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Well...the RX-8 engine that was supposedly replaced 3,000 miles before I bought the car was NOT actually replaced. Long story for another time.
I had it compression tested yesterday and got these values at 250 rpm
1: 5.8, 6.0, 6.1
2: 6.0, 6.0, 6.2
I get those are bad, engine could go any minute, etc.
The odd thing is that it runs extremely well, starts right up hot or cold, doesn't smoke or sputter and I don't have any CEL codes. (Goat has been sacrificed to ensure this continues).
The car was hardly driven for about three years before I bought it. Coils, plugs, wires, starter and battery are all in good shape. Is there anything besides failing apex seals that might cause the readings to be that low?
I've resigned myself to a new engine somewhere down the road, but curious if anybody has ideas on what might be causing it. Could it be due to carbon?
Is the tester accurate?
Its not unheard of, but rare to not have symptoms with those readings. Drive it til you start having actual hot start problems.
Also compression is compresseion. Whether its apex seals or side seals doesn't matter, opening up the engine means replacing all of them. "Apex seals" is what people who don't know rotaries assume the problem is.
...I had it compression tested yesterday and got these values at 250 rpm
1: 5.8, 6.0, 6.1
2: 6.0, 6.0, 6.2
The odd thing is that it runs extremely well, starts right up hot or cold, doesn't smoke or sputter and I don't have any CEL codes....
...Is there anything besides failing apex seals that might cause the readings to be that low? Could it be due to carbon?
Compression is what compression does: principally cause the engine to start and run normally, ...which you're saying yours does.
Low compression c/b yes...apex, side or corner seals. Depending on how it was maintained & driven ...also carbon build-up. You could try a de-carboning treatment, e.g. Seafoam, etc. IFF the issue is carbon...it will help.
Otherwise enjoy her while she lasts, and plan for a rebuild in the future.
Is the tester accurate?
Its not unheard of, but rare to not have symptoms with those readings..."Apex seals" is what people who don't know rotaries assume the problem is.
Thanks for the responses. The shop has a great reputation, so I would be surprised if the tester was wrong. And certainly guilty as charged regarding rotary knowledge but glad to have knowledgeable people like you guys to help out.
I am going to start premixing on advice from the shop and run it harder. I will also look into sea foaming, but that's not so mething I would try myself. Will keep you posted.
Rotor 2 results were
PSI: 65, 69, 57
Kg2: 4.6, 4.9, 4.0
Bar: 4.5, 4.8, 3.9
kPa: 449, 479, 390
More simply put, those numbers are so low they are practically not even on the compression score chart most commonly referenced
So yes, a failing (even severely failing motor) can fire up and run seemingly just fine. Will you be notably down on power? Oh yeah. I had no leaks, nothing that would suggest any sort of issue. Granted, probably helped that my starter cranked the motor between 310-340 rpm consistently and both my fuel and ignition systems were very healthy.
Last edited by Ricky SE3P; 10-15-2019 at 01:31 PM.
I think that hot start actually doesn't sound healthy...
No it doesn't sound very healthy, but not as bad as some Rx8's we have seen with much better compression than mine. The point I was just making to the OP was considering how much worse my compression was than the OP's and it still ran and drove fine with no oil, coolant, or any other leaks and no CEL isn't really a great indicator of engine health nor good/bad compression which my Rx8 was a perfect example of. It's great to be optimistic, however low compression is simply a result of degradation of the seals and combustion surfaces and a decarb- treatment wont be able to resolve those types of issues. Hence why I pointed out it being the engine that -was- in my Rx8, it recently received a heart transplant a few weeks ago by myself to get it back on the road. That was the solution, nothing else.
...I had it compression tested... I get those are bad, engine could go any minute, etc.
...Is there anything besides failing apex seals that might cause the readings to be that low? I've resigned myself to a new engine somewhere down the road, but curious if anybody has ideas on what might be causing it. Could it be due to carbon?
Originally Posted by Xero Ryuu
...however low compression is simply a result of degradation of the seals and combustion surfaces and a decarb- treatment wont be able to resolve those types of issues. ...
Xero - The OP stated understanding that his compression #s aren't good d/t "failing apex (or other) seals", AND that he's resigned to needing a new engine in the future.
Of course the decarb process isn't intended to resolve his low compression..., but given the uncertain maintenance and driving style(s) of previous owner(s)...; it is a low cost attempt at making the best of the engine's current condition...until the inevitable.
Xero - The OP stated understanding that his compression #s aren't good d/t "failing apex (or other) seals", AND that he's resigned to needing a new engine in the future.
Of course the decarb process isn't intended to resolve his low compression..., but given the uncertain maintenance and driving style(s) of previous owner(s)...; it is a low cost attempt at making the best of the engine's current condition...until the inevitable.
I agree with you 100%, and my intentions were to try and give the OP hope that despite his motor having failing compression, he can still take as good care of it as he can to get the most out of it for as long as he can just as I did. The only reason why I hard-parked my 8 was due to my numbers eventually getting so low that I simply didn't trust the engine to be my daily form of transportation any longer (and my numbers were FAR lower than the OP's).
Best of luck OP in keeping your 8 running as long as possible. Staying on top of care of wear and tear items and regular service will go a long way in keeping your 8 on the road. Decarb may help however I would focus closely on premixing, using a quality oil, monitoring coolant temps and replacing items like your water pump or thermostat preemptively, watch your coolant tank and radiator for leaks and replace your coolant tank AND radiator at the first sign of leaking. Also keep an eye on the catalytic if you aren't currently catless, and service/replace your ignition system annually.
I agree with you 100%, and my intentions were to try and give the OP hope that despite his motor having failing compression, he can still take as good care of it as he can to get the most out of it for as long as he can just as I did. The only reason why I hard-parked my 8 was due to my numbers eventually getting so low that I simply didn't trust the engine to be my daily form of transportation any longer (and my numbers were FAR lower than the OP's).
Best of luck OP in keeping your 8 running as long as possible. Staying on top of care of wear and tear items and regular service will go a long way in keeping your 8 on the road. Decarb may help however I would focus closely on premixing, using a quality oil, monitoring coolant temps and replacing items like your water pump or thermostat preemptively, watch your coolant tank and radiator for leaks and replace your coolant tank AND radiator at the first sign of leaking. Also keep an eye on the catalytic if you aren't currently catless, and service/replace your ignition system annually.
Genuinely appreciate the info from all of you. I am indeed more hopeful of getting some decent life after your vids as my hot start is much better than your old engine.
I find anything over 6 bar will run reliably with a good ignition system. I got one 8 averaging 6.5 bar and I just daily drive it with no issues. In fact, it's faster than the automatic i have that is averaging 8.8 bar.
Brandy, the green one, would track all day long, pass cars, daughter lapped a Z06. I noticed a slight loss of straight top speed (2 mph), and figured that, since the engine was older, and I did not have a history, I would check compression. In psi, one face passed, 2 were at 50 psi, 2 were at 60 and one was in the middle.
So, have fun until it annoys you, then worry. Run good gas through it, premix a touch.
A quick update on this. From out of nowhere, the car wouldn't start Sunday. No dash lights, couldn't even use the remote trunk release. Jump started it, drove it home. Same thing again. I checked the battery voltage across the posts and had 12.5 volts. Hmmm. So I checked voltage across the connectors and had 6 volts. Cleaned some corrosion off the postive connection and had 12.5 volts, starts right up.
My question is...could the low voltage from the battery have impacted compression test? I know compression isn't dependent on electrical, but if the battery wasn't sending adequate voltage to the starter would that affect things?
I apologize if it's a dumb question, but I don't know how compression tests are calculated.
Rotary compression tests adjust for 250 RPM so even if it wasnt cranking hard, doubtful it would change your numbers significantly. You still have low compression but you may have an issue with the battery or the connectors. Did you load test the battery? Just because voltage is good doesnt mean the battery itself is fine.
Odd thing happened. I drove home from work, car sat for an hour. I drove it two miles to the store. Came back out, no dash lights again. Nada.
I lifted the battery connectors and reconnected. Interior and dash lights came right on when I opened the door.
Tried to start and it barely did, but did died right within 1 second. Tried a couple more times. Let it sit for 10 minutes.
Tried again, got it started although I had to feather the gas to keep it running for a few minutes.
So two questions.
1. Any ideas why the output from the battery drops so low, and lifting the terminals makes it work again?
2. Does it sound like I flooded it? (never been there...so far)
If it started then it isnt flooded. Make sure the battery wiring harness looks good. Best to remove the battery box and intake to have a better look at it. Make sure the wiring looks good and the fuse in the positive battery terminal is ok.
Gonna rat myself out here, hopefully it helps somebody else to not panic when they see weird electrical stuff.
When I cleaned the battery terminals, I apparently didnt get them far enough down on the post when reconnecting. So they weren't tight allowing them to wiggle around and get loose while driving.
Hopefully this solves the problem.
10-15-2019, 11:04 AM
, but given the uncertain maintenance and driving style(s) of previous owner(s)...; it is a low cost attempt at making the best of the engine's current condition...until the inevitable. 
