Have a 2009 RX-8 Touring. It has original engine with all scheduled maintenance and at just under 94,000 miles. It's a 6MT.

This winter it started having a misfire for a few seconds after starting, only when the engine is cold (below 50 F) and sitting for several hours. The CEL flashes and occasionally sets OBD code P0302 (cylinder 2 misfire), which I assume means rotor 2 (rear rotor?). The misfire lasts typically only for 5-10 seconds, then it suddenly becomes butter smooth. The misfire never lasts more than 30 seconds, even when I started it the other day at 15 F. Engine uses about 1 quart of oil every 1500-2000 miles, and has always been the OEM recommended 5W-20 synthetic blend. Car has never overheated, never has a problem starting, and never had any issues at all really - I think the only complaint would be my taillight with some burnt out LEDs (and no aftermarket replacements available).

I read the sticky and have done the following:
- Performed a cold compression test after sitting overnight (at 45 F) and hot compression test (about 20 minutes after shutdown). Cold was 8.1-8.3, hot was 8.7-8.8. I was told that these were good numbers and to look elsewhere for the problem.
- Replaced old battery with new AGM battery.
- Replaced OEM coils with BHR coils, wires, and NGK plugs (all from BHR). The plugs taken out didn't look bad.

Nothing had any effect on the issue. Still, only when cold, misfires for a few seconds after starting then completely smooth in less than 30 seconds. I know a lot about piston engines, but honestly not an expert on the rotary, so I'm not sure where to go from here.

Thanks in advance!

 

I would suggest getting fuel trim data from the startup when the misfire occurs. It's possible it's a fuel delivery problem, that would be my best guess.

 

Finally got back from a long trip. I left it outside overnight in the 30's so it would get nice and cold, then started it up with an monitor this morning. Both the Short Term and Long Term fuel trims are 0.0, which makes sense since the engine runs in open loop until the first RPM light goes out. I dumped a bottle of redline fuel system cleaner in and filled the tank just in case there is a "sticking" fuel injector when cold.

Anything else I can check without having to start randomly replacing parts?

 

It doesnt sound like a part issue. Misfires are generally related to ignition, fuel or clogged cat. Not sure why its misfiring on cold startup. Maybe someone else can give some suggestions.

 

Thanks. I did have to replace the cats earlier in the Spring. The check engine light came on like a week before inspection was due (of course). I replaced the O2 sensors at the same time for good measure.

 

I mean, it sounds like you did everything to fix the issue that I would try. Only other thing would maybe be an issue with the MAF or intake. Perhaps not getting enough air? Did you check your air filter? Even if it was the case, it seems strange that after it warms up a bit it's completely fine. I'm a bit stumped honestly. Possible to make a video?

 

Yeah, the filters are all good. Maybe I will make a video tomorrow morning. It only happens for a few seconds after starting in the cold. Runs like **** for about 10 seconds, then it's like someone flips a switch and it's smooth as can be. It doesn't even have to warm up to go away, just a few seconds of running. I'd ignore it TBH, but that pesky CEL keeps setting code P0302 and that's annoying to look at.

What's the possibility of a leaky oil injector in the second rotor dripping in and causing a problem, which then burns off after running a few seconds? Is that even possible?

 

Do you notice white smoke coming out of the exhaust on startup? Possible could be a minor coolant seal failure. You could try a block test to see if there is anything wrong with your coolant seals. I'm always up for testing and diagnosing rather than throw parts at it hoping it fixes the problem.

 

You may be on to something... I do get a lot of smoke (or steam?) on starting in the cold, and it is white, but I wouldn't say anymore than on any other engine though and it's not really more than there ever was.
BUT, I did have to add a little coolant this winter after the light came on for a few seconds on a cold start. I have been wondering how it got low.

I too like testing to find problems first as well. How would I go about a block test? You've just ventured beyond my knowledge. And if it is a coolant seal... is that rebuilt territory or is it something replaceable?

 

You need a block testing kit, can be purchased on Amazon or even an auto parts store. It's quite easy to do, directions are widely available online. If it is a coolant seal there is no way to replace it without removing and rebuilding the engine unfortunately. I do know that people have had success using Alumiseal or Blue Devil but even those are risky if you have the funds for a rebuild. Possible they can gunk up the engine a bit and make a rebuild harder.

The only real "correct" way to fix a coolant seal problem is an engine rebuild. In your case however, you could reuse everything except your coolant seals. Would keep the costs down, especially if doing the work yourself. Definitely need to test for a coolant leak based on what you just wrote.

 

Thanks. I'll do that and report back.

 

Hi,
I suggest there is a good chance its a water leak,
I suggest trying " Holts radweld Plus",sealant,
has worked on a number of similar issues here.

 

I'm going to do the block test to be sure, but I bet it probably is. Is the radweld safe for the rotary? That'd be a good thing to try. I think if I were looking at a rebuild I'd probably just sell instead.

 

it is thought that mazda uses something similar to this on the test bench to "seal"the engines when first built as reported by a guy working in the reman centre in the US
anyhow a 06 rx8 here in Aus has 60+ 00 ks since using radweld,
including a few track days,
,
who knows really?

 

It depends. It may work and fix the issue or it may not. If it's a choice between selling the car or trying this stuff, I would just try it.

 

I've got the tools for the block test, will be doing that in the next couple days. In the meantime I was looking to find a bottle of the Holts Radweld Plus, but it looks like its only sold in Europe? Anyone know how I can get it in the US?

 

Block test came back negative, but I definitely have the little white puffs from the exhaust while it misfires. I think the seal is leaking, but possibly only very little causing a one-way leak due to negative pressure after shutdown.

I'd like to try this Holts Radweld Plus I hear so much good things about, but can't figure out how to get some. Anyone know how to get in the US?

 

Definitely leaking but only a small amount. Still problematic. I dont know if I would risk using that product as yet. It could do more harm than good. I'd just live with it.

 

I would too, except the CEL won't stay off, so can't pass emissions. Every other start sets the code again. And it's definitely eating coolant, I got the low coolant light yesterday morning and had to top it up. Don't have much option but to try and see what happens. I figure if I'm looking at it needing a rebuild now, or extending it to when I might be able to afford it... the latter is preferred. I really only need another year or two out of the car anyhow... maybe another 20k or 30k miles at the most.

 

I don't like any "fix in a can." Even on a $1000 99 Corolla, I would think a thousand times before pouring it in.

It's like taking an Advil whenever you have a headache without actually finding out what's causing it.

It might require you to replace more components down the road.

 

I look at it more as a Band-Aid in a can. But I'm open to other ideas that don't involve spending thousands of nonexistent dollars?

 

There is not much else to say. Once the damage is done, there is no fix short of a rebuild.

This isn't a piston engine where you can just swap out the head gasket. Coolant seals aren't serviceable like that. Sorry to break the news to you.

 

If you have no other choice you could try it. Very risky but it may work for you, may make things worse. As I said earlier, if it’s either the red weld or sell the car I’d just try it.