrashing and it rewarded me by stopping in a load of traffic, I pushed it up to over 3000 rpm going up a hill, it chucked out a load of black smoke and stopped, I had to wait until the ECU shut down and the displays went off before it would start again.

I thought I might swap MAFs with the 406, having removed them both I find that the C5 one is about 1/4" bigger in diameter, so that was a waste of time. They look identical until you put them side by side.

Still haven't got my Lexia back so don't know what to try next.

The 406 is doing a funny now, gutless until you reach 3000 rpm then it takes off - although sometimes its OK, when it did that before it was the MAF sensor, I fitted a secondhand one and it was fine, I'm wondering if that has failed now.

Peter

PeterN wrote:The 406 is doing a funny now, gutless until you reach 3000 rpm then it takes off - although sometimes its OK, when it did that before it was the MAF sensor, I fitted a secondhand one and it was fine, I'm wondering if that has failed now.

Peter

That does sound very much like MAF failure symptoms, especially if there are no warning lights (EML etc)

Yep that's your maf sensor Peter 99%, if you want to make sure just unplug it & take it for a test drive
all the best DDD.