bad fuel economy

F

FLATSPOT

Member
Hi
i have had my 2004 3.0 z4 for 4 months now love the car but im having some issues when the engine is cold and i start it up the revs go up to about 1200rpm then drops right down and the engine nearly stalls but it then idles ok when the engine is warm and you start it its fine idles nice straight away also it seems to be using a lot of fuel i do 9.2 miles a day average speed 35 miles a hour 6 days a week and im putting £50 in every 2 weeks i also own a nissan 350z which is tuned i can use that for 2 weeks for £30 im not sure if the to issues are linked does any body have any ideas
 
9 x 6 x 2 = 108 & £50 gets around 6/7 gallon so quick tot up suggests under 20mpg :o agreed that aint good
The Nissan more like 26/27 is about right ? The Z is very wrong somewhere :? Have you tried running a brimmed tank for 30 miles then brimming again ?
 
RickRob said:
Your's is making my ///M look frugal!
Click to expand...

Doesn't make my maser look frugal.
But it ain't far off. :o

High thirst/tickover when cold is sometimes a symptom of a gummed lambda sensor or blocked air filter. Check the air filter first, but have you tried an Italian tune up?

Disconnect battery for 30 mins, reconnect, take it for a decent 20-30 mile blast over open roads using bouts of full throttle, then disconnect the battery for 30 mins again?
The battery disconnection thing could be peculiar to Italian electrics, but it is supposed to allow all the electrickery bits to reset and relearn.
The other bit is just an excuse to go for a hoon. :wink:
 
The air filters fine that's the first thing i checked i will try disconnecting the battery for 30 mins trick and see what that does
 
Firstly, clean your exhaust before you commute and check for black soot on arrival. If its dirty inside (it shouldnt be after 10 miles) the engine is running very rich. You can also use your nose to check if it smells rich (smells fuelly). The M54 engine typically runs quite lean.

If rich, 2 main culprits will be the MAF and the pre-CAT Lambda(s). Clean your MAF with Brake/Carb cleaner spraying in the direction of air flow. The MAF is not brittle on these cars unlike others. If that fails, i would then change the lambda sensor. If a lambda has gone bad, it will likely only affect a bank of 3 cylinders (each manifold has its own IIRC), so i would pull the spark plugs to see if the front 3 look any different to the rear 3.

Also check your brake pads aren't binding - i know this wont affect your idle, but it will certainly affect MPG! You can usually feel this when rolling to a very gentle stop, but also by the convected heat from the wheel arches.

Does the car use much oil? Some M54's can have a it of a thirst problem which could also affect the spark plugs.
 
Check intake boots for splits. Remove them and check properly rather than a quick once over. Take everything off from the MAF to the throttle body.
 
the car does not use any oil it does not smell rich either it has full BMW service history and had a full service just be for i purchased it i was thinking it might be the lambda sensors they look like a right nightmare to change i will try cleaning the MAF and also check all the intake pipes for splits and clean the exhaust and see what thats like after my commute thanks for your suggestions every one
 
Took the MAF off yesterday and all the in take pipes to the throttle body no splits in the in take pipes clean the MAF it look clean anyway took the plugs out seems to be burning normal disconnected the battery for 30 mins reconnected took it for a 20 mile drive disconnected the battery again for 30 mins reconnected started the car revs went up then right down nearly stall so it still seems to be running the same as it was be for I am running out of things to try
 
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