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Old 12-10-2010, 02:20 PM   #1
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Signs and symptoms of low compression

Ive been worrying lately that my 8 has low compression. i know that one of the signs is hard hot starts which is a problem i do have. compared to when the engine is cold it takes much longer and sometimes barely starts when hot. other than that however there doesnt really seem to be anything wrong. perhaps im just not aware of those so is there anything else i should be looking for?
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Old 12-10-2010, 10:00 PM   #2
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Hard or slow starting when warm is a sign of weak compression. Based on my experiences so far, the rx8 pcm is quite picky about it's engine's compression in order to run properly as compared to the older platforms that could more easily tolerate a weak compression engine.

Unfortunately by the time the symptoms show up the seal wear has already been ongoing for years and there isn't much you can do at that point except save your pennies and keep running it, hoping for the best. A fat 2 cycle ratio in the gas can help bump compression slightly and prevent further wear so it might be worth a try to see if it makes a difference for you.

Now of course it goes without saying that compression is not the only cause of weak starting. Plugs, wires, coils, fuel injectors, battery, starter, these are all significant contributors to starting that should not be overlooked. Barring a failure of one or more of those areas, weak compression is more often than not the cause of hard hotstarting.
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Old 12-10-2010, 10:40 PM   #3
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Very good but also how is your idle when hot or cold?

Last edited by Old Rotor; 12-10-2010 at 10:42 PM.
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Old 12-10-2010, 11:19 PM   #4
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Does the car hold idle well?
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Old 12-11-2010, 12:18 AM   #5
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Originally Posted by RotaryResurrection View Post
Hard or slow starting when warm is a sign of weak compression. Based on my experiences so far, the rx8 pcm is quite picky about it's engine's compression in order to run properly as compared to the older platforms that could more easily tolerate a weak compression engine.

Unfortunately by the time the symptoms show up the seal wear has already been ongoing for years and there isn't much you can do at that point except save your pennies and keep running it, hoping for the best. A fat 2 cycle ratio in the gas can help bump compression slightly and prevent further wear so it might be worth a try to see if it makes a difference for you.

Now of course it goes without saying that compression is not the only cause of weak starting. Plugs, wires, coils, fuel injectors, battery, starter, these are all significant contributors to starting that should not be overlooked. Barring a failure of one or more of those areas, weak compression is more often than not the cause of hard hotstarting.

RR saved you a lot of searching with this.
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Old 12-11-2010, 12:32 AM   #6
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I too would like to know the signs of low compression besides hard starting when warm?
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Old 12-11-2010, 12:55 AM   #7
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I too would like to know the signs of low compression besides hard starting when warm?
Multiple choice:

A) who lives in a pineapple under the sea?

B) SEARCH NOOB

C) difficulty starting, difficulty idling, stalling at idle, slight power loss

D) low compression values when THE ENGINE IS ACTUALLY TESTED FOR COMPRESSION

E) Both C and D

F) picardfacepalm.jpg
Click the image to open in full size.
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Old 12-11-2010, 03:47 AM   #8
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What a waste of space. If you can not give a good answer there is no reason to be an *******!




Quote:
Originally Posted by RotaryResurrection View Post
Multiple choice:

A) who lives in a pineapple under the sea?

B) SEARCH NOOB

C) difficulty starting, difficulty idling, stalling at idle, slight power loss

D) low compression values when THE ENGINE IS ACTUALLY TESTED FOR COMPRESSION

E) Both C and D

F) picardfacepalm.jpg
Click the image to open in full size.
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Old 12-11-2010, 12:14 PM   #9
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What a waste of space. If you can not give a good answer there is no reason to be an *******!
you really need to be careful how you talk about me on the forum, i dont appreciate it. tone down the disrespect, i dont know where you're from but where i am from, we dont tolerate that. dont even reply to this, just keep your mouth shut. consider yourself warned.
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Old 12-11-2010, 01:30 PM   #10
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The idle is fine, no problems there, regardless of temp.

Plugs, coils, wires were just replaced a few thousand miles ago so the issue isnt there either

Like i said, the only problem ive noticed is the hard starts when the engine is hot. It makes me wince every time i start the car when hot and it takes so long to do so. My poor engine...
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Old 12-11-2010, 01:50 PM   #11
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Having just gone through a similar issue, I'd advise that you or someone else check out your starter. I too was having issues with hard starts when the engine was still warm and especially so after the cold weather set in. I feared low compression too, but it turns out a dying starter was to blame. A speed test confirmed mine was spinning about 100 rpms slower than spec, which should be around 280 rpms. Bear in mind this was a revision b starter without a lot of age or miles, so they can, and obviously do fail. Now, it's click-whir-done and my problem is gone. My compression test showed great numbers and the tech even commented on how healthy the engine was. Just because you're experiencing a "symptom" of low compression doesn't make it so. There are far too many variables with these cars. Run through your ignition system, eliminate some of these variables, and then start considering low compression if you're still not seeing results.
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Old 12-11-2010, 01:54 PM   #12
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The idle is fine, no problems there, regardless of temp.

Plugs, coils, wires were just replaced a few thousand miles ago so the issue isnt there either

Like i said, the only problem ive noticed is the hard starts when the engine is hot. It makes me wince every time i start the car when hot and it takes so long to do so. My poor engine...
I suffer from the hot start issue ... however I'm not convinced it's low compression but without a comp test I have nothing to stand on ..... but anyway what I do ....

If after a 2 or 3 seconds of cranking the car doesn't start ... I stop and let the starter disengage.
I then try again .. and typically it starts right up ....

Yeah yeah I know this prolly means one of the rotor faces is suffering low comp .... blah blah .. blah "I can't hear you"
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Old 12-11-2010, 02:00 PM   #13
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The idle is fine, no problems there, regardless of temp.

Plugs, coils, wires were just replaced a few thousand miles ago so the issue isnt there either

Like i said, the only problem ive noticed is the hard starts when the engine is hot. It makes me wince every time i start the car when hot and it takes so long to do so. My poor engine...
Most members know by now that the search function on this forum is not too efficient.
Most people here also know, or have learned, that being scolded for NOT attempting to search is simply a rite of passage around here and it should not be taken personally.

Mazda issued TSBs for hard hot-starts years ago and those TSBs steered Mazda techs to check/replace the battery and starter before they then examined the coils, spark plugs, and wires. The new starters are WAY better than the old ones but one may wish to have their compression values checked once in a while as part of a regular regimen.
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Old 12-11-2010, 02:18 PM   #14
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also .....

Hard starting - suspect your starter motor ? check this out
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Old 12-11-2010, 02:19 PM   #15
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The idle is fine, no problems there, regardless of temp.
If the car is idling fine, then I'm assuming its not dying on you at stop signals or so.

There is one issue that popped up recently with a friend's RX8...and it started exactly like your problem... with hot starts. After the hot starting, the car started vibrating, was unable to hold idle and was making pinging noises at high revs. Plugs and coils were fine, so this all sounded like the engine failing. However upon the suggestion of another friend, he changed the fuel and it all went away. The car started running like new again... compression was also tested later and it was fine. He also started premixing after this and says his car is running better than before.

If you've changed your fuel recently, you might want to look into changing back.
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Old 12-11-2010, 02:46 PM   #16
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When i get a chance i will post up a couple videos highlighting the significant differences between a cold and hot start
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Old 12-11-2010, 05:22 PM   #17
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What a waste of space. If you can not give a good answer there is no reason to be an *******!
he actually did give a good answer mixed in with a little sarcasm and humour .
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Old 12-11-2010, 09:40 PM   #18
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he actually did give a good answer mixed in with a little sarcasm and humour .
Evidently, only spoon feeding, hand holding, and other forms of coddling are permitted here now.
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Old 12-13-2010, 12:21 AM   #19
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RR is about to beat some ***....

but I understand it statement about searching...it's there for a reason...why do we need 192848490509394 posts about this issue when we can have ONE GOOD/GREAT/INFORMATIVE post
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Old 12-13-2010, 01:05 AM   #20
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there should be a forced "how to/where to find" tutorial video for anyone joining a forum.
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Old 12-13-2010, 01:08 AM   #21
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Search is probably so inefficient because we have so many duplicate threads. Also, google with "site:rx8club.com" works pretty good.
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Old 12-13-2010, 02:47 AM   #22
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Evidently, only spoon feeding, hand holding, and other forms of coddling are permitted here now.
That has always been the case.

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What a waste of space. If you can not give a good answer there is no reason to be an *******!
He did actually answer your question.
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Old 12-13-2010, 11:29 AM   #23
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is it me, or are some posts missing?
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Old 12-13-2010, 11:33 AM   #24
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I removed some of the posts and I should probably edit these last two also.
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Old 12-13-2010, 12:20 PM   #25
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I removed some of the posts and I should probably edit these last two also.
That's a moderator doing his job!

Can we just get back to the fact that this guy might have a bad engine.

OP: If you search, you will find a lot more useful information than the replies that you will get to this thread. Your question has been well answered many times, and I think that there is a sticky thread here someplace. Try the Tech section.

RR - Get over it. It isn't that big of a deal.

And Maniac, I am disappointed that I think I have seen that profile pic. before. It must be time for me to leave this forum if I have hit the top of your profile pics again.
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Old 12-13-2010, 12:20 PM
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