Massive drop in fuel economy - help????

[guest3759]

I have a 59 reg 1.2 honda jazz.  I have never been impressed with the fuel economy (the most I have ever got is about 49mpg). Anyway this has now dropped to 39-41mpg!!!! I had it serviced (full service) 8 weeks ago at a local garage.  It only has 14500 miles on clock.  Neither my journeys or my driving style has changed - I am a very fuel concious driver.  I took it back to garage to explain the drop (I normally get around 200miles on half a tank but am now getting around  115 miles). They checked brakes were not binding, clutch was not slipping and there were no fuel leaks.  They could not come up with any other reason for this drop in economy. Can anyone help me with why this may have happened??? I dont know anyone who is a mechanic and am frigtened to just take it to another garage in case they rip me off. Please help??/

[guest3721]

Have you changed where you get your petrol from, ie Supermarket?

Dave

[Alan]

You state that you are not impressed with 49mpg so what are your expectations as compared with other cars of the same engine size.
Are you not aware that in the winter when temperatures drop all cars use more fuel compared to the warmer summer months.
You state your consumption is down to 39/41 which equates to 184 miles per 1/2 tank and yet you you state you only get 115 miles per 1/2 tank.
I would respectfully suggest that you run a proper fuel consumption test by brimming your tank, taking note of the mileage and then going from full tank to full tank taking note of the mileage each time and then you can calculate a proper consumption figure by dividing the mileage by the amount of petrol you put in.
One further tip I would give you is DO NOT use Tesco petrol as you will use up to10% more petrol depending on the driving conditions a fact proven on my spreadsheets for my last 3 Hondas. 

[chrisc]

Pardon what might seem a silly question, but why would supermarket petrol be any different from that from a garage?  (supermarkets are not permitted to sell petrol here yet)

[guest1521]

1.2 '59' Jazz. Supermarket petrol (Morrisons) almost every time. 'Whole of life' mpg at 28600 miles showing 54.3 on the display. The car is used for all sorts of motoring, sometimes fully laden. Driven 'sympathetically' but not slowly, air con always on.

Claire, I wonder what grade of oil they put in at service. If it's a much thicker grade than it should be, it'll raise your mpg 'cos of more drag in your engine internals.

Just like tyre pressures lower than should be raise fuel consumption but I'm sure you know that.

Cleanish air filter and clear air route to it? It's easy to check. Constriction raises mpg.

Brakes binding... a simple DOUBLECHECK you can do yourself is simply to feel/touch each wheel on its steel/alloy after a few miles steady driving. Rears should always feel cooler than fronts, as most braking pressure (friction) is on fronts. But if one wheel is significantly wamer than the others, you can bet that brake is binding. If you can drive a couple of miles without the need to brake, maintaining at least a steady 30mph no wheel should be even a tad warm to feel. Not in this cold weather.

Is the car warming up as it used to do? From a cold start the automatic choke should 'switch off' completely after round about a couple of minutes. Idle rpm from around 1000rpm should then fall to 750rpm. If not, it'll be using more fuel.

If the above doesn't help, I'd suggest you get a computer diagnostic test. If the problem seemed to appear not long after the service, I'd get that 'diagnostic' done elsewhere.

It'd be great if you let us know what you find. It adds to knowledge on the forum.



 

   

[guest3759]

Thanks for all the replies.  I have had the car for 3 years and have done the proper testing of MpG by filling to top then resetting milometer and working out how many mpg it does by the litres required to fill again from empty and what I am getting now is significantly different to a few weeks .  i will try the touching the tyres test next time i am out - thanks all

[Alan]

Pardon what might seem a silly question, but why would supermarket petrol be any different from that from a garage?  (supermarkets are not permitted to sell petrol here yet)

There must be differences in the various petrols which is born out by the car's performance which is far better on both Sainsburys and Morrisons petrol as compared to Tesco petrol. 

[guest334]

Pardon what might seem a silly question, but why would supermarket petrol be any different from that from a garage?  (supermarkets are not permitted to sell petrol here yet)

In the UK local & national price wars often break out for petrol/diesel sales - the huge supermarkets obtain their fuel from various UK & EU refiners, in the recent past one large Supermarket had a problem with lack of additives this resulted in many motorist claiming for engine damage. Tests in the motring press have shown SHELL & BP fuels do deliver consistant grade fuel and performance engines benefit from running the premium grade fuel  - my Subaru turbo went well on Shell V-power.

[guest1521]

Claire..."i will try the touching the tyres test next time i am out"
You probably meant the alloy or steel of the wheels - but just in case. 'cos the rubber of the tyres won't warm up at all in the way I mean...

[guest1262]

Many years ago my SAAB 96 lost a lot of power. After many attempts to find the reason a cracked spark plug was found - one cylinder (of the three) was not firing. A new spark plug sorted it.

Your jazz has 8 spark plugs (I think). Were they checked or were any replaced when the car was serviced?

Just a thought.

[guest3208]

As a driving instructor, I need to check running costs - with my 1.4Si, I am getting an overall figure of exactly 36.63 mpg. I fill to the brim each time to ensure consistency and all the figures I calculate on an iPhone app called MPGCalc.
This is with a combination of all types of retailers - I do of course do a lot of stop start driving in towns, but there is plenty of open road stuff and dual carriageway. My best figure ever was 42.32 and worst was 31.64.
Useful little app as it calculates instantly the cost per mile which is the figure I need to know, so in a range of 14 - 19 pence per mile.
Due for a full service next month so it will be interesting to see if it improves.

[chrisc]

In South Africa we have Shell, BP, Engen and Total, the petrol is refined from crude oil.  Then we have SASOL, where the petrol is made from coal.  Not popular at the coast due to the lower octane rating but sells very well in Johannesburg (2000 metres elevation) since it is cheaper than regular petrol and 91 octane petrol is all you can use due to the altitude.  However, the exhaust smells funny and when you step out the airport in Johannesburg, you can smell it straight away.  Kinda makes your nose twitch for the first 10 mins.

Cars in Jhb produce 17% less power than cars at the coast and you can feel it especially with a car with under 1400cc engine

[guest3250]

So many things can effect the economy - but I am sure you have looked into most of those
A sudden change is more concerning
Mine has dropped a little for the Cold snap of the Winter - but still very good/acceptable
I am religious in keeping tyre pressures correct and also try not to drive to madly most of the time.
Oil would make a difference - if they put a higher viscosity rated oil in - it might change it a little
Let us know if you get to the bottom of this - would be interested to know the outcome
 

[guest3735]

friend of mine was in the tanker trade before i worked with him and claimed that all petrol is graded 1 to 5 when picked up..... bp and shell was always 1 or 2, tesco wasn't.... might be some truth in it... plus, who knows whether their holding tanks are full of sediment? i stopped using tesco for fuel and consumption may be much the same, but the engine seems to run a lot smoother...

[guest3250]

Now you all come to mention it....
I used to think I was imagining it - when after 3 cars I found whenever I filled up with
One particular brand (not mentioned so far) my car would start to run a little rough
or 'off tune' I thought it was the car - then the next car did the same - and then a third
we are talking about a span of nearly 20 years here!! I avoid that brand these days as I don't
want my Jazz to run rough
I have used Sainsbury's Petrol with no probs and also filled up in Leeds with Asda Petrol and managed,
admittedly with careful driving, 63 MPG driving home to Surrey.
So - there dose seem to be something going on here...
Would be interested in brands that people find particularly good (generally)
Meanwhile - I am hoping this particular issue has a reason/solution that we can all hear eventually.