2004 Impala SS question

Hello everyone, recently I noticed a pronounced drop in my fuel mileage, and while it's nowhere near some of the other horror stories I've read on here, I wanted to explain my situation, and get some advice...

I have a 04 SS with 50k miles on it. Typically driving around mostly highway, I would get 425 miles on a tank. Thought everything was great. I changed oil at 2500-3000 miles intervals and kept the hull clean.

In October, I started noticing my mpg falling from an average of 30 to 27 on the computer. Now I've read in here that the computer doesn't always tell the truth, so I watched the mileage per trip... sure enough, it was dropping.

First thing I thought was a bad oil change... nope. Fuel filter... nope. Air filter... nope. I needed new tires, so I got Michelin Pilot Sport A/S and tried them at 30 psi and 35 psi... nope. Wash and Wax... nope. Brakes are about a year to a year and a half old.

So now, I'm thinking either/both spark plugs and wires, and possibly the O2 sensor might be going bad. After doing some reading in here, if I replace the wires and plugs, I'm going with the Iridium plugs, but I'm not sure what wires to use.

First question, the plugs, etc are suppose to be good until 100k... is this normal to have to change them at 50k?
Second, what wires and plugs are the best for the car? There are 2 Iridiums on the website, the OE Laser and the Iridium IX
Thurd, What about the O2 sensor?


Thanks everyone!

i know they say plugs are good for 100k miles, but i don't like leaving them in there that long. truth is alot of them are "bad" sooner, but the computer has adaptive strategies to help it run like it should. as for brand and type, what the factory put in is the best, A/C Delco plats or iridiums.

O2 sensor: if the sensor is bad, i.e. not reading correctly, then the MIL(check engine light) will come on. from 30 to 27 is not that bad of a drop, unless it keeps dropping. try switching to another brand of gas and see if it starts going back up.

as far as being at 50k miles, trans flush is due, would go for a fuel/induction cleaning as well. best of luck

Thanks Hippy

I've done different gas brands, and I forgot to mention that I've done the fuel system cleaning and also done the trans fluid changeout.

Right now, my mpg is down between 22-25. I'm thinking a trip to the dealer might be in order.
I'm not getting any check engine warnings, so the only thing I can think is that maybe my plugs are dirty or something. I don't know... I'm just confused, and I can't spend a lot of time with this since I'm starting flight school

I have a 2005 ss and have the same fuel milage problems. Changed filters, plugs, cheaned throttle body, I use only good premium gas....any ideas

O2 sensors fail gradually to the rich side of their calibration. This means they signal the computer that there is still unburned Oxygen in the exhaust stream and the computer will comply by adding fuel. An O2 sensor is not a working or not working device, it ages slowly in most cases, and too many people miss its association with fuel milage. I have an 08 LTZ with 42k on it and I will be checking this on my own car. If there is only one Oxygen sensor it pays to change it, it will pay for itself quickly, at todays gas prices. Some vehicles have 2 or more and they are expensive, but at gas prices today, it will still pay to change them.

I have read that some O2 sensors need changed as early as 25K to maintain fuel milage. Just think about the enviroment in which an O2 sensor works.

Just a helpful FYI There are two o2 sensors on our cars, but the rear one is only used to monitor that the CAT is functioning.

And if you've ever ran seafoam through your engine, it also can sometimes help foul up the o2 sensor.

I have a 2005 ss and have the same fuel milage problems. Changed filters, plugs, cheaned throttle body, I use only good premium gas....any ideas
Changed both O2 sensors...no help. My around town mileage is down to 11 mpg...no codes, checked by the to best shops in town???help anyone