Hi all,

bought my 2004 RX8 with 115000km (70000Miles). 3 month later, I tested the compression because I had access to a rotary tester.

75,74,80 psi @ 248 rpm

81,82,82 psi @ 252rpm

To my understanding , it's below spec by a good margin (98 psi min). But she does'nt exhibit classic low compression symptoms I read. No hot start (tested a lot since I knew it was low comp), no significant loss of power, no idle shaking with A/C, no hesitation.

What I can see is the idle is'nt smooth. The RPM needle does'nt move a lot (750 ±50 rpm) but I can feel the pop every 3-6 seconds. Like a little backfire. The fuel consumption is on the high side with 15-16 mpg in city, 19 mpg on pure highway. This winter, she was hard to start when very cold (-20 degree C, -4 F or colder)

I changed the coils, wires and plugs but I don't see how it can change the compression.

My question is: Should I wait to have more symptoms or should I consider a rebuild right now before the internals goes south?

 

Robz, thats your decision to make. The longer you run the motor the more wear and tear your putting on it. Do you happen to have a copy of the compression sheet?

 

No, I don't have a copy since it was an individual not a garage who did it with a TR-01. I took a picture of the tester for each reading and I was with the guy the whole time. I saw the tester. Your question about getting a copy is to make sure a shady garage was'nt trying to screw me?

Also, I tried with a normal compression tester back at home because the test with a TR-01 was made without depressing the throttle. So I tried it while depressing the throttle and it gave me about the same result. Obviously not 3 faces per reading but the max it could reach was about 83 PSI on each rotor.

What I wonder is how low can it go (in PSI) before breaking or showing more symptoms? I mean, how low are the engine when they show major problem (stalling, poor idling, won't start hot, etc) I searched a lot and did'nt find a failure mode for this symptom (just low compression and not really anything else major). Overall low compression would indicate the apex's are not broken. Where does it leak compression? I did a coolant pressurisation test and it was OK so the water jacket are probably holding on.



Oh and please excuse my grammar, english ain't my primary language.

 

It's not really an easy thing to estimate exactly how long your motor will last. With you showing 83 PSI per that tells me your already sitting below the healthy threshold. Now are you planning on purchasing a new motor, or are you going to rebuild the one you have?

 

Well, at this point, I just hope I can do summer with it. It's my second car for the weekends, not a DD.

It was my impression too that it's not wise to make the engine suffer more than necessary. I wanted to confirm this.

I live in Quebec City, Province of Quebec, Canada. There's no shop in here that do rotary. The nearest that touches rotaries is Montreal (3h drive) and I don't know if they rebuild engines. After that, it's Toronto (9h drive). So shipping will be a bit steep for a 250 lbs package.

Being the type of guy that's not afraid to try and I document myself a lot, I figured I could at least get the engine out and disassemble it. I have access to some mechanics tools so I could verify what parts are in specs. And from there, I'll decide which path I'll go.

I considered buying a used engine but I'm afraid to buy from a junk yard since I can't really tell the health, unless there's a way I don't know.

So I'll probably end up trying to rebuild it myself. I know there's a lot of horror stories but I think some people sucessfully did it. So it's doable with the right tools and time. Since it's not my DD, I can take my time.

What do you think?

 

If memory serves me well, your engine is still in healthy state. anything lower than 6.9 is below specs if I remember correctly.

For your rough idle, clean your MAF sensor, ESS sensor and also check your air filter, it may have been covered in oil. Yes, excess engine oil.

You mentioned replacing coils, plugs, and wires, may I ask where did you source them from. If they are original Mazda parts than you should be good for another 30k on those, if from ebay, then you maybe at risk of running some crappy knock off products.

Also, try decarbing (Seafoam, or Zoom Zoom cleaner) that may also help boosting compression.

Hope you find this of assistance

 

I got Intermotor coils from a local shop. I read a bit before purchasing them and it seems they are OK. I planned to change them every 30K miles since they are not that expensive.

I did the seafoam procedure. I took my compression before / after and got a grand total of 0 improvement.

I cleaned the ESS and MAF about a month ago. It changed nothing so they were ok i guess. And I do have a kind of greasy stuff in the bottom of the accordeon intake. I'll try to clean in the next few day. Would brake cleaner be too agressive for this "plastic" part?

 

Only reason I can see for rebuilding early is that if something breaks, it will screw up parts you want to re-use. Problem is, until you tear it apart, you do not know if things are already worn.

Best I got.

 

I have heard that adding 2 stroke oil to your gas can help with the seals and actually raise your compression a bit along with increasing the lifetime of your internals.

Check out my signature for an estimated premix chart that ive put together.

Remember to use smokeless unless you wanna look like you are on fire lol

 

More than likely it's oil coming from the fill. Im assuming you DO NOT have an oil catch can?
A picture would help


As for the compression numbers you will need this..

https://www.rx8club.com/attachments/...sion_chart.png

83 PSI @ 250RPM averages out at 6.0kg/cm2= Failing compression #'s


Just as an extra bit of knowledge i use 1/2 oz. Idemitsu 2 stroke Premix/ Gallon of 93
Premixing helps with the lubrication of the Apex seals.

 

Yeah, I came to accept the fact that it fails Mazda's specs. I do premix about 250ml per tank (about what you suggest). And I don't have a catch can. Thanks for the chart, saw it a while ago but could'nt find it anymore.

My question is more about waiting for major symptoms like hot start, stalling, vs early rebuilding. Hard to say from the answers I got, I wanted to have the opinions of the gurus here.

Well, thanks everyone, when I'll rebuild it (if I don't change it) I'll try to document as other have done. It's always nice to see internals.