Slump in mpg?

[Parki]

Hi all - my 1.9 Auto recently had an MOT which failed on ' major leak of exhaust gasses ' & ' exhaust emmission Carbon monoxide content after 2nd fast idle excessive'.
I was told the exhaust needed welding & car was then passed ok. Driving away,I noticed mpg display was low 20's but put this down to engine idling to warm up & for test. But after filling up the next day & doing my normal journeys mpg is about 26mpg - about 6 less than it has been (about 32). The question is what would they have done to reduce the excessive carbon monoxide & could this have caused the drop in mpg? Consumption is slowly improving so perhaps its this reset issue that has been discussed elsewhere?

[DC]

Hi all - my 1.9 Auto recently had an MOT which failed on ' major leak of exhaust gasses ' & ' exhaust emmission Carbon monoxide content after 2nd fast idle excessive'.
I was told the exhaust needed welding & car was then passed ok. Driving away,I noticed mpg display was low 20's but put this down to engine idling to warm up & for test. But after filling up the next day & doing my normal journeys mpg is about 26mpg - about 6 less than it has been (about 32). The question is what would they have done to reduce the excessive carbon monoxide & could this have caused the drop in mpg? Consumption is slowly improving so perhaps its this reset issue that has been discussed elsewhere?

I get a lot better mpg than even the 32 you quote in my 1.9 auto so could be a dodgy computer.
Have you checked the mpg by brimming the tank twice then doing the maths? You might get a nice surprise.
The A9 is the nearest to a motorway in these parts and lots of hills and winding A and B roads but I've averaged over 40 quite often.
Dave.

[Lancsbob]

Do all Z3s have a computer to tell the MPG or is it an option
as i dont know how to look for it.

Cheers Bob

[Brian4]

Lancsbob Press in the end of the left hand stalk and the display changes on the console screen where the clock is. mpg, av speed, miles to empty, outside temp and some others.

[Brian4]

Quote 'The question is what would they have done to reduce the excessive carbon monoxide & could this have caused the drop in mpg? Consumption is slowly improving so perhaps its this reset issue that has been discussed elsewhere?'

The mpg can be reset as the reading on the computer is average not instantaneous. If it has been idling or running at the MOT it will reduce the mpg quite quickly if the car has been reset recently.

If you scroll through to the required display and the press the S/R button that item is reset. works for mpg and av speed.

[Lancsbob]

Lancsbob Press in the end of the left hand stalk and the display changes on the console screen where the clock is. mpg, av speed, miles to empty, outside temp and some others.

Mine has an analogue clock not digital so no display, as i say was the computer
an optional extra or is there some other way to show MPG.

Bob

[lightning]

As above really.

Reset the MPG readout or change it to "instant" as it's probably on the "average" setting.

Also the garage may have disconnected the battery to weld the exhaust and the ECU may then take a while to re align itself to the engine.

[Del]

Unless you have an ineffective Cat, a high Co is normally a sign of an overly rich mixture. For an exhaust system to cause this I would assume a leak between the exhaust manifold and first, pre-cat lambda sensor. The "exhaust hole" in such a position might suck in air from outside and thus give a "lean mixture" reading to the pre-cat lambda sensor. The sensor would then erroneously enrich the combustion mixture.

I would guess that the on-board economy measure in these cars is not that sophisticated and might take time to give an accurate reading as mentioned already.

[BladeRunner919]

Mine has an analogue clock not digital so no display, as i say was the computer
an optional extra or is there some other way to show MPG.

Bob

It was an option. And even then, unless you calibrate the computer's mpg reading to the car's actual mpg (which most people don't) it's rather pointless.

See here for calibration info: http://www.bimmerfest.com/forums/showpo ... ostcount=3

[Parki]

Thanks for all input - I'm sure its not a reset/calibration issue as fuel guage is half full after only 120 miles so its definitely running rich.

As del suggested this could have been due to leak in exhaust system but has only started after MOT so its likely to be something the garage has done.

I will go back & ask about this but its good to have your inputs to give me an idea of what I'm talking about. How can mixture be adjusted?

[Del]

There is no ability to manually adjust - it is controlled by the programme (map) put into the car's ECU. Things that could cause an overly rich mixture could include:- poor condition spark plugs: weaker spark at the plugs caused by faulty coil/HT leads (1.9 car): a faulty sensor typically a MAF or lambda sensor: the car is running too cold or the ECU thinks it is running too cold due to faulty feedback from a sensor. An old injector can leak like an old tap which would be evident by looking at the spark plugs i.e. one might look black and sooty compared to the more healthy, light brown others.

Spark plugs are supposed to last 30K/60K for copper/platinum but to be honest I've always noticed an improvement when changing after around 20K regardless of copper/platinum.

[BladeRunner919]

How can mixture be adjusted?

I'm pretty sure I didn't say that!

[Del]

I'm pretty sure I didn't say that!

No you didn't - I've removed - must be a gremlin

[Robert T]

No you didn't - I've removed - must be a gremlin

Is that some kind of Gizmo?

Cheers R.

[Parki]

Again thanks for inputs - I think some of the possible causes would have reduced mpg gradually & before MOT but electrics & bits can go overnight. My Haynes manual suggests cheapest route is to get complete diagnostic check by Dealer & go from there? Will let you know how I get on.

[Parki]

Diagnostic showed fault with oxygen sensor upstream of catalyser code (P0130) dealer estimated £300 to replace sensor!
Local garage warned sensor could be ok but picking up other fault but replaced for less than £100 (bit of a bugger apparently)
Just done 70 mile round trip & mpg display is improving so it looks like could be ok?
Again thanks for help