Chevrolet Lacetti Station Wagon auto - Poor mpg - KCSRenault


My everday car a chevrolet lacetti 1.8 auto sx, is now doing terrible mpg used to get around 30mpg average, now only around 12mpg. Used to get 350 from a full tank now only around 210 miles. The car has been checked, no fault codes, went to a specialist locally who is put it into a full diagnostic mode and nothing came up as being out of sync or not working. Took the plugs out and they are very black and sooty, clearly car is running rich.

Does anyone know what would be casuing this, everyone seems to be stumped around here.

Car is really good other wise and has been well serviced,

Any help much appreciated,

Chevrolet Lacetti Station Wagon auto - Poor mpg - elekie&a/c doctor

I would suggest run the car into a garage/mot station that has an exhaust gas analyser and check the actual exhaust emissions.Is the engine running up to adequate temp?

Edited by elekie&a/c doctor on 13/12/2017 at 09:14

Chevrolet Lacetti Station Wagon auto - Poor mpg - Railroad.

There are an endless number of possible reasons for an engine running too rich, which it sounds like it is if the plugs are black and sooty. Forget fault codes. The presence of them or otherwise doesn't prove anything. Just because there are no fault codes does not mean there are no faults. It only means that as far as the ECM is concerned nothing is outside of its parameters.

Essentially it means the engine is drawing too much fuel or too little air. First you need to do some fuel trim checks with a suitable scantool to see if the ECM is adding fuel to compensate for a lean running condition, or taking fuel away to compensate for a rich condition. Then if so you need to figure out why. Possible causes would include a blocked or dirty air filter, faulty mass air flow sensor, stuck open EGR valve, blocked or worn fuel injector nozzles, blocked exhaust or faulty O2 sensor.


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