07 2.5Si Sport N52 fuel usage!!

[Montjeu]

So I've owned my car for over two years now. The fuel economy has always been a concern since purchase. I took it back to the dealer, they claimed they carried out all the checks that they could and because it was meeting manufacturer minimums, if I wanted any further investigation they'd take it to bmw but at my expense. I didn't take that up.
Originally my commute involved free running motorway and I averaged 25mpg. Recently I moved work, now no motorway but mostly free running A roads with a bit of traffic home, and I'm averaging 18mpg. This is calculated manually over 2 months using Shell V Power.
I reckon this is way too high and about time it was sorted if at all possible. Since purchase I've had an Inspection II, brand new tyres all round, and a hunter alignment.
Do people think I should simply get my trusted indy to do a full investigation? Is there a good starting point? Just break my heart a little and move the car on?

Thanks

[itguy]

Absolutely ridiculous

If you're based anywhere near me i'll help you check over everything that could be an issue but here is a list to go at;

Rolling gear - are the wheels all 'free running' when you jack the car up? Any wheels binding shows either a brake calliper that is sticking (common) or handbrake adjustment that needs to be looked at possibly.

Tyres / tyre pressures - are these all ok and checked? Wheel alignment also

Engine - when was it last fully serviced (by fully, i mean fuel filter, proper decent spark plugs, air filter etc) and with what oil?

Does it have a 'sports' air filter fitted, an oil coated type like KN? If so, the oil may have contaminated the Mass Airflow Meter sensor and it may be mis-reading, over fuelling the engine

Does the engine warm up correctly on the gauge to dead centre? If not, thermostat is probably dead or the sensor is faulty (rare). I haven't looked but it is also possible that the ECU temperature feed from the engine is from a different sender to the gauge, so may be worth checking also.

An OBD2 usb dongle and free software on the laptop will help see if there are any problem with sensors / fuel trims etc too.

18mpg or even 25mpg is just crazy. My 3.0i (not the Si, which is even more efficient) does 37 mpg on my motorway commute and only down to about 30 when trundling in heavy town traffic. I've NEVER seen less than 25 even after some proper heavy foot action.

I can almost categorically say that there is something wrong with your car so don't despair - it can be sorted, just a process of elimination.

If it was in my garage, i'd be doing the rolling wheel stuff and then looking at fuelling (i.e. MAF, temp sensors etc). The kind of drop in MPG you have over standard is dramatic and typically only things like a thermostat or MAF issue would cause these big drops.

Happy to help you find it if you are local

[Montjeu]

Thanks for the kind offer itguy, I may well take you up on that I'm in Milton Keynes so not a million miles away from you. Must warn you though my mechanical knowledge is amazingly basic, and I can just about change a wheel on my own

The wheels should be free running, never tested myself but they've all been off for the tyre swop and the fitters never said there was a problem. Tyre pressures were fine, I can check those again. Hunter alignment carried out earlier this year.

Full service was about a year ago. I was hoping that might improve economy, but if anything it dropped off a little. I've never had to top up the oil in over two years of ownership.

The air filter is standard afaik.

Temperature gauge goes straight to centre and doesn't move once warm.

My indy hooked it up to his computer, he said the only thing it was throwing up was a very tiny mis-fire on one of the cylinders.

[itguy]

Ok, well it sounds like you've got a good basic place to start from then.

My next steps would be to get the car to normal operating temperature and look at the stats from the OBD2 interface via a laptop. I'd be specifically looking for long term fuel trim %ages (as this can show if the MAF or lambdas are failing) and see if the temperature being electronically given to the engine is the same as the gauge is indicating.

Obviously scan for any codes etc but i suspect there won't be any from what you've said.

How black is your exhaust? 18mpg is really quite rich especially if you haven't been thrashing it everywhere!

Happy to take a look with you or can point you in the direction of the usb interfaces etc if you want to explore yourself?

[Montjeu]

This is the receipt from sevice, so oil is castrol edge 5w30

[Montjeu]

Took a quick snap of the pipes (Monkey tips) I hope none of the detailers look in!

[Montjeu]

Checked tyre pressures:

NSR : 34
OSR : 31
NSF : 28
OSF : 31

So could do with leveling up on the same axle. But don't think that's the cause of the problem.

Couple of points to note, there's usually a sign of liquid drops on the ground when I've parked up etc. The dealer told me it was normal, could have been a fob oc!

And every now and then I get a smell of fuel in the cabin when entering from cold.

[itguy]

Ok - do you know what those few drops of liquid are?

If you run the aircon all the time they could well just be water through condensation on the pipes.

But, if they are fuel, you need to get the whole car fuel lines checked over ASAP. The smell of fuel in the cabin really isn't right and could be a minor fuel leak.

How are you getting the 18mpg-25mpg figures calculated? Is it manual calculation or via the on board computer? Have you checked the on board computer accuracy? I ask as sometimes the gauge can be stuck in american MPG not english (i.e. size of gallons)

How many miles can you get on a full tank?

[Montjeu]

Calculated manually. 387 miles from two tanks brimmed, 95.77 litres of fuel. I've tried to smell the liquid, and couldn't get anything so likely it is water. I could try turning the AC off and see if it's still there.
I can purchase a OBD2 usb dongle as it may come in useful for other things as well.

[itguy]

That would be my next step for sure. I have one of the basic elm327 ones and came with some free software that does the job ok.

[Sim_Simma]

I get much better figures in the M on those sort of runs. Definitely sounds out. Most people do seem to exaggerate their figures so can be hard to find out exactly how well it should be doing. But my week consists of about 100miles. I'd say my commuting would be classed as a combined run. But in rush hour it's nearly all stop start. No motorway driving and I'm averaging 21 on the clock I the m.
So to summarise, buy an ///M

[Zookoo25]

Hi - I have the same car... And get around the same values in mpg as read by on board computer. Also the same random whiff of petrol very occasionally in cabin and occasional water drops on floor.

No advice to offer though just a sympathetic ear. I am going to take mine to a garage next week for something else but will also mention this.

[shortfuse 2]

the smell of petrol is not good get that checked my m never drops below 22 mpg but I do live in the country

[TonyP]

2.5si here too, but with fuel consumption averaging low 30's on a mixture of town & dual carriageway driving, that's according to the trip computer, never brimmed the tank to do a comparison so cant be sure how accurate it is, any way, I don't use the car a lot so tend to stick £25 in when the fuel light comes on rather than filling it right up.

No smells of fuel here either so I would say you have a problem, but I hope it's a quick & cheap fix.

[jamesgarbett]

Hi, same engine here and just for comparison I usually get 325-375 miles from a tank, that's a mix of motorway and urban driving. Hope you get it sorted.