Drop in MPG...causes?

[designforlife]

My old mitsubishi is experiencing a noticeable drop in MPG in the last 2 or 3 months.

It's 22 years old with 142k on the clock, but has been meticulously serviced, isn't due a minor service until august.

MPG has gone from 28/30 with a fairly leaden foot, down to 25 with me driving conservatively....certainly a difference enough to notice...pretty sure if i was driving it like i usually would, i'd be getting that figure down to 22/23.

Any suggestions as to causes of this drop in MPG? Car is otherwise running fine, if a little sluggish acceleration wise. No dashlights, and doesn't feel like the block is running rough.

[stevieturbo]

Presumably petrol ?

Are you comparing against a dash reading, or actual brim to brim fuel/miles ?

Same tyres sizes throughout ? Same filling stations ?

[designforlife]

stevieturbo said:

Presumably petrol ?

Are you comparing against a dash reading, or actual brim to brim fuel/miles ?

Same tyres sizes throughout ? Same filling stations ?

Yup, petrol 1.6 Mivec. No dash reading, brim to brim calculation using the same V-power from the same filling station I always use.

Same tyres too It's been enough to notice a difference, gone from getting 270 odd miles to a tank, to 200.

[Mignon]

Possible but unlikely mechanical cause would be a binding brake. Most likely electronic cause would be a faulty engine temp sensor making the ecu think the engine is cold and causing a rich mixture.

[Steve_D]

Beat me to it on the coolant sensor but I'm liking the binding brake option.

Steve

[designforlife]

Binding brakes would have been my call too, but It's got new calipers, discs and pads all round and nothing is sticking.

[HustleRussell]

Check if the lambda sensor has a replacement interval. Off spec lambda readings will generally throw a code but in some cases may not.

If it's not mechanical, fuel, driver or journey profile related then that leaves engine management the likely culprit.

[Auntieroll]

HT leads,if fitted could be another possible cause, Mivec system misbehaving is also a possibility.

Edited by Auntieroll on Friday 28th April 19:14

[GreenV8S]

Have you checked the tyre pressures recently?

[E-bmw]

Auntieroll said:

HT leads,if fitted could be another possible cause, Moved system misbehaving is also a possibility.

HT leads if not fitted could also be a problem............

What is this "Moved system" of which you speak?

[Auntieroll]

B####y predictive text ! Obviously it should read Mivec !
re HT leads,was at work and unable to check whether conventional
Rather than coil on plug ignition system used on OP's vehicle.

[designforlife]

Thanks for the replies.

to address a couple of raised points, tyre pressures are correct, and MIVEC is engaging exactly as it should at 5k rpm and pulling right around to 8k...usually if MIVEC plays up it would cut at 6.5k or not engage at all.

As for Leads, it has two regular leads, and two Coil-on-plugs...the leads are easy enough to source, but the COPs cost a fortune new, and aren't that easy to track down used.

Interestingly, took the car up to nottingham and back from st albans last week, and got 40mpg out of it, which is normal historically for motorway runs in this car.

So, basically it only seems to be exhibiting poorer than usual MPG on short runs, rather than longer motorway trips...not sure if that is telling in some regard?

[HustleRussell]

Is the car getting up to temperature quickly and staying there?

[designforlife]

HustleRussell said:

Is the car getting up to temperature quickly and staying there?

yeah the temp gauge isn't showing any funny business, sitting at its usual spot once warmed up, and not taking longer than usual to get there.

[fido]

O2 sensor?

[GreenV8S]

designforlife said:

seems to be exhibiting poorer than usual MPG on short runs

Isn't it completely normal and expected to get lower MPG on short trips?