What causes a reduction in MPG?

[TriumphVitesse]

Hi All,

Quick question. My missus has a W Reg 1.0 Micra (don't laugh), which she has always found cheap on fuel and a tenner's worth seemed to last for ages. Recently She mentioned that it seemed to be using more & more fuel of which I replied that fuel had gone up & that you get less fuel for a tenner now. However, I agreed to take it to work one week as She was convinced it was getting noticeably thirstier. She is right, its no better than my MX-5! Its done 81 miles to 15 quid of petrol which according to my calculations is around 29mpg which for a 1.0 Micra is pretty ste! So whats causing this dip in economy?? Its regularly serviced, its on new tyres, it hasnt got any sign of a leak anywhere, the engine management light isn't coming on. Its done 82,000 miles so I wouldnt say its excessive engine wear, the engine is smooth & quiet and overall the car drives perfectly.

Any ideas?

[TheEnd]

Get a fault code read, there can often be problems about that don't put the light on.
Misfires, sensors, airleaks etc, and then there is external stuff, blocked/broken cats or binding brakes.

Tyre pressures can do something similar, but probably not to that extent.

[5lab]

I'd be testing top speed out (presumably it should only be 80 or so anyway) to see if its lost anything significant

in winter, heaters (esp rear screen), lights etc all put more drag on the engine, as do more colder, shorter runs - this is up to 20% difference. If you've taken all the account, maybe one of the brakes has binded on or something?

[SLCZ3]

Check for leaks in the fuel system/piping/tank

[citizen_smith]

first guess would be lambda sensor.

[anonymous-user]

99.9% MAF sensor, very common on the Micra.

[TriumphVitesse]

I have checked the brakes etc and nothing appears to be binding, its not down on power (not that it had any anyway!), there are no leaks that I can see & no smell of petrol anywhere suggesting a leak.

How do you check lambda sensor (is it a garage plug it in job) and if its faulty is it a relatively cheap fix or is it big £ signs???

[TriumphVitesse]

yonex said:

99.9% MAF sensor, very common on the Micra.

You can clean MAF sensors can't you? If so how?

[renrut]

TriumphVitesse said:

yonex said:

99.9% MAF sensor, very common on the Micra.

You can clean MAF sensors can't you? If so how?

I've always found carb cleaner to be good for it. But be careful some MAFs don't like it and die instead.

[Starfighter]

TheEnd said:

Tyre pressures can do something similar, but probably not to that extent.

I had a right-rear drop to 30psi and the tell tale was the MPG figures dropped by 10 overnight. Quick stop and to up on that one as the others were fine and all was back to normal.

[maniac0796]

yonex said:

99.9% MAF sensor, very common on the Micra.

Depends though. W reg is around the changeover.

If it's got the earlier Hitachi MAF sensor, then there's three joints you can resolder, which fixes it. If it's got the later Bosch sensor, then that's a sealed unit and you'd need a new one, but I was only aware of the Hitachi ones failing normally.

Does it run very lump at idle, or loose power randomly when driving?

[littlebasher]

As mentioned above

http://www.micra.com.au/technical-articles/cg13de-...

Look forward to reading about your efforts!

Edited by littlebasher on Monday 22 November 19:37

[The Moose]

I don't mean to be silly, but is she still driving the same amount of mileage.

I've had the same situation with a girl I know - a tank of fuel used to last a week. Then suddenly it ended up with 2 tanks a week and she asked me as I 'knew about cars'. Did the Tyres etc etc etc. Turns out her weekly mileage was doubled due to work!! Sad, but true!!

Cheers

The Moose

[TriumphVitesse]

The Moose said:

I don't mean to be silly, but is she still driving the same amount of mileage.

I've had the same situation with a girl I know - a tank of fuel used to last a week. Then suddenly it ended up with 2 tanks a week and she asked me as I 'knew about cars'. Did the Tyres etc etc etc. Turns out her weekly mileage was doubled due to work!! Sad, but true!!

Cheers

The Moose

Yes still doing roughly the same, as I said I have even taken it to work for a week & find it no better than my Mk1 MX5!!

To answer another question, it does not run lumoy at idle or lose any power (any more power loss and it would stop ha ha!!). It runs & drives perfectly thats what I don't underststand.

Also as its a W reg on the changeover, its still fitted with a distributor cap so I guess it has the earlier MAF sensor.

RANT - Whats the point in fking MAF sensors, or any sensors for that matter!!!

[Prof Prolapse]

TriumphVitesse said:

RANT - Whats the point in fking MAF sensors, or any sensors for that matter!!!

I'm cynical so I expect that it's because even if you manage to diagnose a fault yourself you still have to come to the dealer for massively overpriced parts.

If it is the throttle body and you can't clean or repair it. A company called BBA does them.

http://www.bba-reman.com/content.aspx?content=rema...

I've not used them myself but have heard good things.

[dirty doug]

I used to have a '02 1.0 Micra for a DD. I used to get 55mpg if driven carefully & they can achieve 0.935 leptons.

I cleaned my MAF though quite early doors in ownership as very low mileage & sporadic s/h. 15 min diy job as long as you have security torx bits.

Made the car quite a lot perkier after - well worth doing.