After putting the 195 t-stat back in to see if it'* the cause of my recent crappy gas mileage, I was running the car to check for leaks. I decided to rev the engine, mostly because I like the way it sounds. I did it quickly but noticed a sputter.

So I slowly ran the rpms up and right around 4k, it started to mess up. It was a shudder in the car and the rpm dropped a bit. This happened about once per second while I held the pedal there. It'* hard to explain, sorry if it doesn't make sense. Is there some type of rev limiter for when the car is in park?

It felt and sounded like a spark plug miss, but I would have expected it to show up before 4k rpm. And if it is missing, I can only imagine how red my cat converter is getting as I'm driving down the highway, with unburned fuels and such.

Could this be the culprit of my missing mpg too? I'm waiting for the car to cool down now so I can go reseat all of the plug wires.

By the way, this was sitting in the driveway, in park, park brake on, a/c running...just in case it matters.

That'* the rev limiter kicking in to prevent engine damage. It will not allow higher than about 4k in neutral or park.

Edit - Have you changed your O2 sensor? That would be the first suspect (after a good tune up) of bad gas mileage.

:P Whew! I'm so glad it'* supposed to do that. I have not checked/changed the O2 sensor. I'll look for a way to test it (if there is one). Aren't there two on this car? an 05 SE.

Thanks for the help Archon.

There are 2 - one in the rear exhaust manifold, and one behind that cat. No need to replace the rear one which is there to make sure the cat is doing its job, unless you get a code. The only way to check it is to watch it on a scan tool.