It will be running lean for a short while until it re-learns the new parameters, imagine an asthmatic being given new lungs and asked to do a 400m sprint... thats what your engine is going through at the moment....
You can help it along a bit better so it learns quicker by doing a soft ECU reset:
Disconnect the battery, both terminals and touch them together. Wait about 45 minutes for any charge in the ECU and loom to dissipate.
Turn ignition on and leave it for 90 seconds
turn ignition off but keep key in for 90 seconds
turn ignition back on for 90 seconds (this is known as a throttle position reset)
Start the engine from the last part of ignition on right away and don't touch the throttle pedal
Let it idle for 15 minutes...
Now take it out for a spirited drive for 20 minutes using all different rev ranges and slow/ medium cruise speeds and full throttle...
Go home and turn off, job done.
Try cleaning your MAF before doing a reset.
Here is how to:
Stuff that you need to arm yourself with is:
Isopropyl alcohol spray (maplins) for cleaning the Maf sensor wires, DO NOT TOUCH THE WIRES AND RESISTOR, JUST SPRAY AND LET IT EVAPORATE good stuff, not as harsh as Carb cleaner and WILL NOT damage the wires, it WILL remove all the contamination.
Cotton buds (raid the wife's make up kit... I did)
To remove the 5 pointed security screws on the MAF you can use a pair of long nosed pliers on the outer edge of the screw head. A bit of a chew on but will come out,they aren't in too tight. Once you have removed them, get replacement standard cross point screws from B&Q etc... Take the security screw for comparison of thread and length and speak to one of the wise jedi- like pensioners who work at B&Q, they will sort you out mate.
Glass cleaner (raid the wife's cleaning cupboard..... I did) for the angled mirror inside the MAF sensor
Now you are armed with the essential MAF and total induction cleaning kit (will last years BTW) you can tackle the job, no more than an hour total
Easy as pie : http://www.alfa156.net/fr_index.html?/tech/MAFguide/
Written by Jeff Porter no less and a God when it comes to technical know how and saving money on buying MAF's when they aren't needed
When you put it all back together, dont forget to rub a little bit of silicon spray on the rubber sealing ring with your finger on the MAF to make sure it gets an air tight seal and use the isopropyl alcohol to clean the electrical contacts on the MAF (once refitted in) and on its multiplug to ensure a good electrical contact.
Get stuck? give me a holla and I will try and help as best I can mate ok
Induction hose splits:
Remove the rubber induction hose and inspect for slight splits.
Use three rubber inspection gloves and elastic bands to seal up the openings and dunk it in a bowl of water to check for any slight leaks in the rubber, air bubbles!... Just like you would do with an inner tube
If you find any, use the glue (vulcanising fluid ) from a cheap bicycle repair kit and rub it into the splits and hold together for a few minutes... It will end up stronger than it was before as it "welds" rubber together
Clean the throttle body butterfly valve
with carburettor cleaner spray while you have the induction tube off... Do not touch the butterfly
Turn ignition on and get a helper to press and release the throttle pedal to partially open and close the butterfly while you spray the cleaner in... Don't go silly or you will flood the tops of the inlet valves and it will backfire when started up... As Del boy says (a little dab will do ya!)
Now while your helper holds the pedal down, very carefully use a clean rag and gently rub the excess dirt and grime away from the edges of the outer ring that the butter sits against when closed... If you can't get a rag in, then use cotton buds soaked in the cleaner but be careful not to drop them in the throttle body! (Remember: Do not touch the butterfly itself, no matter how tempting it is)!
You have now given your induction system a thorough going over and wont have issues with it in the near future... Repeat every year!