My 2009 RX-8 will be 9 years old in March (and has about 68,000 miles on it). For the last two years or so I've been noticing that it feels down on power, especially at high RPM. I also keep track of gas mileage for every tank of gas via a Google Sheet. In 2017, the fuel efficiency was down about 2mpg for my commute to work (84 miles each way, almost entirely expressway) compared to new, or about 10% down--I used to get 21 or 22mpg. Now I'm lucky to hit 20mpg. I use cruise control set to 70mph and have the entire time I've owned the car.

I was thinking it was time for a compression check, but I'm wondering if I'm being too paranoid. It's expensive at the dealer, and I don't want to waste the money if I'm barking up the wrong tree. I have none of the typical symptoms of low compression. The car starts fine under all conditions and runs fine (other than what I *feel* is a lower power output compared to new, and slightly reduced fuel efficiency).

I have yet to replace the coils. Last spring or early summer, I paid to have the dealer do a full test on the OEM ones, and they claimed they were fine. I know that it's time to replace the spark plugs again. I was planning on doing coils and pugs as soon as I get it out of storage this spring. I also had them check the cat, and they said it was fine, although I don't know how they determine that. After the last time I replaced the plugs, there was no noticeable improvement in power, as I had noticed the prior time--it brought the car back to life, but I may not have waited as long to replace them the first time.

I'm trying to decide if I should bother with the compression check, or if I should do the things I was planning to do regardless: coils ("C"), plugs, and BHR catless midpipe. Then after that see how things are. On the other hand, if my compression is low, I was planning on having a new engine built and custom-tuned by the nearest place, Chip's Motorsports (or maybe Elite Rotary, which is quite a bit further away). If I do that, I guess I could have them use the new plugs, wires, and midpipe.

Thoughts?

 

I would start with the compression test so you know where you are with that . You can do one yourself by buying an ordinary cheap gauge. I have a procedure for using it so you get the results you need if you go that way.

 

Get it tested for peace of mind, but I would much sooner blame the ignition coils. 68000mi is double the recommended replacement period round here.

 

It's always time for a compression test.

 

Thanks all. I'll stick with my original plan to get the compression checked. I want winter to be over!