Greetings, I have a 2008 Anniversary Edition (6 speed MT) that is making me scratch my head a little bit. It has ~22,000 miles on the clock, original engine. I am the second owner.

Compression tested good last month, 7.7-7.9 on all faces, tested by Mazda, all recalls addressed. Maf readings at idle seem correct (5.5-6.5), cleaned it anyways with maf cleaner, made no difference.

Fuel trims seem to have something going on though. Long term trims at idle seem to dance from 6-10%, and from 6-12.5% when cruising. AFR seems a little lean too.

Exhaust pipes aren't caked black or anything, but definitely have some dark residue at the bottom on both sides. At least on one occasion I have gotten a good whiff of rich exhaust smell in the garage.

Took a can of carb cleaner and poked around for a vacuum leak with no success, though I'm not sure if this is the symptom of a vacuum leak since the issue seems to still be observed while in motion.

Idle isn't bad, but gets a little vibrate-y when the compressor kicks on though. Am beginning to notice some gurgling if I blip the throttle while parked.

I did get one missfire code a few weeks ago at high RPM. It didn't kick the CEL on, but blinked it and monitored for the code. Hasn't happened again.

I'm going to rig something up to do a smoke test in the engine bay, but my line of thought considering the missfire at high RPM and the mileage of the car is something like the ignition is starting to get weak -> O2 sensor got fouled(?) and is reporting a leaner condition that what is happening?

Am I totally off base here? I've read through the other fuel trim threads, and I'm proceeding based on those discussions, but still wanted to see if anyone had any ideas I might be missing?

Attaching a CSV log with my fuel trims, O2 sensor readings, Maf readings, etc.

Really sorry if I overlooked something obvious here, but wanted to ask the experts. My primary concern is identifying the issue before it snowballs into anything more serious.

I have a BHR ignition system I'm going to install, but I'd like to figure out whats going on before I start introducing new variables.

 

You need to break the log file up a bit more.
Trying to go thru 6k+ lines of data makes my face hurt.


Break the log files up in to chunks.
example

Do a 10 second idle log

Cruise log at 3500 rpm in 3rd gear for 10 seconds
Cruise log at 4500 "" << Might be in Open Loop in this log.


I don't think your LTFT is that bad at 6-7% ... I think I saw some in the 12% but even still not too bad.


The only thing I can see from your post you believe is an issue is the misfire.
Likely Coils or plugs or both. Sounds like you have that handled.


I suppose you could have a vac leak but 6-7% LTFT on the RX8 isn't nothing to worry about.

 

Oh geeze. I guess I didn't realize how big the file was, I have just been looking at the graphs.

I'll grab some new logs once I get the coils in.

I wasn't sure if the fuel trims were normal or off. I see a lot of posts that state that idle should be 0% for the long term, etc.

It likes to settle around 6-7% at idle, 7-10% cruising, and it hits 12.5% pretty consistently right when you let off the throttle while moving. Seems to be fairly consistent regardless of RPM.

I'm probably being too OCD about it, just don't want the engine running lean when its already 100F outside.

 

The guys that are scaling their Maf's are likely to try for a LTFT of 0%
But really anything either side of 0% by -3% to +3% is fine.


I would be willing to bet you would be hard pressed to find any stock RX8 that doesn't have a LTFT of around +-6% or more



Is the car stock?
No aftermarket intake or anything?

 

Got it. That sounds more in line with what I am seeing then.

Car is bone stock, minus the BHR coils I'm putting in right now. Plugs looked a little white when I pulled them to put new ones in. Will post pictures once I get everything back together.

 

When you change the RPM, the car will go into the open loop and your fuel trim will be out of whack.

The chunks of data with constant RPM don't look too terrible to me.

And for the most part, STFT, LTFT, and their sum should all be within ±10% with constant RPM.

 

Got the BHR system installed finally, and took it out for a spin. Seems to idle a bit better, and feels more confident at up at the top.

Fuel trims look to be acting about the same, based on the short term trims. It does still seem to shoot up some times, but as it's been pointed out this may just be when its switching between open and closed loop.

Old spark plugs look a little white to me (lean?) though.

Heard the vacuum break when I had to disconnect one of the lines to get to the coils, so between that and not seeing any smoke when I did my smoke test, if there is a vacuum related problem, I'm leaning towards it not being a major leak.

I'll probably just keep an eye on it for now, run some injector cleaner through the tank, and make my way into one of the local rotary shops when I get a chance.

On the plus side, this was a good chance to test out my new quick jack, so that was fun.

 

Yup those plugs look the the normal wear and tear you would expect to see on RX8 plugs.
They needed to be changed, obviously.

But the light brown ash deposits are typical.

 

quick jack rocks!!

With new ignition, try the 20 brake stop reset, then start cold, let it idle until warm, then drive easy (the fast learn process). If you ignition was heading south, it was not burning all the fuel. This leaves excess O2 in the exhaust, which the computer reads as lean (it measures un-burned O2, not fuel), and will bump up the trims.

But, as said above, +6 is not unusual. The fun thing is watching it change based on where you buy fuel. My local Mobile stations run at about -3, almost everyone else is at +2. That is a 5% difference.