Lately I've been having a low rpm misfire, the engine doesnt run as smooth as it did (it never really ran THAT smooth...) and if its in 5th or sixt, if i give it 3/4 or full throttle it backfires a little. Also my gas mileage has dropped to about 16 city/25 highway, it was 21 city or so before and at least 30 on the highway. I took it to autozone to get the codes scanned and there was nothing stored. From searching I figured it was the 02s (its just about to hit 80,000 miles) but there was no code, do they go bad without setting a code? Or could it be something else? I did pull the front driverside spark plug to take a look at it, maybe I didnt get it back on all the way or something. its an m6 2000 Z28. Thanks!

 

Similar drop in MPG, numbers are like yours, no code at 40.000 miles.
I would like to know why?


LS1 M6, no mods, so far changed air and fuel filter

 

low rpm misfire? while under load or at idle? makes a big difference. give more details.

 

what rpm are you giving 3/4 and full throttle in 5th and 6th cause if you do that below 4k then its expected to bog...at least a lil bit.

 

It is while under load, like I said if I've got it in a higher gear at say 1500 rpm or so, itll buck every two seconds or so. I know it will bog a little but there is a noticeable difference iin the past week or two. Also, as with 2001z28man, the car is stock to the paper filter.

edit: it also doesnt seem to do it as much (if at all) when the engine is cold then when it is hot. Thats why i suspected 02s

 

Ok well I tried it in the morning when it was real cold and apparently, it does do it when its cold...

 

BTW my engine slightly vibrates, sort of rough idle, so it started to be a little bit annoying, misfire or what?

 

i had a similar problem.. happened to be the spark plugs.. i just replaced mine with TR6's because im running n20 and since then its been alot smoother