406 Hdi fuel consumption

[Peter.N.]

I have two 406 2.0. hdi's and neither of them are giving the sort of fuel consumption that my first one did . They are all '51 reg estates. My original one was written off due to water ingress to the electronics and is now my spare parts dept.

It always averaged around 60 mpg in the summer and a bit less in the winter, my first test of consumption was from here to Exeter where it always gave me around 60+ mpg, mostly on the A30 dual carriageway, the two I have now are doing nearer 50 mpg or a little over according to the ambient temperature.

As I have been having to travel to Exeter for hospital treatment every day for the last few weeks I thought it would be a good opportunity to to confirm my findings, one of them is doing 1 or 2 mpg better than the other but both around just over 50 mpg, the performance is what I would expect from both of them.

Yesterday I tried an experiment, I disconnected the MAF, to my surprise the consumption has improved by about 3 or 4 mpg, I tried it again today and it is averaging around 55 mpg. The other one has a known good MAF (it came from my original one) but that is only doing around 50 mpg.

I assume from the results that the MAF on the first one is faulty, I haven't looked inside it yet, I know its usually fairly obvious if they are broken or coated in oil, I could try swapping them over but will a good one improve my mpg even further.

Is there any way of testing them, does anyone have the correct resistance readings for them? Or could there be another problem?

Any polite suggestions gratefully received.

Thanks

Peter

[Paul-R]

Peter, I've always had my reservations about the your reported fuel consumption figures. If you're comparing the readout figures of one car against the readout figures of another car then there's every chance that they aren't comparable. Have you compared the brim to brim fill-up figure with indicated figure over the same distance? For each car? Many times over?

I treat the figures from the dash read-out as indicative rather than definitive. And the only one I trust is the one I work out.

Hope I've been polite enough!

[boristhemoggy]

I've had my Boxer which also has a 2.0 HDi for 13 years almost and I've 'measured' the mpg since I bought it. Basically fill up to fill up I match the mileage to the fill up and see what I got.
It always came out at about 30mpg until a few years ago when it slowly dropped to 22. It's been 22-24 for a long time now and although once in a blue moon I get 28, 29 or even 30, I can't sustain it. The engine has just been refurbished as has the gearbox (138,000) and it's exactly the same so I've replaced the MAF sensor and the EGR valve which has made no difference whatsoever. The next thing is to put new injectors in it and see what happens. It's been suggested the turbo could be slightly worn and that could be the fault but as that's the most expensive thing to replace I'm leaving that for now.
If you find out what's affecting yours I'd be keen to hear about it.

[Peter.N.]

Hi Paul

I don't blame you for being sceptical about my fuel consumption figures but its because of the boring way I drive, I rarely exceed 60 mph or 2000 rpm and tend to drive in the slow lane with the lorries even when going to Scotland, I accelerate gently and use the brakes as little as possible, when my wife drives the car the fuel consumption takes a dive. I used to regularly get 50-55 mpg from my XM estates on long journeys and I had about six different ones over 15 years.

I also do a brim to brim check and the figures match up near enough to the displayed ones although the first 406 I had did exaggerate a bit but one of my present ones is actually using less fuel than it displays - not very much though. We did achieve 70.1 mpg on a trip to Yorkshire last year but much of that was in 40 and 50 mph speed limits and the average consumption was nearer 60 mpg but it was hot.

I find that when cold the engine will use about 50% more fuel until it reaches working temperature so if you are doing short journeys it is a lot less, but we don't do many short journeys because of our rural location neither do we often get stuck in traffic. Our round trip to Exeter each day is 64 miles, the hospital pay me 15p per mile and I'm making a profit on it

The improvement in mpg I got by disconnecting the MAF shows there is room for improvement, I could get 60 mpg from the 406 last summer when it was warm but as I say this dropped to around 50 mpg in the winter.

What I really want to know is exactly what effect a good working MAF should have on the figures.

Boris.

I would also be very interested to hear the results of changing your injectors if you do it, I have never had one actually fail and have two sets of spare ones here.

Peter

[lexi]

As you are mad keen on the big mpg Peter, I would love to see you own an Insight or Prius. My 8v HDI C5 does not see 50mpg going up the Highlands. I do drive for economy. They must all be set up different and varied in wear on injectors IP''s etc. The 1.4 Jazz gets more. It has 100bhp too. They are jiggly on the ride mind you. I suspect the only way to get a top MAF is to buy the Bosch one.
I have no idea if mine is set up or otherwise. I could take it over for a Lexia to young Simon's.............would probably blow his machine with hundreds of fault codes!

[mickeymoon]

My 2.0HDi C5 on an 02 plate saw 68.1mpg on a run from Hartshead Moor Services on the M62 to Twyford near Reading, on cruise control at just faster than lorries go!

I was so bored I stopped for a nap at Keele!

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[Peter.N.]

Hi Lexi

I think I could get good mileage from an electric vehicle especially one that has the flappy paddle braking giving maximum regen going down hills but I don't think I am ever likely to be able to afford one.

My C5 was pretty good in Scotland although not as good as the 406 and yet the weight is almost the same, what I lost in consumption on the single track roads I recovered when I got on the A9 or other fairly level straight sections.

Mickey.

I have been driving like that for half a million miles or more so I've got used to it now - very relaxing. I have had my boy racer moments but they were 50 or 60 years ago in Vauxhall Velox's and the like.

Peter

[mickeymoon]

I do it most of the time now Peter. As someone recently said to me "the road is never busy if all the cars are going faster than you"..

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[demag]

Well I was saving up for a Prius but blew 2 grand on a three piece today so it's going to have to wait now![emoji53]

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[RichardW]

It' spossible the MAF was duff on the original one and underfuelling it. Most people would have noticed the lack of power, but your driving style probably masked it!


I hope I never get behind you on the A9.....

[Peter.N.]

Its OK on the dual carriageway bits - as long as you don't get stuck behind a lorry! They opened that road in bits starting in the '70s, we watched its progress over the years, now all the bridges are open its knocked about 50 miles off the trip to wick and about 3 hours!. It was brilliant at first, practically empty but getting dangerously crowded now, are they going to dual any more of it?

The power seems to be about the same on all of them, my classic test is to floor it at 60 mph up the Axminster bypass which is quite steep, they all will accelerate slowly in 5th, at least the two we have now do, the scrap on did. You are quite right though, I drove my C5 round in limp mode for several months before I noticed it, wouldn't rev above 2,500 but then I rarely do.

I will do some more experimenting with MAFs and see what happens.

Peter

[elma]

Maybe they just need a good thrash to clear out the old airways? Diesels seem to go lame when driven sensibly for too long. I'd suggest lending them to someone a little less patient than yourself for a little while then repeating your economy test first.

[Peter.N.]

I have tried the 'Italian tune up' but not for very long and a surprisingly small amount of smoke.

Peter

[ekjdm14]

Our 1.4 HDi 206 responds well to a good thrash up to 4000-4500rpm now and again, I often find that power is down after the missus has been driving it for a while as she tends to stay around 2200rpm max and never uses full throttle. However being used to larger engined cars I tend to work it harder to get up to speed with the rest of the traffic (generally speaking, it's larger diesel BMWs and the like in local traffic) & it does seem to make a difference after I've beasted the poor thing a few times up the local A roads.

A proper "Italian tune up" would mean you getting it to operating temperature (I'd want to have done at least 8-10 miles before redlining a Turbo Diesel) and then accelerating from standstill to governor speed in all gears up to 4th at least 3 or 4 times as well as running around the 3-3500rpm mark at a cruise & definitely floor it every time you accelerate. Our 1.4 blows virtually no smoke after one run through the gears (where it leaves a light haze between 3500-4500rpm) but continues to feel "happier" with further thrashing...

I doubt it'd need that kind of abuse very often, couple of times a year maybe to keep the airways free. Also worthwhile checking the EGR valve is sealing fully as they can stick partially open without flagging a fault (at least on the 1.4, not sure if the 2.0 has position feedback) which won't help keeping the tubes clear.

As always, when advocating heavy use of an engine, I'd point out that your results may vary & to make sure that the servicing (esp cambelt) are well up to date to avoid hurting anything. Properly maintained there's nothing to stop one driving flat out every day without damaging the engine but any deficiency would be more likely to make itself known under heavier loading...

[Peter.N.]

Yes, echo's of early MOT testing. There is something weird going on with both these engines, with the MAF is disconnected the consumption improves by about 3-4 mpg but on one it knocks the low speed grunt. I have the data for testing the MAF's so that will be my next job.

Peter