Interpretation of compression test results
 

So I am having starting issues and am worried about low compression. This is ONLY when the engine is hot, it starts fine when cold. I have brand new battery, plugs, coils, and they didnt help at all. 2004 MT with 75K on the clock, and an occasional seafoaming.

My friend and I (thanks Vik!) used his compression checker (the one twisted rotors makes) and got the following results from the trailing ports:

Front: @182rpm: 71, 71, 70 (in PSI)
Rear: @ 184 rpm: 77, 77, 74 (in PSI)

Now I know this has to be compensated for the very low starting RPMs. Can someone just let me know if I'm doing this right?

If the difference in PSI between 250 RPM and 182 RPM is about 22psi, then is it correct to extrapolate and say that the mimimum spec at 182rpm should be about 22psi lower than the normal minimum spec (98.6 psi @250 rpm)?? So that would be 98.6 - 22 = 76.6 ?? In which case I'm a few PSI under spec....

I've attached a modified graph from the service manual. Am I doing this right?

I need to bring it in to mazda anyway, because if I stall on the road, it doesn't start up for 20 seconds, and it's dangerous. But I'd like to be armed with as much info as possible and be sure I'm interpreting this right before poking around for a new engine.

Thanks everyone....
-Andrew

Is the value in PSI on that tester...or kPA?

If it's PSI its really low...even corrected

If its kPA it would be OK....

No, it's in PSI so yeah I think quite low.
But they are fairly balanced, which is a good sign, so I'm confused.

Though the RPMs are lower than I've seen generally reported here, so there's a lot of correction involved, which is why I'm looking for some 'experienced' eyes on this. It still has the crappy 2004 starter before they were upgraded.

OK I just found this graph, which for some reason shows the plot for lower RPMs, which the other graph did not.
From this post: https://www.rx8club.com/showpost.php...&postcount=123

So I guess I'm......just below borderline?

Nice find with that chart. I guess that puts my engine fully between standard and minimum (with my cold engine numbers so my real numbers are probably lower but still probably above minimum).

Keep us posted on what Mazda says.

The low RPM ratings here could mean that your starter is dying. This is exactly what happened to a friend of mine.

Your results are still low for that RPM.

FYI The correlation is not linear, it's a function, so you can't just add 22 psi.

^ The second chart he posted is a nonlinear chart. It has the appropriate curve and low rpm ranges.

I agree that the starter could be a point of failure here, but can you explain why the starter would have a harder time starting a hot engine than a cold engine? I saw it first hand, he had to crank the engine for a good 20 seconds before it started. Later when it was cold, it started right up instantly.

Yah.....i'm going to mazda tomorrow for an official compression test. I've actually ordered a new "uprgraded" starter [cheap too! ask me where....] But I will leave the original one on for tomorrow, 70psi looks shitty, regardless of RPMs.

OK so get this: I bring it in this morning and tell them about the hard starting and dropping mpg.

Mazda does a compression test:
5.4 5.5 5.4 203 rpm front rotor
6.2 6.3 6.2 205 rpm rear rotor

Front rotor is below spec, even compensated for RPM. The tech agreed with me on that (and said that the computer adjusts the reading for RPM) but said "mazda wants us to try everything before going down the road of a new engine, and we need to get the RPMs up to 270rpm to know" Tries to sell me the $400 new starter.
I tried to stay respectful....but made it clear that i think it's obvious that if the reading is properly adjusted for RPM by the computer, then the motor is not up to spec, period. I tell them I'll be able to buy and install a starter on my own a lot cheaper, and went home.

Now - I called mazda USA who are really really nice, every time I've called them by the way. She was helpful and sympathetic, but said she can't second-guess if the mechanics have a certain diagnostic path they want to follow. Sounds kinda weird but....OK.

Anyway, I'm kinda of annoyed I need to go to extra trouble, but I've ordered a used upgraded starter and am *expecting* that when I bring it back it will fail the test again. At least it better! I'd really rather just start fresh at this point...

It's not about warm vs hot, it's about the starter dying in general. Sometimes it'll work, sometimes it'll struggle.



Bullshit. They want to test it, let them put the starter in and test it.

I wouldn't have said that unless you were planning on replacing your starter anyways. No reason to volunteer the expense.

I replaced spark plugs and coils with brand new OEM stuff one month ago. I told them that from the beginning to "help them along".

This is what's frustrating. And I even said I'd buy their starter and have them re-do the test if they'd agree to cover it in the event the engine still fails @ higher RPM. They wouldnt go there....

so I'll just play their game I guess...put my own faster starter in. Assuming I can read graphs correctly and understand this right, then it should still fail the test?

If I hadn't found a faster starter for really cheap I woulndt have been so nice. But I did so I'll play their game.

Seriously, how do people without car knowledge survive dealerships, if I know what I'm talking about and I can barely not get ripped off?!?

Wouldnt it be nice if MAzda-certified technicians KNEW that a starter wont affect compression?....

Also if this is useful for anyone else, he spoke as though the computer automatically does the reading, and adjusts for RPM, and gives the appropriate results. So you don't have to rely on some guy reading a graph improperly.

I believe the mazda compression tester does automatically adjust to 250 rpm.

It doesnt make sense.
The only thing that sorta makes sense is that a new engine is only the "short block", they move everything else over from the old engine. this includes the starter motor, and the guy mentioned that "either way you need a new starter, or you will be vulnerable to flooding"
So yeah, unrelated and stupid, but I sorta need to get a new starter one way or another regardless.

I just hope this helps out anyone else in this situation....you have to be persistent, but I know trying to force something might backfire.... It'll cost me a week and $100, and then I should have a new engine coming to me.

Looking forward to hearing the result. Good luck. :)

Installed the faster starter tonight, seems to work well. Heading back to the dealer Wednesday morning.

BTW, this single post alone is worth it's weight in gold. Written by a VERY experienced rotary mechanic and engine builder.
https://www.rx8club.com/showpost.php...6&postcount=41

"The engine must also be tested when warm, to get accurate numbers. Rotary engines make more compression when they are cold, and lose about 5psi when they warm up."
I'll be sure to rev the shit of her on my way to the dealer wednesday... I have my doubts as to how closely they follow procedure over there, and it will probably sit for an hour before they actually test it...

Yes that's true. What you have to realize is that warranty engine replacements are practically slave labor for the techs. Mazda compensates them a fraction of what they would make if the replacement were done off warranty. If you are going to lower his paycheck, he's going to make sure he's exhausted all options before he gets stuck with doing unprofitable in lieu of something that will put bread on the table.

Have you ever worked as a dealer technician? Give the guy a break, he's probably not an idiot.

So they did the decarb and it brought the compression back up above spec. Kind of annoying, frankly. I'll post the numbers when i get a chance later.... Unfortunately the post-decarb #'s, the RPMs got cut off, which is really annoying for someone like me who likes to look at the raw data. And at this point I dont know if I trust them to do things properly (Mazda 112 in Medford NY), although they were very "nice".

I paid $110 for the diagnostic service time and compression test, they told me they talked to Mazda NA after the first fail (pre-decarb) and mazda covered the cost of the de-carb procedure. Still seems kinda weird...

Also - Does anyone who really understand this know if seafoam and the mazda zoom-zoom cleaner do EXACTLY the same thing? Is the mazda cleaner more potent? Because I've seafoamed my engine 3 times now, about once a year for the last 3 years, and I assumed it was fairly de-carbonized. So I was surprised that their procedure had a significant effect.

Those are kgf/cm2 values

Corrected to 250 RPM:

Front Rotor
6.4 / 6.5 / 6.4

Rear Rotor
7.1 / 7.2 / 7.2

Standard 250 RPM kgf/cm2 values should be 8.5, with a Minimum of 6.9

You need to calculate chamber and rotor difference numbers also. Your values are better than mine currently. :fingersx: But I don't give a crap if you followed my build.

Get it replaced ;) We want a spare :)

I guess with the current economic slow down + people hate Japanese car. Mazda(and its dealership network) is working hard to look for ways to avoid any possible costly warranty work ...

not good ...

I called Mazda Customer service to obtain more information on the engine warranty, like what components are covered since the warranty letter states the following:


Rotary Engine Core

Rotary Housing and Internal Parts

Internal Seals and Gaskets

That's about it !!!

My question is, what components are included with this warranty, there have been a few member's which have been charged for some parts related to the engine, and some other members who havent payed a single penny, I do know it will depend on the Mazda Dealer who offers the best service and which ones will help the customer as they can, but to be clear on the subject it's best to know what's the reality cause for some it's not fair to others it's a blessing not having to pay nothing.

Elsyum I feel ya having to pay a retarded price on parts that can be found for half the price... it's why I'm also frustated cause every customer service or Tech I talk to have nothing in common to fix or know about the problem with the Renesis Engine...

thanks jmc23200, i guess that answers my concern, i was happy when they called me with the good news my car is ready, and later got pissed at the front desk with an invoice of almost $500, i got charged for the labor on some external gaskets, plugs replaced.. but hey at least it's running like new.... at some point the dealer always win

I wanted to wait untill 98 to 99 for my test. I a currently at 70 but the way they can jerk you around I am worried

That sounds like total BS to me. I was told today that my new engine, provided under the 96/100k warranty, would cost me nothing.