Poor fuel mileage

[speedyx14]

I overfilled my Firebird with oil(due to a leak) and had excessive smoke coming out of the tail pipe. Black soot was coming out of the exhaust due to the excess in oil. At first I didn't know that this was due to an oil overfill, so I let the car sit and turned it on for short drives just to check on it for a span of 2 weeks. Took the car to a shop, they told my what was up and got the oil to the proper level. The power returned and the exhaust stopped oozing smoke.

The issue: Since the incident, my gas mileage has been shot. The excessive oil has ruined my gas mileage to the point where I will burn a half a tank in less than 20 miles. What did the excessive oil disrupt?. What can I do do get my mileage back? Any help would be greatly appreciated.

[ssms5411]

Re: Poor fuel mileage

I would think your O2 sensors would of took a dump since you were putting all that oil burning through your exhaust. Also EGR would need to be checked.

[loser]

Re: Poor fuel mileage

I would think your O2 sensors would of took a dump since you were putting all that oil burning through your exhaust. Also EGR would need to be checked.

both possiblity's.

i wonder if becuase there was too much oil, and the motor not being able to turn though that much oil( lack of power ) the pcm thought it was under much more load, chaning air/fuel setting to compensate,ect. this again kinda goes allong with the 02 sensors.

[ssms5411]

Re: Poor fuel mileage

I also thinking you should pull a spark plug or 2 to see if they are soaked in oil, that would cause a loss in spark.

[speedyx14]

Re: Poor fuel mileage

^ I actually had someone(shade tree mechanic) change the spark plugs, and this was before I knew that it was an oil overfill causing the trouble. The plugs were oil fouled, so he put new ones on. He gave me a compression test and all cylinders were 180 except for one that measured 170. Sprinkled oil in it and it hit 180 also. I continued to suffer the excessive blow by, so the new plugs put on are probably oil fouled too.

But I will get the plugs and sensors changed, so hopefully that will end my problem. I recently seaFOAMED the car so maybe all that oil is dried up now.
Thanks for all the help.

Side note: another thing that's strange is when I unscrew the gas cap I hear this hissing sound as though pressure is being released. I don't remember noticing this before. Is it an issue?

[loser]

Re: Poor fuel mileage

Side note: another thing that's strange is when I unscrew the gas cap I hear this hissing sound as though pressure is being released. I don't remember noticing this before. Is it an issue?

no, thats fine.

change the plugs and o2's and check back.

[FadeToBlak121]

Re: Poor fuel mileage

how much did you overfill the oil? Ive never heard of overfilled oil causing that big of an effect. but ive never seen oil overfilled more than like a half a quart.

[speedyx14]

Re: Poor fuel mileage

2 -3 quarts overfilled :(

[auto666]

Re: Poor fuel mileage

also unhook your battery for an hour or so to reset the computers peramiters that are deffinataly skewed from blowing so much oil threw the exhaust id pretty much bet your catalitic converter is pluged pretty badly to both with greatly effect fuel economy

[GinsuGuy585]

Re: Poor fuel mileage

Do you usually drive the car for long periods? (1+ hrs) You can probably cook off whatever residue is left in the cat and sensors.

Try new plugs now that the oil problem is long gone, and see if that helps. Check them after 2 weeks for more fouling. If they re-foul there is another problem. Also do an ecu reset (post above mine) or use a reader to do it.