I've been trying to figure out what all to check on my 08 Caliber SXT to figure out why it dropped about 100 miles per tank over a six month period. I have since changed the spark plugs, done fuel treatments (Sea Foam), and added an after market muffler. It has improved, but I would really like to find the initial cause to the mileage drop to hopefully get it fixed.

Any thoughts as to what else I could check would be appreciated. I'm probably going to have the cat cleaned by having air blown through in reverse.

 

Welcome to the forum
Not sure where you are located, but winter blends of fuel are less efficient than summer blends and can lose a couple MPG. Also if your gas has 10% ethanol, that can cost a nother couple of MPG. My truck loses about 2 mpg from winter fuel.

 

Thank you, I very much appreciate getting some feedback.

I'm in Houston area so we don't get cold enough for the higher ethanol, but I bought the car brand new and this has happened in the fourth year of owning it. Ran the same and just fine the first three years.

 

How many miles are on the car now?

 

My SXT hates certain gases, like HESS and Shell. If I put those in I lose quite a bit of mileage over all on my tank. Sometime I just forget about it and end up putting it in. It's not like I drive a race car.. LOL!! My car does love chevron and Sunoco though. Always better tanks with those. The only additive I run is a fuel injector cleaner through it at every oil change. That's it. Nothing else goes in my car. I have 37k miles on it and it has been running the same since I bought it. I do lose a little bit of mileage too in the winter months. I live in South Florida where it is warmer than the Houston area. All gas is winter blend. They don't adjust it for certain area codes. Someone can take their car to colder climates so it is safe across the board.

Back to what lxman asked, how many miles do you have? What kind of oil are you running in it. If you are running a cheap conventional oil, you might have a sludge problem. The 3 year mark and it is now just starting to notice the performance may indicate that. On a side note, I have been running synthetic Pennzoil or Mobil1 since first change.

 

Shes a little over 108,500. Actually now that I think about it, I had used Pennzoil High Mileage on the first change of 2011 because it was a bit after I cleared 100,000, but I was told by two people that it wasn't necessary yet so I went back to regular.

I'm saving up to get transmission fluid now since the manual says to change the transmission filter at 120,000 and I already got that, but we weren't expecting the fluid to be so expensive.

 

It is a special fluid and is quite pricey compared to regular fluids.
As for the MPG issue, you might try unhooking the battery for about an hour or so to reset the adaptive memory in the computer. It can then relearn driving habits and may help the MPG as well.
At over 100K, it may not hurt to replace the front )2 sensor as well. Sometimes as they age, carbon builds up in the sensor and they may cause the computer to run a little richer mixture thus use more fuel.
I would personally try the battery disconnecting first. It's free.

Also, I would stick with the std 5w20 oil over the high mileage stuff if the engine isn't burning any oil between changes.