Low compression cause?
 

So I just received the dreaded low compression results, and I'm most likely going the rebuild route. I am posting mostly out of curiosity, and am wondering if anyone has any insight about the specific mode of failure of my engine, just from the compression scores. I realize I will find out once it is torn down, but I'd be interested if anyone can predict what went wrong without looking at the internals. Perhaps pointless, but hey I'm curious. Here are the scores:

kgf/cm2 Rotor 1 Rotor 2
1 4.7 7.7
2 5.8 7.8
3 7.7 8.6
RPM 289 298

The massive difference in the faces of Rotor 1 seem to me to point towards an apex seal problem. What's interesting though is that the engine was rebuilt with new apex seals only about 15000 miles ago. That should also mean that excessive carbon buildup is unlikely, right? Just spitballing here.
Any ideas?
04 MT about 116000 miles, 15000 on engine

Yes, the big rotor 1 difference points toward an apex seal problem. Being rebuild 15,000 miles ago suggests that it wasn't a proper rebuild, and more stuff was reused than should have been.

More than likely something was out of spec or close to it when it was rebuilt. That or the side seals were not cut perfectly (if cut to fit were used) or shabby aftermarket parts were used. I have seen brand new Atkins seals come scalloped from Atkins. Guessing won't help though, it needs to be torn down.

After only 15k miles? Who rebuilt it?

If I were guessing, I would say one of the apex seals is hosed on rotor 1. Either broken, which is unlikely because without a peripheral port, the pieces hand around until they destroy something, or just stuck and not moving in and out.

If it were my car, I would pull the lower plug and look through the plug hole as a buddy slowly turns the engine over. You want to get the seal under the plug hole. I would poke it with a screwdriver and spray some seafoam on each one. Try to make sure it wiggles around. I would NOT use a hammer or anything like that. If I found one that did not move at all, and then got it to move, I would be a really happy person.

There is a video out on youtube of a guy doing this with a 13B that had no compression. He hooked a compressor up to a plug hole and it just leaked out. He did this on both rotors and when he hooked up the compressor, the motor spun around and around.

It is a long shot, but will only take a few hours. Beats a rebuild if it works.

Of course, if one of the seals is broken, or missing, you are hosed.

Good luck.

Good idea, I might give that a shot. Thanks!

Interesting info everyone, unless 04Green's method works out I'll let you guys know what the results are when it's torn down!

Even if you free it up if it is stuck due to carbon build up, the fix will be short lived because it will just happen again. It needs to be torn down before it ruins parts that may be reusable if you tear it down now.

OK well I just checked it out, and all of the apex seals seem to move OK. Some might have had a smaller range of motion than the others, but I can't be certain. What is interesting is there is definitely a coolant leak into the rear rotor, when rotating it a substantial amount of brownish-tinged coolant seeped out of the spark plug hole. Strange though because my rear rotor compression numbers looked OK (wait just making sure, rotor 1=front right?).

if it was just coolant, it would be green/clear, depending on what you have in the system. If it's brown, that means it isn't just coolant leaking, and it is oil as well. That oil is likely increasing the compression test results, as it will cling to the seals and help seal them better...until faced with combustion.

Regardless of compression scores now though, that engine is definately and conclusively dead.

Still kinda wondering... Just as an addendum. So some of us who are interested in future rebuilds know who to steer clear of.

It may not be a rebuild problem.

Has it ever been overheated since the rebuild? Even for a minute? Be honest.

I agree the engine is dead, but do you think the brown tinge could be from remnant omp oil from the numerous failed start attempts?

It wasn't a professional rebuild, so you needn't worry about avoiding anyone. It was a friend of mine who has experience with rotaries, and I helped and watched. We most likely messed something up with the build, still glad we did it though. Truly fascinating to see inside a rotary, they're much more finicky then piston engines that's for sure! We also followed the Atkins rebuild DVD just for safe measure, which is why I'm a little dissappointed that the rebuild seemingly failed :(

I can honestly tell you it never overheated, ever since the rebuild I spent more time watching the temp needle than the road! ;) I know how sensitive rotaries are to heat, and I was paranoid that it would happen. The needle never moved for a second.

Surprisingly they usually handle overheating better than normal engines when it happens, the temperatures are just so much higher that they are constantly at risk.

Just an idea, but could the brownish liquid be coolant mixing with light carbon buildup and coming out together?

OP,

Rotaries ARE VERY sensitive when it comes to heat actually. Even a slight overheat can warp a housing. Once that housing is warped even slightly, the water seal no longer seats properly and will or will eventually start leaking into the combustion chamber.

The "temp needle" you refer to is almost useless, as soon as it budges, you've overheated.

Thats why most purchase aftermarket temp gauges to read the actual temp.

I have dealt with the "brownish" liquid, but mine was due to the previous owner putting DEXcool in with FL22... It became...Chunky...


Hope that helps clear some things up.

Travis

Not accurate Legot. Most piston engines can survive 250F-270F, if not more without any lasting damage. If the engine is old or weak the head gasket may fail at these temps.

The rotary can easily sustain damage as low as 220F, and our needle doesn't start moving until 235F. By the time it gets to 240F, engine damage is virtually garuanteed. Not certain, it's possible to avoid the bullet in the russian roulette game too.

All consumer engine fluids are designed for operating temps of 200F, and manufacturers use thermostats to set the target temp as close as possible to this. So if everything is working correctly, any modern car will be running at approximately the same temperature. So the rotary doesn't run any hotter.

The difference is that the rotary is dumping more heat into the cooling system than piston engines, so our cooling systems are under more strain and stress than piston engines, and when that stress catches up and things start failing, everything goes bad in a real big hurry.

Ah, sorry.

Rebuilding an engine is a difficult task for most. The hardest part is determining what should and should not be reused.

Well I finally got the engine apart today, and here's what I found. Turns out the problem with the first rebuild wasn't bad parts or incorrect installation, but something much more trivial (yet clearly equally catastrophic): assembly lube. During the build, we had used a significant amount of grease to keep the seals in place during rotor install, as well as to provide initial startup lubrication (also because the atkins DVD said to). I had thought that after running for a few minutes, all of that grease would burn off and leave a clean, happy engine. Instead, it clearly all just turned into a massive amount of carbon buildup in and around all the seals on the rotor. This caused many of them to get stuck in place. The apex seals seemed least affected, but the side and corner seals where so frozen by the carbon that I almost broke them trying to dig them out of the rotor.

Now the grease I used was a general purpose automotive grease, not specifically designed for engine assembly lube. I really should have been more careful and used a more appropriate product, but I figured the intense heat from the engine would be more than enough to burn it out. I'm currently doing more research into the different kinds of lubes and greases, and why the stuff I used acted in the way that it did.

Just goes to show you, the smallest details....

Well duh. The DVD said to use grease? Typically Vaseline is used to hold the corner seals in and side seal. And assembly used can be used elsewhere and specific break in engine oil if you choose.

Atkins video is a half a..... rip off. Thats my opinion since I bought it and was expecting something more thorough...Anyways, This may or may not be of use to you...but watch this video. It's an rx-7 rebuild 4 hours long for free. From tear down to re assembly. Just search rotary rebuild on YouTube and watch vid series by Blake Martinez.

yes thats odd all the rebuilds vids ive watched use Vaseline. have not seen the Atkins video.
I am building a engine and will make a slidshow once done this picture is interesting and shows why the engine had low compression