Fuel economy nosediving...

[highlander]

Hi all,

My MOT bloke gave me an advisery on a "minor exhaust system leak" - not enough to fail the MOT, but he felt it necessary to add this to the MOT slip.

A little before the MOT took place, I had started to notice I was getting less and less mileage on each tank.

I know that sitting in traffic isn't a great measure of fuel economy, but I've burned a third of a tank so far and gotten about 70 miles from it - noticably less than usual. I haven't taken her out on the dual carriageways much lately, but on average I've been getting 350 or so miles to the tank (as much as 550 when motorway cruising - one one trip early this year I got 650+).

Is the exhaust gas leak likely to cause that much of a dent in my fuel economy?

If not, what else could be buggering things up, and what should be my next move? I'm getting the kind of mileage I'd expect from a V6, but without all the fun acceleration!

The car had a 12,000 mile service back in July (full oil change, air/oil filter change, spark plug replacement, brake fluid replacement), and the MOT was in early October. Tyres are brand new (Bridgestone B250s are great!) and are pumped up to 34 psi.

And no, I haven't put on 10 stone in weight since the start of the year

Any help much appreciated, as always!

Cheers
Graeme

[dom07968]

Hi,

The leak in the exhaust may have a minor effect, i noticed that i was pressing harder on the accelorator, so used more fuel when my back box was on the way out so that may explain part of the problem.
Have you considered over cooling as a cause. I generally notice during the winter months that may car becomes more thirsty and put it down to longer warm up times due to low ambient temperature. I've considered covering part of the radiator after being inspired by watching ice road truckers but waits in traffic made me nervous which are a regular part of my commute.
Possibly investigate the operation of the cooling fan.
Switch off the air-con if you've got that. Consider implications of lack of a/c use before you do. I did turn off the a/c and it did help as on my old car the fans are infront of both a/c and coolant rads which kick in from when the a/c button is pushed and so contribute to excessive cooling at all times, espcially in winter. I understand on modern cars that the a/c works only after the engine has warmed up. Your car may have a different arrangement so you'll need to check.Oh..implications of lack of a/c use, mine is rubbish after switching it off over winter. It is an old car and i'm lucky it functioned for as long as it did.
Also consider brake binding as a cause. I've had binding for two weeks on a rear drum and i'd lost about about 10% of my typical mpg.
Sorry i can't be of more help but hope you get find out what the cause.
Dom

[cardaft]

Could be a number of things really, air temp sensor, maf sensor, your exhaust hole, weather been using a different make of fuel recently?

406s have rear drum breaks?

[BruceT]

I'm putting it down to winter.

Im down about 4-5MPG this month over last month doing the exact same commute

[highlander]

Could be a number of things really, air temp sensor, maf sensor, your exhaust hole, weather been using a different make of fuel recently?

Been using Shell's normal unleaded for ages now; I seem to get better mileage with it for some reason. I occasionally chuck in some Redex Injector Cleaner, but haven't done for a while.

When you say the air temp sensor, do you mean the thing on the passenger-side wing mirror? That hasn't worked properly since I got the car (tends to believe the temperature is between 35C and 50C, and occasionally just shows --C).

The car insists on turning on the AC whenever I start the engine. I normally turn it off again, but sometimes the windows steam up straight away when I do that. I am honestly getting worse mileage now (with AC off) than I was during the height of summer (when I had to put the AC on just to stop myself from dying).

What's a MAF, and why would the car need to sense it?

406s have rear drum breaks?

Yep. The V6 models and the Coupes have front and rear discs/pads (I think the SRi does too, but I could be wrong). Being a 1.8 LX owner, I have rear drums. They look naff, but they have a working handbrake - from what I've been told on this forum, the 406s with rear discs/pads rather than drums tend to have really poor handbrakes.

[Welly]

Probably just the cold weather. Also it seems to have been very windy out there of late - this has a huge impact on economy.

[cardaft]

Wow, im surprised by that, i was surprised that my 206 2.0 had rear drums but on a 406 thats a bit mean of Peugeot. Mind the one i should be going to see tomorrow has a close up picture of the rear wheel, i thought the dics looked ruined behind the alloy, but its just the drum then I cant thing of another big car which has drum rear brakes

Anyway, back to your topic:

Shell is the best fuel to use and you will get more mpg from that than tescos value fuel, i would never recommend or use any cheaper brand of supermarket fuels. That statement often causes conflict but fuel is not just fuel, some are way better than others and around here at least shell and bp are no more expensive to buy than any supermarket. Top brands (shell and BP) have lots more additives and lubricating factors than a lot of cheaper brands. HDis are very fuel sensitive and their injectors can suffer if run on a fuel which is low in lubricant factors.

Anyway ...

Your car will have an air intake temperature sensor, this tells the car how cold the air is so it can change the mixture of fuel it puts into the engine. Often this can fail or become less effective, sometimes making the car put in more fuel. Ive had this many times on diesel cars with the MAF sensor going faulty, not enough to put in a fault code but enough to effect economy and to some extent performance - oh, only ever fit geniune sensors, ive had lots of trouble with cheaper ones from motorfactors. So that could be one thing.

How do you use your car? Mostly town driving? Motorway? Do you drive it hard or easy?

Id recommend you get some ait intake spray and clean out the air intake with this, forte air intake cleaner is the best stuff i can find, you can probably get it on ebay if your interested. As your temp sensor is also in your air intake system it should give that a clean too. However its been a while since ive had problems with a petrol car.

A MAF is a Mass air flow meter, which does the job i described above.

Please forgive my basic mechanical knowledge

[jasper5]

I would guess that if the car passed the mot on emissions, the MAF and everything else related to mixture settings would be working fine.

My 406 HDi ran 64,000 miles on Tesco diesel without a single hiccup.

There are lots of good injection system cleaners on the market every bit as good as Forte but a third of the cost.

Ford Mondeo have drum brakes on the rear up to about 2000.

[cardaft]

I have found the MAF sensor can go faulty without effecting emissions too much and without it storing a fault code. Also ive found them to be a problem if they have been replaced with a non genuine MAF sensor. Though often a good clean with some contact cleaner has them working better, i always do this on every service.

Id not use supermarket fuel, there are differences, its not like it comes from the same place, and its no more expensive generally to use a good quality supplier, shell etc. Ive used supermarket fuel in the past and have found it doesnt return the same mpg.

Yes there are lots of injector cleaners, though again i cant find one which works as well, though i was meaning an air intake spray, which would clean the air intake system - throttle body, temp sensors etc.