Just after a bit of a techie huddle to see if anyone has any suggestions I haven't tried.
So. Scenic 1.6 RXE 16V 2000 petrol K4M engine. Fuel tank goes down very quickly, no signs of leaks all the way from tank,filter,up aux belt side of engine and to the injector rail or on any injectors. Car runs smoothly and accelerates fine. MPG reading via the computer is about 37-40mpg, which is about right (had the car since new). I change the plugs, injectors, coil packs, air filter, oil filter and noted the dates (last changed Nov 24 due to suspicion of rich mixture).
The plugs didnt really look like a rich mixture (they looked fine - ill attach a picture for others to comment on).
The injectors where spraying pretty well but i replaced them with some cheap ones (kept the originals, and all the ones changed over the years after i cleaned them). Coils are firing fine it seems but i replaced them anyway (kept the old ones).
The car has been smelling rich at the engine end (not really noticeable out of the exhaust) - no smoke black or white out the back.
The carbon vapour cannister which sits down near the abs module (front drivers side of engine) seems to be able to blown through one direction (to vent) but not the other (dont know if thats relevent to the smell)
The tank going down so quickly with the rich smell has me thinking its running rich but diagnostics say everything is fine - I changed the MAP sensor (on the intake manifold). The Intake Air temp sensor in the front of the intake manifold seems to be giving a correct temp at startup then shows about 15-20c above ambient when the car has been running for a bit (not sure if that is normal either). O2 sensor seems to be responding to revs but not sure if its ever been changed (when i unplugged it, i got an error code for it so the heater/readings seem legit. I got a replacement O2 sensor anyway as i can see its not too difficult to get too from underneath - but even after having the car running/exhaust getting hot, I couldnt get the O2 sensor out - more a case of lack of leverage i think and 25 years of 'welding' itself in there.
The tank level sensor again seems to be reading properly, i fill up to the brim, do what the computer thinks is 40mpg for 100miles, then fill to the brim again and do the maths it works out like I get only about 20mpg (rough figures but obviously a long way off 40)
Ive got some new fuel pipes coming but i cleaned all the pipes underneath all the way to the tank to look for any signs of leaks and there is nothing - no puddles where i park etc either. Had it on a ramp at camp with the engine running to see if it leaks under pressure - but again, nothing (and no smell from exhaust all the way until the engine end.)
As for vacuum/air leaks - I took the air filter cover off and put my hand over it - it sucked hard on my hand and then stalled - i think that suggests its not leaking air in elsewhere?
Bit of a waffle but hopefully makes sense.
- Spark plugs
