Chevrolet Repair: gas mileage, air filter oil, gas mileage

Question
hello . I've got a 2004 silverado with 64000km and i was wondering if you knew how to fix my problem . when i bought my truck it would get 500 km to a tank and now i only get 290 average . what should i look for ? i just changed air filter . oil . could it be o2 sensor or what else would make it change that much ? thanks dietl

Answer
Hi Dietl,

Gas mileage can go bad if the O2 is worn out.  I will explain in a bit.  But first I want to tell you what else can cause the problem.  Bad plugs, wires, leaky fuel injector, not shifting into OD, low tire pressure, timing (not adjustable), egr issues, computer getting misinformation from sensor and adjusting the fuel delivery too rich, fuel leak, and on and on.  

The O2 works in this way.  It is constantly giving the computer a numerical reading.  It will tell the computer a number between 0 and 999.  It will drift high and low every second.  A number above 500 tells the computer the fuel is rich.  Below 500 says lean.  So, the computer sees 876 and leans the fuel delivery for that second.  Then see 397 and riches the mixture for that second.  This continues every second the engine runs.  If the O2 says totally rich and never drop below 500, the computer will run only rich.  It sees nothing wrong with this since it is responding to the O2 input.  Therefore, no check engine light will come on.  And your economy goes way down.  

Hope this helps.

C J S