think P0400 is egr valve code.i would do a compression check.depending what the oil is like if its black or not i mean mine on 208,000 miles and still going,but read the piston rings on the 1.8 was of a cheap brand.dose it smoke on startup or if you in gear n go down a hill in gear with foot off throttle then at bottom of hill hit the throttle dose it puff blue smoke??i treated mine to a wyns engine flush last oil change
I will try the down hill suggestion. Not seen that before. Are you talking of engine braking, and then floor it?