Hi. I have a 53 plate JTD multiwagon. I had a glow plug fail about 6 months ago and noticed a severe reduction in mpg ( down from 55 to 44 ). Having had one plug replaced and a service its now back up to 49 but i cant squeeze any more out of it. I have tried disconecting the MAF sensor and seeing if this changes anything, it made the performance a bit more lively but no increase in mpg. Also when accelerating hard, lots of black smoke is emitted but this goes away once i'm back to normal cruising. Any ideas what to try next to get the mpg back up to normal????

blackrat001 said:

Hi. I have a 53 plate JTD multiwagon. I had a glow plug fail about 6 months ago and noticed a severe reduction in mpg ( down from 55 to 44 ). Having had one plug replaced and a service its now back up to 49 but i cant squeeze any more out of it. I have tried disconecting the MAF sensor and seeing if this changes anything, it made the performance a bit more lively but no increase in mpg. Also when accelerating hard, lots of black smoke is emitted but this goes away once i'm back to normal cruising. Any ideas what to try next to get the mpg back up to normal????

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That's a fascinating post as I can't see how a duff glow plug could affect mpg enough to be measurable

That said, I'm suffering poor mpg and I get intermittent heater plug errors (plugs are good so it's most likely the relay) and hence my interest in your post.

MAF is the item to change if you suspect poor mpg (although not guaranteed) especially if the car runs better with it disconnected. You'll always get bad mpg with it disconnected so never use this as a yardstick. Try a clean first using proper electrical contact cleaner.

This is my second winter with the Stilo and do find over Winter I get a drop in mpg, recently though I had a noticeable drop in mpg and it ended up being the o/s/r brake binding.

I had a serious drop in MPG recently for a matter of about 6 months - even after changing the (very clogged) EGR with a new one (about £85 from Vauxhall), and replacing the MAF with a new one from Fiat (about £100) I was still only getting about 45mpg, although the car was running reasonably smooth.

I ended up resolving the MPG problem by buying a new MAP (manifold pressure) sensor about 3 months ago - about £30 from Fiat (blaady bargain as my local Bosch specialist wanted £70 for the part!). Before, my car had become a bit of a pig driving at low speeds/tickover such as parking or at junctions - now it behaves absolutely brilliantly and my MPG is back up to 50-55mpg consistantly (very minimal black smoke too!). Incidentally, I had the problems after cleaning my MAP sensor with carb cleaner - initially ran smoother but over the course of a few weeks my average MPG plunged to about 40, so I guess that probably killed it!

Very highly recommended that you try that you have any of the symptoms, particularly MPG drop... let us know how it goes if you swap it - I think it may benefit from a battery-off ECU reset after too

chances are if black smoke sort the erg valve as mine smoked like a sod when the egr was blanked off. Have A Look and report back.
Glow plugs will not affect fuel consumption what so ever!!!

matty_b said:

I ended up resolving the MPG problem by buying a new MAP (manifold pressure) sensor about 3 months ago - about £30 from Fiat (blaady bargain as my local Bosch specialist wanted £70 for the part!). Before, my car had become a bit of a pig driving at low speeds/tickover such as parking or at junctions - now it behaves absolutely brilliantly and my MPG is back up to 50-55mpg consistantly (very minimal black smoke too!).

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Good suggestion - I may try that

Muppet1 said:

Glow plugs will not affect fuel consumption what so ever!!!

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I wouldn't say whatsoever. Spending 10 minutes trying to start an engine at sub-zero temperature wont do your fuel consumption much good

Have you tried the obviouse things first when was the air filter last changed?
And the most obviouse are you running on supermarket diesel it’s a waste of money.
I cover about 1000 miles a week and have noticed supermarket diesel will give noticably less MPG make the car slugish and smoke on acceleration.
The price difference on a full tank of Esso or Shell is less than £1- 00 which is compensated by the extra MPG and a better drive.
I saw a post somewhere that supermarket diesel has a lot less sulpher added and produces a cooler burn which is much less efficent, don’t know how true that is but it makes sense to me.

Top tips everyone thank you for your time. My car is due its MOT in April, so will get my friendly spanner twirler to have a go with the MAF and MAP sensors, cleaning first. The service was done in November so don't think its filters. Will report back asap.

Thanks to blackrat's post I can now say my search for the cause for poor mpg over the last 3 years or so has finally ended

My mpg started to drop off about 3 years ago and ever since I've tried everything I could think of to get it back to what it used to be (my best ever run was 130 miles @ 62.5mpg - but that was very much in the past ). I've had heater plug errors on and off during that whole period. To start with I had a couple of plugs go (genuine faults) but finally the problem started to become intermittent (plugs all read good) but since I never got an error on cold mornings I basically just considered it an annoyance that one day I'd get around to sorting.

Recently I did resolve the problem by changing the Heater Relay Module. I also cleared the fault code - which of course now stays cleared.

The difference of mpg has been dramatic

Previously, I'd be lucky to see 50+mpg but on a run to Weymouth and back today the JTD returned:

• 62 miles @ 62.6mpg :spin:
• 30 miles @ 67.5mpg :spin: :spin: - this is one way only, and is a new best, but is cheating slightly since Weymouth is a little lower than my starting point in Poole.

So what's the cause as clearly a duff heater plug can't affect mpg much I can only really guess but I think the ECU must drop all leaning parameters and load defaults when it becomes aware of a reported error (even if it's a glow plug error) which means my JTD has only just become free of all errors after years :doh:

Out today I could almost feel the engine become more economical (by watching on-board mpg display). For instance, it's become much smoother at 30mph in 5th driving in towns (normally would have to drop to 4th) which itself provides a further boost to mpg.

INTERESTING POST.

i also have a heater pre heat error the plugs are all new. i get a spanner symbol which i believe is a loose connection with this error, it goes off after a min or so cant find a loose connection anywhere,probably the heater relay....

i wouldn't have thought that this would cause any drop in mpg as once hot and started they turn off any way.....

i have been to Wales twice in the past 3-4 weeks
that's a round trip of about 430-450 miles i used £40 worth of fuel so that would average about 55-60 ish mpg.

just a thought have you checked for leaks or possible ignition timing issues....air filter....etc....

ricnfiona said:

i have been to Wales twice in the past 3-4 weeks
that's a round trip of about 430-450 miles i used £40 worth of fuel so that would average about 55-60 ish mpg.

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What does your on-board computer say

I'd say yours is fine - although if you ALWAYS get a preheat fault then I'd get is sorted as you may well see even better mpg.

Remember if you get 3 good starts (without an error) then the fault should be cleared (mpg not degraded) but otherwise it will stay logged.

Only guessing about what's going on here but I'm sure not dreaming it

Argonought said:

I can only really guess but I think the ECU must drop all leaning parameters and load defaults when it becomes aware of a reported error

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NumanR, who I believe was/is a Fiat EDET, told me some while ago that the JTD ECU doesn't have any learning capability.

ricnfiona said:

once hot and started they turn off any way.

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Argonought said:

So what's the cause as clearly a duff heater plug can't affect mpg much

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JTD heater plugs continue to operate after the engine has started until it reaches a pre-defined temperature. During the after heating period, I'm not sure whether they stay on constantly or pulse on and off. I presume this is to assist with combustion whilst the engine is getting up to optimum temperature.

Even with this extra heating, I doubt it would have a dramatic effect on mpg though.

Davren said:

Even with this extra heating, I doubt it would have a dramatic effect on mpg though.

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It's out of the question - the change is far too dramatic.

So what is the explanation

That's two reports from unrelated people which I'd say must exclude coincidence (the real problem magically fixed itself at the same time the heater plug fault was fixed).

Not wanting to fallout with NumanR but the most obvious answer is as I've described unless we get into the realms of really crazy stuff.

Have you not noticed that the JTD runs differently for a while after a reset (power removed) ?

I can't think of any other explaination.

From what you've said, it would appear that there must be a relearn function, although I've disconnected the battery and the ECU on mine several times and never noticed any difference in performance when reconnected.

It would be extremely interesting to know what really is going on with the ECU (someone has to know :chin: )

I have contradicted myself a little though.

If I've had this problem for say the best part of 3 years, then if a re-learn function does exist then it would have been disabled any way for the best part of that period - which means, like you, I shouldn't have been aware of it.

The other thing I don't understand is why Examiner hasn't picked up on this over the years I've had the car looked at - I mean surely you'd expect a flag to be set within the ECU which Examiner can at least report

i always thought that when a fault is logged in the ECU it goes to a basic setting if you like, obviously as not to do as much damage, like limp mode but not as bad.

this may explain why after you've had the battery off the car you've experienced better performance as it will have reset itself.

Sooo...

Would 40-42 combined mpg be a bad return?

I get 50+ on motorways...never 60+

MAP & MAF you say...??? ECU had a reset not long ago due to a brand spanking new battery...

I am interested in how to get more miles from my diesel for my hard earned £££!

NA

NotAgain said:

Would 40-42 combined mpg be a bad return?

I get 50+ on motorways...never 60+

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I'd say that's bad unless you only drive around town.

I got 41.9 mpg today but that's all town driving (with a lot of people stopping in front of me and a traffic crawl - and not to mention some joker stopping on a green light just as I timed the lights perfectly )

I think (and remember this is still unproven) the critical point is to make sure that no errors are logged within the ECU at all.

Well no errors apear on the Dash, only the usual airbag warning.

I do mainly Motorway driving and 40-44 is combined, as I said 60+ is a dream for me. My Motorway speed is hardly ever legal limt +10% ...such is my desire to save the pennies...I am now worried something else is wrong...

NA

As already suggested in this thread check really basic stuff like how dirty is your air filter and you could also try changing the MAF.

Whilst looking out for a cheap one, you could try cleaning the MAF sensor with proper electrical cleaner as a starting point (you'll need a 5 point tool to remove it).

If that doesn't work then you could try the MAP sensor.