Low MPG

[edkillick]

Since I've been using my car I've noticed it drinks a serious amount of petrol. I've got a 1.2i saloon that hits about 26mpg at best, seen a few other novae getting 39ish?
It's a four speed and I never really do more than 65 on motorways because its ****ing loud haha. I don't drive aggressively either as I really try to save fuel but it still drinks loads! I mean it might be a dodgy fuel gauge as well as every time I turn the engine off at night lets say my petrol gauge will show its half full but then I go out in the morning and I've got just over a quarter tank so unless some ones driving my car over night, I am baffled and skint haha. Any ideas how I can use less or anything that could be causing the over fuelling? Cheers

[Jack]

Check for leaks if its disappearing overnight!

[edkillick]

There's been no puddles or signs of petrol anywhere I've been parking which is what's worrying haha, probably having my fuel siphoned lol

[MK999]

fuel evaporates.

[burgo]

Tank could have even gone porous

[edkillick]

It's not gonna evaporate over night that much over night haha

[paul james]

If fuel is disappearing overnight then I really would check for leaks first, jack the car up and have a good look at the fuel lines, the fuel filler neck and its connection to the tank, the tank itself maybe rusty. A small leak that you may barely notice will lose you a lot of money in fuel.

If its genuinely using a lot of fuel when running, then it shouldn't be too hard to narrow down as the 1.2i is a pretty simple engine set up. A dodgy lambda sensor perhaps? Check the basics like the spark plugs, air filter, fuel filter, is the exhaust leaking anywhere?

Do you ever fill up the tank? the tank sender units aren't great, so you might just be thinking you are getting poor MPG judging by the fuel guage which could be inaccurate anyway. The senders seem less accurate the less fuel there is in the tank, so if you only ever half fill the tank you may never see an accurate reading from the gauge.

[Calamity Josh]

i get around 50 to the gallon out of my 1.2 carb 4spd on motorways at 70...

also when are you looking at the fual gauge, mine moves around tonnes when i'm accelerating/ decelerating, i'm guessing thats common lol

[edkillick]

Cheers Paul ill have a look. Just had a powerflow backbox fitted which does chuck a small amount of black fluid out now and then. I've been told about my lambda sensor before so it could well be that, never looked at the fuel filter myself so will check that out. Just had a new air filter put in at rs MOT in December so I can't imagine it will be that. I calculated it from a full tank last time I filled up right down to empty and got about 200 miles from a full tank which worked out at 26.8mpg but I will jack it up next week and take a look at it all. Thanks for the help mate really appreciate it! Josh that's what makes me think there's something up if you can get that sort of mpg haha, my gauge reads empty when I go round roundabouts lol so you're not alone!

[meritlover]

If you have been told about your lambda sensor before, surely it would make sense to change this first? they have a pretty massive effect on fuel consumption and also prevent you from cooking your cat.
if you go through 3-4 full tanks of fuel, the difference saved in MPG will likely pay for a new lambda sensor. So it's a false economy to put off replacing it.

Your method of calculating MPGs is flawed, if you have an erratic gauge then you will never know when its empty unless you actually drove it until you ran out of fuel.
The best way is to neck the tank (after you've checked for leaks) drive around and then neck it again and use the pump to record how many litres it took to fill it back to the same point. Then do your calculation.
A couple of litres either way will throw your calculation and your average MPG.

Its likely there is an issue causing low MPGs, but until you work out what the figure is accurately you might be thinking its worse than it really is.

[edkillick]

I wasn't told about my lambda sensor in relation to fuel consumption, I had out a new air filter in and my dad was saying it will ruin the lambda sensor? Well I've worked out the mpg per tank which has been roughly the same, I used a calculator I found online you just put in how many litres of fuel you used and how many miles you covered and it does its little thing like that. Reason I haven't replaced my sensor is because a few mechanic friends of mine have said that it won't be that causing it

[meritlover]

i think you are perhaps missing my point. It doesnt matter how accurate the calculator is, it only works if you can enter the exact number of liters of fuel in.

regardless of what the lambda sensor was in relation to, it measures O2 and lets the ECU control fueling which will therefore impact MPG.

were they good enough mechanics to tell you what WAS causing it then?

[edkillick]

Oh I see what you are saying now. I only text them about it, so they couldn't get to my car lol but its got worse as of late which is why I'm asking on here as well to get more opinions on it than just two who probably couldn't be bothered to tell my why it could be the lambda which you guys have told me, I don't know enough about cars to tell myself what it is

[meritlover]

it is a difficult thing to diagnose. As said, these engines are about as basic as FI gets so there is very little to go wrong, but it still means that something like a coolant temp or lambda sensor could be faulty, the base trigger timing could be off, cat blocked/melted, low compression, brakes binding/caliper stuck, low tyre pressure, air filter, driving style etc..


...or you might have nothing wrong at all.

[edkillick]

I've noticed my engine has never got properly warm since I've had it, even on 3-4 hour drives it never really leaves the blue bar on the temp gauge which could have something to do with it? Only thing I really can do is just replace said bits one by one until its sorted or just accept I will have to be skint lol

[meritlover]

The gauges are never accurate, even as a guide. you really need to measure the engine temp by a more suitable means. it is possible the stat is jammed open causing over cooling and enriched fueling but i would stick to what you know first.
blindly replacing components normally adds to the problem.

what were the circumstances that made you think the lambda sensor was faulty?

[edkillick]

I didn't have a problem with it really, but as I said I was told that its the sensor that could be doing it, two people said because of my fuel consumption and my dad said because of the air filter but my dad seems to go into extreme measures on things like that, ie telling me that putting 16" wheels on my car won't let me go above 50mph lol I could replace it but as you say replacing them willy nilly could just add to it. What you're saying about the temp makes alot more sense than the lambda sensor, but I will have a check over it all with a mate and see what we find, cheers mate you've been helpful :)

[meritlover]

the air filter is a perfectly reasonable suggestion.
larger wheels will also make your speedo read less than the miles you are actually doing and will affect your consumption calculation slightly.
there are lots of factors.

good luck with finding where your missing fuels are going.

[MK999]

It's not gonna evaporate over night that much over night haha

Really? Empty a complete jerry can over the drive and see if it's still there tomorrow.

No wet patch doesn't mean it isn't leaking.

[edkillick]

If you fancy paying for it lol

[burgo]

My car didnt get hold through the emissions on thr mot cos the lambda was knackered. It was stupidly rich. Just grab one from the scrappy. Also a new thermostat is only a couple of quid and makes the world of difference

[meritlover]

Its a one wire lambda. its not going to be expensive. probably safer and cheaper in the long run to buy new.

[burgo]

Im sure it was £70 I got quoted. Could try 100 from th scrappy for That

[meritlover]

Id be surprised. a Bosch LSU 4.2 wideband is only £55.

[meritlover]

I wont post links. but even a cursory glance on ebay shows them between 14 and 20 pounds.

[therealnovaboy]

before you by a lambda, check there is no holes in the exhaust first.

[Adam]

Single wire lambda is about 20quid, can replace it for even less if you get a "universal" one and just swap the plug over.

[edkillick]

Putting my car over a pit tomorrow and gonna check the fuel lines and see what's happening with that and go from there, and I'm gonna get a new stat as well.