I know there are lots of threads and lots of possibilities but I wanted to start one that hopefully will stay on track and highlight what to check and when, if this problem arises for anyone in the future.
Firstly, my Ep3 has been my daily since early September since selling my Fn2. It started with a similar mpg to the Fn2 of around 31mpg but has slowly got worse, I'm guessing it's around 24mpg at the moment. Oil usage is a bit more than normal, but I have changed the oil and transmission fluid since September.
90% of my driving is the commute which is 18 miles each way on an open A road with a mile or two of city driving (stop/start). Speeds average 45mph, a few bursts of vtec from junctions etc
No EML, no loss of power, no misfire or rough idle. Tyres are good and set right, no brakes binding although I am getting a whiff after driving which leads me onto possible problem number 1. One of the o2 sensors could be on it's way out. Number 2, it could be the cat on it's way out.
Other causes could be....
3. Bad spark plugs - I checked when I bought the car and they looked recent
4. Faulty injectors - I need to pulse test and also check with multimeter
5. Dirty Air Filter - the air filter on the teg was clean when I installed it, don't think it will be this
6. Throttle body - need to check it's clean/not stuck open
7. Valve clearance - I really need to get this done, could be a cause, especially when the oil consumption is up too
I'm going to do a compression test when I re-check through plugs, it can't hurt
Any other causes or opinions? Please add