The cause could be a whole manner of things, but in the interest of safety, DO NOT drive the car in such a way as to make the lambda meter read lean on boost, assume that it is correct and that the car really is running lean; that way if it is a dodgy lambda sensor you have just driven the car more calmly than you otherwise would... if you assume the sensor is wrong and the fuelling is right, but it turns out that assumption is wrong, then you may end up with one or more melted pistons, which is very wallet unfriendly.

The best way to figure out what is really happening is to get the car connected to a SelectMonitor or a copy of DelatDash. There are three main things that could explain the readings... a) tired lambda sensor b) tired MAF sensor or c) tired fuel pump. Figuring out exactly which one it is should be fairly straightforward...

Do a quick run and check the engine load and MAF voltage seen by DeltaDash. If it's "low" then odds are that the MAF sensor is goosed and should be replaced IMMEDIATELY. If the load reading is fine then it's not the MAF so it could be the lambda sensor or the fuel pump. The best way of figuring out which it is would be to fit a fuel pressure gauge, and take the car for a run. If the fuel pressure maintains roughly 3 bar difference between it and the boost pressure then it's fine (ie should read 4 bar at 1 bar of boost). If it falls away but the boost remains, then the pump is goosed. If it remains but the lambda reading falls away then the lambda sensor is goosed.

In any event, you'll have identified the failing component and can replace only what's necessary. If it's the MAF or the pump you MUST replace it... or you must not drive the car until it has been replaced. Don't risk it, it's not worth it... I'm currently supplying some bits to a chap who ran his car with a dodgy MAF.. result: burnt a hole in No. 2 piston, complete engine rebuild and new turbo required! Ouch!

If it's a tired lambda sensor, it would be very wise to replace it but not absolutely necessary. A tired sensor will still allow the ECU to control fuelling on cruise, and since the ECU doesn't listen to the lambda sensor on boost, if it's low it's irrelevant. What you do risk, though, is that if there were another problem that really did make it run lean, then you'de never know.

Hope this helps,

Pat.

 

Thanks Pat, greatly appreciated

I havent been driving over 4k rpm, I dont most of the time anyway, cept maybe 1st & 2nd gear, but even then not that often

In the 6 months Ive had the car, ive covered 12-13k miles, there is absolutly nothing different to the car's running from the day I picked it up, fuel consumption the same, idle the same, performance the same, engine noise the same (all average for a car of this year)

I understand what you are saying, but if one of the above was knackered then I would have noticed something different

It is however booked to go the select next saturday morning, as I dont have Delta Dash, nor do I know anyone with it either, it is something ive looked at buying, but if I need a new Lambda & Maf then I cant afford Delta Dash, or If I buy Delta Dash, I cant buy a replacement Maf & Lambda, I also dont know anyone's who I could borrow to test if mine is up the spout

Yesterday I inadvertently started the car, and backed it off the drive, and the MAF was unplugged (I had earlier removed the OE Airbox, to check my plugs (but my plug socket aint deep enough, as my dealer suggested, checking the plugs is a sure fire wire to tell if the car really is running lean) and I had forgot to reconnect the MAF Sensor

Needless to say, the car coughed & spluttered, wouldnt idle properly and cut out on me, reconnected the MAF, and the car was sweet as a nut again

Im gonna see if I can find some screened cable too, and do as JamesN suggested, as its the only other thing I havent done, bar replace the sensors

Steve