toplessFC3Sman

After a number of track days and 93k miles (10k under my ownership), over the course of a day of daily-driving (all in-town, dropping my son off at school, going home, running an errand etc) my S2 RX-8 started misfiring sporadically, then almost constantly above 50% throttle, until the low oil buzzer went off as I was getting home and I pulled the intake off and found that it was completely oil soaked. I check the oil level regularly, and had changed it about 600 miles before (that mileage spanned 6 months and two HPDE days), so I know it didn't start off low. The catch can that I had installed when chasing down a similar issue at a track day a couple years ago was completely full too (the catch-can has been dry almost since I installed it, the issue was topping off the oil in haste before my session with the engine idling - some got sucked into the various vacuum lines). Anyway, even a small amount of time running would result in pumping a lot of oil out of the crank case and into the catch can, then into the intake. The engine started up pretty well (as long as the MAF and plugs were cleaned of the oil), and idled fine, but rotor 1 was low on compression on all faces. Anyway, after taking apart and cleaning the intake, oil fill manifold, and the associated tubes a couple times, I had convinced myself it was rebuild time.

So, on to the rebuild, the rear rotor looks pretty much fine... a bit of wear evident on the housing plus 2 minor gouges (not sure what those were from), and one of the side seal springs had broken apart into a few pieces, but all the pieces were still contained behind the side seal. The front housing was a bit more roughed up, and two of the three side seals on the center iron face of the rotor were at least half-missing, with no springs left behind them and only a couple tiny bits of metal found on the trailing plug face and in the exhaust port. The oil seals on all rotor faces were intact and appeared undamaged.

Obviously, this isn't good news, but would this pattern of damage result in pressurizing the crankcase to a large degree? I'd expect poor starting, running, misfires etc that accompany low compression and too much internal exhaust gas residual, but not blowing tons of oil out the PCV tubes. I'm trying to make sure I'm not missing something else when rebuilding the engine that will set me back with these same issues.

Thank you!

Loki

iTrader: (1)

If the side seals are missing, where does pressure go on the compression "stroke"? To the oil channels and oil pan after it. So that sounds consistent.

Possible rotor 1 saw detonation for some reason? Losing seals catastrophically isn't really usual..

toplessFC3Sman

Since side seals on two of the rotor faces were missing, I guess I would expect it to leak from one face to another, or get trapped and push a whole bunch of the fuel/air mix into that giant crevice area where it would not want to burn. I didn't think it'd push past the three intact oil control rings to get to the crankcase, at least not in the amounts that it seemed to, but who knows - it was all on the central iron so if the pressure were enough to push the rotor towards the front iron, it could find a gap right into the pan I guess.

As far as the sudden nature of the failure, that I'm still puzzled by. Over the winter I had done a little bit of ice racing with the car (basically like a long rally-X stage on a frozen lake), but I'd make sure that the car was well warmed up first and wouldn't be just hanging out bouncing off the rev limiter for the whole time, that's not the way to find grip. Other than that, it was normal driving around about twice a week (usually I'd take the van for DD duties, using the '8 on occasion of fresh snow or something). There was really no "event" that seemed to trigger this since it had been about 3 weeks since the last ice race and the car had run wonderfully for that event, the 2 hour drive home, and DD duty since then.

XDragon8

Great detail upon teardown. I suspect most of us that have been through the rebuild process after a catastrophic failure never really find out which bit let go first. Afterall often many bits often go through the engine taking others with them.

My high km 2005 definitely had all the low compression symptoms but it still went great until the day it let go in the countryside. As it had high km i never really planned to use much of the old engine in the rebuild anyway. Turned out it was toast of course. If i had not diven it until BANG would anything have been different during rebuild? Probably not!!

toplessFC3Sman

Ok, I guess its parts-damage-opinion time...

First, I'm planning on new housings front & rear - the front has some obvious damage from bits of the side seals getting driven into the exhaust-face of the housing, so I don't think this one would be reusable even if it could be re-surfaced.


Otherwise there are minor scratches, most of which I can't quite catch a fingernail on. There's also a bit of a different wear pattern in the center vs sides along the housing, with some slight chatter marks, but none of this seems unreasonable for an engine with 93k miles on it. Oddly, there are some notable grooves running around the whole inner circumference of both the front and rear housing that don't seem to be mirrored in the apex seals, and the rear housing also has some similar evidence of something getting driven into it by the rotor on the exhaust portion of the surface. Maybe bits of the front rotor side seals made their way into the intake and over to the rear rotor? Either that or this engine has been apart before and the housings were re-used. There was a lot of RTV squeeze-out between the housings, irons, and sump, and the clutch is pretty new, so maybe?

On to the irons - the wear pattern looks pretty normal I think, with the exception of a couple spots. The trailing edge of some of the ports looks a little chewed up by bits of broken seal that got caught there, the finish on the trailing edge of the exhaust port does not seem as smooth as elsewhere, and particularly on the front face of the middle iron, there are some deeper gouges and divots presumably from the side seals coming apart (this was the face that the broken seals were riding on).

Front iron, mostly good except for some chewing on the trialing edge of the lower intake ports and different finish on trailing edge of exhaust port:

Middle iron, front face, definitely some damage at the trailing edge of the intake port and on the sliding surface from breaking seals:


Finally, the rotors. The rear rotor looked fine - some carbon deposits but thats not unexpected. The front rotor was a bit more scuffed up, including scrapes around the rear face presumably from the side seals coming apart.

Also, on the leading edge of the apex seal groove there was a bit of metal junk building up. Its definitely sticking proud of the machined surface, and doesn't seem like its been pushed up from inside the apex seal groove (not sure how this would happen, especially on the leading edge of the groove and not on the trailing edge), so I'm guessing that this is what happened to the missing parts of the seals and their springs... most of them got chewed up and squashed by the rotor here, getting stuck as the rotor spun and dragged them along until they hit the apex seals. This is on all 3 apexes of the front rotor, only on their leading edge, and is not apparent at all on the rear rotor.

The apex seals mostly look good, and I'm on the fence about re-using them, but figure if I'm getting new housings, may as well get new apex seals to try to get as much life as possible out of the rebuild.

So, anyway the plan is to try to get good-condition used front and middle irons, or barring that get the ones I have re-nitrided. Clean up the rotor surfaces and that debris on the front rotor apex groove, get new housings, apex seals & springs, side seals & springs, corner seal springs, and all the soft seals (all the oil seals and bearings look like they're in good shape). The corner seals themselves all look good, but I still need to look through the FSM to see if there is a spec to measure them to.

XDragon8

Not sure about the irons but I would suggest you choose your preferred colour scheme on rotors and housings for 'Garage Art'. Possibly a coffee table piece if your partner likes to be reminded of where all that cash wemt!!
I somehow ended up with 3 spare rotors so went with 1 red-chrome/ 1 polished/ 1 polished with full set of seals superglued in situ. Happy days.

toplessFC3Sman

Well, the engine is going back together now - new housings, apex seals, side seals & corner seals. There were definitely wear marks on the sides of the corner seals where the side seals would butt into them, and moreso on the face of the rotor that saw the side seals fail. Hard to tell whether these were caused by those two seals getting too hot and expanding into them, getting jammed into them when the seals got broken/torn out of the grooves, or whether they were too tight in the first place. I got a used front and center iron in much better shape than mine from a friend, so I'm planning on using those as-is.

The new side seals are all clearanced to 0.1 - 0.15 mm, using 0.15mm shims to hold the side seals to the inner face as appropriate for the RX-8. I made a little fixture to hold the dremel and allow the angle of the side seal to be more consistently controlled when grinding it to size, and I've triple-checked all the clearances to make sure that none of the m end up too tight.



I also suspect that this engine was apart before, due to some minor impact damage on a face of the rear rotor (which did not have any seal failure) without corresponding damage in the rear housing, and a suspicion that those side seals were too tight. It also seemed like a lot of RTV was used between the "legs" of the housings & irons, and for the oil pan - there was a pretty consistent grey sealant bead that was squeezed out in all these locations that seemed a little sloppier than I'd expect for a Mazda-built motor (not that I have much experience with mazda OEM quality, it just seemed a bit sloppy). However, there was still the wax pellet in the end of the eccentric shaft which many people seem to want to replace with solid - I'm planning on keeping the wax. Are there any other obvious ways to tell if its been apart before?

toplessFC3Sman

Many months later and I've got some compression numbers. These are all warm numbers, corrected back to 250 rpm, measured in a similar way to the DIY compression tester on here with a dataq 1100 data logger, 200psi pressure sensor and adaptor for the spark plug hole that is only a couple ml bigger than a spark plug. The most worrying is the front, which has been consistent at 6.6-6.7-7.2 +/- 0.1 from 5 miles in to today at 163 miles and a few trips to redline. This has me worrying about a damaged apex seal or missing apex corner or something...

• Is there any way to check this without pulling the engine?
• would more driving now be more likely to help or hurt this condition and/or damage engine components?

Driving-wise she feels fine, pulling smoothly to redline, starting up hot or cold immediately etc, so a part of me wants to declare victory and hope the numbers improve, but...

kevink0000

With those numbers and the mileage, I would keep driving it and check again later. Not worth pulling it at this point. What rotors did you use? I don't see it mentioned.

toplessFC3Sman

I re-used the rotors that came out of the engine after cleaning them, lightly filing off that metal ridge that had built up around the apex seal grooves (probably bits of the old side-seals), and checking the seal groove gap sizes. Unfortunately I don't have comparable pre-rebuild numbers since I only put together the pressure test setup recently, but I'm most worried about the difference between faces instead of the absolute numbers anyway.

kevink0000

It's too soon to tell. If this was mine, I would run it at least 1500 miles and see where it stands.

I will be honest, I was hoping you didn't reuse those rotors. Besides the thin scratches side damage, which may or may not be a problem, the apex seal groove looked pretty wallowed out, lots of displaced metal at the tip.

But, this may also be helpful, if you haven't seen it already:

This guy's numbers came into line, but it took more than a couple hundred miles.

https://www.rx8club.com/series-i-tec...umbers-271334/

toplessFC3Sman

I was a bit concerned about the rotors initially too. A few of the scratches on the sides tended to pinch the side seals and keep them from moving freely, but some very gentle sanding with 200 grit paper wrapped around the blade of a putty knife opened them up enough that they wouldn't pinch. The apex seal grooves didn't appear to be wallowed out based on feeler-gauge measurements - that displaced metal near the ridge was only on the leading side of the apex seal groove and I suspect that it was the remains of the missing side seals getting chewed up between the rotor and housing - I didn't find any large pieces in the exhaust and without a peripheral exhaust port it would be harder for them to make their way there. Anyhow, that was my justification for re-using them - if the engine comes out again I'll re-take those measurements to check.

I saw that thread you linked to and it's encouraging, plus I'm not looking forward to having to tear everything back out again. I'll re-measure after a few hundred more miles and we'll see if this is going to be a winter project.

kevink0000

The displaced metal on the leading edge is put there by the apex seal pushing against it in normal use. It is not deposited side seal metal in my opinion. There will be similar wear in the groove itself on the opposite edge where the bottom of the apex seal "*****" in the groove. Did both rotors have that same amount of moved metal?

kevink0000

You might find this interesting also:

https://www.rx8club.com/series-i-tec...easure-267605/

This was my engine. Needless to say I did not reuse either rotor.

toplessFC3Sman

The build-up was only on the leading edge of the front rotor. The trailing edge was clear of it and did not appear pushed up, and the rear rotor where there were no seal failures or anything and no build-up at all. That (well, and the measurements) is what lead me to believe it was the seal, not rotor damage.

kevink0000

OK I see. How much better/different is the rear rotor at this time?

toplessFC3Sman

The rear rotor was 7.2-6.6-7.5 right after rebuilding, I didn't take the rear rotor this last time since I was doing this in a parking lot waiting for my son to get out of an activity and I forgot the right u-joint extension to reach the rear.

kevink0000

"Sir, license and registration please. And what are you doing to your car in this parking lot?"

"I am checking compression."

"Oh, I see its a rotary. I understand completely. Here's your ID back. Carry on."

toplessFC3Sman

I remembered to bring the right combo of U-joints and extensions today, so I got both the front and rear in the parking lot waiting for my son (now at 520 miles). The front went up overall, but still has a similar difference: 6.7-6.9-7.6 kgf/cm2. The rear is more consistent at 7.2-7.1-7.0 (so that high 7.5 dropped but the 6.9 [6.6 before was a typo] came up). If anything, that 7.6 kgf/cm2 on the front rotor is now looking like the outlier.

kevink0000

If you keep doing that in the school lot, they will call the cops on you, guaranteed. "Chester the Tester".

If it were mine I would keep running it, and check it again at 3000 miles, but not sooner. I bet everything comes into line at that point, to an acceptable degree.

I very much doubt there is much of anything wrong with the build, not worth taking it apart again. Unless you want to, to try to get the best result possible. But more downside to that approach in my opinion.