Hi, I have a 1.4L 16V Fabia I, 2004.

 

I got a CEL and read off a P0170 Fuel trim code. 

 

From then on the mpg has dropped from a consistent 45 to massively lower readings, varying from 20-35. I've seen short term fuel trim push up to over 30% just gently revving to ~2k in the driveway.

 

I've measured the fuel pressure in the rail and it's constant around 47psi, which is fine.

 

I'm getting readings from the O2 sensor which seem within normal limits, and no codes about it being faulty.

 

 

The engine seems to be running fine both at idle and while driving.

 

My next step will be to drain the tank and refill, and use that to calculate the mpg. I'll also use Torque app to calculate, in case the trip computer is giving false readings. However, I think it's being accurate as the needle seems to be dropping a lot faster than usual.

 

 

 

Does anyone have any thoughts or experiences like this?

 

If there is any more information I can give or readings I can take then let me know!

 

Thanks

Reset the ECU and see if the code returns. Also consider replacing your ECT sensor since it's cheap and easy.

Ah, yeah I forgot to mention that I cleared the fault codes and nothing has reappeared, although the economy has remained the same.

I replaced the ECT sensor about 10 months back, and it's giving reliable readings to the OBD scanner. I replaced the thermostat 3 months ago and replaced the coolant, so all of that system seems to be working perfectly. As far as I can tell, all systems are working perfectly, but just burning waay too much fuel.

In that case I suggest the pre-cat lambda has drifted out of spec.

There's only several things that dictate fueling in partial throttle.

Throttle, load (map/maf sensor readings) and compensation tables based on IAT, coolant etc etc, ignition timing to certain extent.

These cheap Obd readers don't have the best refresh rate to accurately read say an o2 sensor when it bounces around stoichiometric afr.

If your idle was bogging or higher or lower than usual then it's most likely o2 sensor in cat failing secondary one that basically checks if cat is still there or rather functioning if this has failed it could fail lean thus car adds fuel or map/maf sensor.

Looking at it a stft of 30s is at its maximum level and if in + it's adding as much fuel as it can thus will trigger a cell as it hit rich threshold, it'll never be 0 but should be low figures, so either vacuum pipe, failing map/o2 sensor etc,as above if high ect or high IAT it will add fuel or indeed remove it (lean out) thus ecu adds low of fuel as a rich condition is safer well just before bore wash lol.