Hello all.

I bought my '07 RX-8 about two years ago, certified used with just under 30k, in beautiful condition. Apparently the previous owner traded it in on a Mazda3 that was an easier fit for the carseat. It's been great, love the way it drives; but after a while I started to notice some odd behavior that would only really show up on/after pretty long drives; like over an hour. It'd feel sluggish, bog down, and eventually a weird irregular rattle started to surface, while under power in the 5500-6200 rpm range. The weird thing would be, once I'd get it home and let it rest overnight, it would be back to normal again the next morning, plenty of power, no rattles anywhere up the rpm range. I've run premium gas (93 octane) pretty much the whole 20k I've put on it, though it's often been discount/Kroger fuel.

Since I was mostly doing short drives, I let it lie for a while, didn't start trying hard to get it diagnosed until I was looking at a crosscountry drive for my move back to California. Needless to say, getting the dealer to know what they were trying to fix was a challenge, since the problems only assert themselves after the car has been going pretty solid for a while. My Virginia dealer's best guess in the time available before I left was sticky secondary intake shutter valves; they cleaned them a bit before I left and the rattle got less pronounced. They put in fresh sparkplugs at the same time at my request; I had spares already

The car made it across the country, but it definitely suffered; oddly enough it actually seemed to get better or worse through the course of pretty solid days of driving (9-15 hours/day). Starting back up at a rest stop just over the California border, it finally threw it's first check engine light (secondary shutter valve stuck open, I believe). It almost seemed to regain some of it's lost power once the CEL was on, but it might have been my imagination on that front.

I've been working through the local dealer out here since then, the Certified Used status came with a 100k engine/powertrain limited warantee, so I'm hoping that should help defray major costs, they've at least been good about getting me into loaner cars as well, which has also been helpful. That said, they haven't really been able to isolate the problem. I'll go through what they've worked through so far.

Replacing the main intake manifold and shutter valves was their first step, since they were siezed up with deposits. On the drive home it threw a new CEL for the lower shutter valve, and started misfiring pretty badly.

The lower shutter was apparently misaligned, the position sensor was supposedly off by 40 degrees or so, they blamed the first dealer for this, but as far as I know, the first dealer didn't touch that part. Correcting that didn't solve the issue either. The misfires were mostly happening on the front rotor, as many as 86 in one key cycle. Next they swapped in a set of used coils in good condition, which helped some (old ones were damaged/arcing). A hot compression test put the front rotor quite low, but not quite low enough to fail the short block by Mazda's guidelines. They also inspected the cat converter and said it looked fine.

I've had their service guy do ridealongs with me when the car was acting up, and he swears the rattle sounds like the engine is pinging, but he's never heard of a rotary doing that.

Again, the car drives well when it's cold, and for as long as 40-50 minutes, then it progressively turns into an underpowered, misfiring mess. Any ideas? Seems like my local dealer is down to trying to negotiate with Mazda for a new short block, so far unsuccessfully.

Thanks in advance for any wisdom any of you can impart!

-Tom

 

Fuel pump failure. Classic symptoms. Your first paragraph had me thinking that, and the conviction only got stronger.

Fuel pump starts to get hot under prolonged use, and when it's near the end of it's life, it will start slowing down and/or stopping. This leads to lower fuel pressure, running lean (pinging possible), stumbling, lack of power, etc... Not the only thing that does these things, but it is the only one that causes these symptoms to show up after driving a while, and the colder it is the longer it stays away.


Please don't pay a dealer to diagnose problems, you just end up paying lots for it.

 

How often do you do hard pulls through the rpm? I'd recommend catch can, premix and better coils - at the very least they are good insurance.

 

Fuel pump is a thought, But, the idea of used coils in good condition is, um, no'ish.

The coils will get hot, when not, they are not happy. If used coils helped a little, a new set (could come from a parts store, not mazda, cheaper that way) might make a bigger difference. Wires and plugs might help as well. They are a little easier to get to than a fuel pump.

I am interested in how this turns out. When mine is hot (sitting in traffic a long time, heat in the engine compartment) it will make a sound similar to what you described. But, I do not see a performance impact of misfire.