Worried about piston rings...

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FC, FE, Mazda5
I bought a 2012 Mazda5 a couple weeks ago (minimal rust, 108k miles, manual trans), and flew a few states away to buy it (not that many manuals available!). Everything seemed to be fine on the drive back to the dealership in the car, but I also had my face mask on when sharing the car with the dealer rep, so vehicle smells were hard to discern. Shortly after buying the car, I drove it 700 miles home, stopping about 100 miles into the trip to pick up a used transmission and rear diff for a project car. After getting it home and unloading the car parts, there has been a smell of slightly burnt oil or trans fluid inside the van that seems to intensify with driving. I can smell it under the hood after parking the car too, and had noticed it on the drive home (but wrote it off as coming from the used transmission).

Anyway, I went to investigate if there were any leaks earlier today, and couldn't find any - oil, trans output seals, dripping from the engine-trans face (crank or trans input seal), or torn CV boots. Looking around for the source, I opened up the oil cap and it smelled slightly burnt. More worryingly, when the engine was running, there was a strong pulsation out the oil cap hole that quickly deposited a nice oil film on my glove. My understanding was that at idle, the crankcase should be under a slight vacuum since the engine relies on that vacuum to purge water vapor etc from the crankcase. The only way that I am aware of for positive pressure to get into the crankcase would be a ring sealing failure, especially given the volume of air that seemed to be pulsing out (quite a puff, not just a slight breeze)

Is there anything that I'm missing, is this normal for the 2.5l engines, or am I a few weeks away from pulling this engine and tearing it apart?
 
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2020 CX-5 AWD
The positive pressure at the cap could definitely be a problem, but I'd be doing other things first, before getting into the major engine overhaul. First off, how much oil is it using? If there's a problem with the rings, then it surely must also be burning some motor oil. Also, how did the oil quality and level look when you bought it?

Next, I'd want to do a wet/dry compression test, followed by a leak down test (if the compression test is not up to spec). And then, if the compression and leak down tests indicate a ring problem, I'd be trying one of the cylinder cleaning products a few times, in an effort to remove crud from the rings. Without disassembly, there's no way to tell if the rings are worn, or just not able to move properly due to deposits. So why not try the much simpler cleaning process, in case the problem is crud on the rings, and can be removed chemically. See if a fly swatter might work, before bringing out the elephant gun.
 
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2010 Mazda 5 Sport
Compression/Leakdown test is a good indicator. Also, just pulling the plugs to see if they are looking fouled/gunked up in any way will tell you a lot.

A bad/plugged PCV valve would cause this or a plugged line from the PCV valve. That is your cheapest fix to try.
 
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2020 CX-5 AWD
It's always been puzzling to me why Mazda decided to squirrel away the PCV valve under the intake manifold on so many of their vehicles. It's annoying to spend 2 hours or so changing a $5 standard maintenance part.
 
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FC, FE, Mazda5
I was planning on doing a compression test later today and inspecting the plugs, and possibly going digging for the PCV. What compression should I be looking for? My 2012 van has the 2.5L L5-VE, but all the service manuals that I can find are for earlier model years with either the 2.3 L3 or non-north-american market MZR 1.8 or 2.0. It seems like healthy numbers are around 180 - 250 psi for the L3, but 140 - 145 was said to be Ok in a different thread for a mazda3 with the 2.3L as low as 110 psi for the 2.5L Skyactiv G (which does use the intake valve closing time to reduce effective compression ratio).

Anyhow, I'd like to start digging in today so I know what to order and can generally start performing any maintenance necessary.
 
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2010 Mazda 5 Sport
It's always been puzzling to me why Mazda decided to squirrel away the PCV valve under the intake manifold on so many of their vehicles. It's annoying to spend 2 hours or so changing a $5 standard maintenance part.

Didn't know that. Is this a Ford or a Mazda engine? I think the 2.3L we have is a Ford engine. Not sure on the 2.5L
 
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2020 CX-5 AWD
.... What compression should I be looking for? ...

For your L5, Mazda specifies the 'standard' (fully warmed up engine) compression reading to be 192 PSI; Mininum acceptable compression is 134; and largest allowed difference between any 2 cylinders to be 28.5. Personally I wouldn't want to be seeing anything close to 134 PSI, but I guess that's just me.
 
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FC, FE, Mazda5
Well, all the plugs looked good - no thick deposits or anything and a nice tanish-reddish color on the ceramic. Dry compression readings were 142, 142, 146 and 148 on a cold engine, I'll have to re-test after warming it up but for the moment I'm not too worried there. Phew!

Next it's time to dig for the PCV and plumbing to check that out. Are there any replacable gaskets that I'll need when putting the intake back together, or are they like the ones in the RX-8 where its like a rubber o-ring captured in the plastic manifold?
 
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2020 CX-5 AWD
.... Are there any replacable gaskets that I'll need when putting the intake back together, or are they like the ones in the RX-8 where its like a rubber o-ring captured in the plastic manifold?
I'd install a new set of gaskets, but lots of folks have reported reusing them successfully, so that choice would be up to you. JMO, but before getting into that fairly big job, I'd redo the compression test with a warmed-up engine, but also an additional test of each cylinder with some oil squirted in, to see how much (if any) PSI increase there is, wet versus dry.
 
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FC, FE, Mazda5
Well, compression when warm is 150-155 psi, so a little bit higher than when cold. There is still a pretty strong oily smell in the van when driving that dissipates when its off with windows & doors open. I think I smell a little bit of oil in the exhaust when idling too, which does not bode well.
 
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FC, FE, Mazda5
Digging into how to remove the intake manifold now - I've got a thermostat on the way, is there anything else that should be replaced while I'm there?
 
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FC, FE, Mazda5
Got the intake manifold off today - lots of fiddly plastic clips for wiring and such that got accidentally broken, but zip ties will hold everything in place on reassembly. However, the PCV looked perfectly fine, and was still functioning as a one-way valve when I sucked and blew into it... so I'm not too certain that it was the issue. The inside of the oil separator looked pretty normal too - no thick buildup, crustiness etc. I'll put the manifold on again tomorrow or Sat - still waiting on a new thermostat to arrive.

I've also been pouring MMO down the spark plug holes and spinning the engine over a few times each day (pulled the fuel pump fuse and disconnected spark plugs and fuel injectors). Hopefully that'll loosen up any deposits that might be causing a ring to stick. It seemed that there was about a pulse from the oil cap for every 2 revolutions, which would indicate that it's a single cylinder causing the issue, but all the cylinders were within 5 psi on the compression test so it's hard to definitively nail down one or the other. The intake runner for cyls 2 & 4 looked maybe slightly blacker and more oily than the others, but 4 was generally the highest testing cylinder, so who knows... The deposits in the intake and runners were slightly sticky, but not super wet or anything.

One other question is how the oil smell would get from the exhaust, if it was really burning in-cylinder, into the cabin. I have not been able to find any exhaust leaks so this bit also has me a little stumped. In the cabin it smells more like wet oil sitting out or soaked into a surface, but all the carpets, seats etc individually look and smell fine
 
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2010 Mazda 5 Sport
So rings can't stick. If they stick...engine go boom.

On the PVC, did you check all hoses from it? The PVC can be OK, but the hoses could be blocked.

Could it be a valve issue? I thought leakdown/compression tests are supposed to detect that problem.
 
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FC, FE, Mazda5
I've seen piston rings get carbon'ed up so that they can't rotate and don't seal well all the way around the bore, or don't wear in an even pattern over a long period of time. This typically happens with a lot of very low-load running, idling, cold-starts etc that don't allow the oil and coolant to really warm up. Freeing them up to rotate by breaking up the deposits will allow the condition to correct itself over time unless it's too bad and the cylinder has already been oval'ed or scored. This would also open up a flow path for combustion gas to get into the crankcase, which would be one way for the pressure pulsations in the crankcase to be caused. I'll be checking out the bore walls with a borescope later today or tomorrow to look for scoring.

The only PCV hose is from the valve to the intake, and this was not blocked or cracked. I could hear the air blown into it coming out the intake port openings. There was also not much accumulated oil there - only a thin film. Inside the separator volume was also clean and not blocked.

While there may be some valve wear that could reduce the compression ratio, I don't see how that would pressurize the crankcase or cause an oily smell in the cabin, which are the main issues I'm trying to address.

The fallback if there is bore scoring or I can't solve things is always a new engine... there are a bunch of wreckers around here with low-mile L5-VE's for $5-700
 
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2020 CX-5 AWD
....
I've also been pouring MMO down the spark plug holes and spinning the engine over a few times each day (pulled the fuel pump fuse and disconnected spark plugs and fuel injectors). Hopefully that'll loosen up any deposits that might be causing a ring to stick. ...
I'm not familiar with using MMO as a direct cleaning agent. Not saying it doesn't work, only that I don't know anything about using it in that way.

Most of what I've seen related to cleaning oil rings involves soaking for a few hours or overnight with a cleaning agent designed to aggressively attack carbon, such as Mopar, GM, or BG Engine Purge (and there are others as well). The problem of course is getting the stuff past the top 2 rings, which is the reason for the soaking. If you end up going that route, you would also need to do an engine oil/filter change afterwards.

Is there any chance that what you're smelling is spilled motor oil (or perhaps even transmission fluid), that was done by someone before you took ownership?
 
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FC, FE, Mazda5
I haven't used MMO specifically for cleaning in this way, but there are a bunch of people online that seem to swear by it, and it was less aggressive than pouring acetone, PB blaster etc (a few other of the suggestions I ran across) down there! It does not seem to be fully draining into the pan over the course of a day, since some invariably squirts out the spark plug holes when cranking the engine over. I plan on changing the oil and filter before starting the engine anyway (although MMO advertises using about 20% of their product in place of engine oil... i don't feel like risking it on a modern engine designed with tighter clearances for thin 0W20 oils). I will probably run some Seafoam through the intake too - another product I've anecdotally heard good things about but have never used myself.

I keep hoping it's actually spilled oil inside the car somewhere, but the carpets are literally like-new with no stains whatsoever, and most of the rubber floor mats are good too. There's a suspicious stain on the passenger seat, but it doesn't smell like oil. I was literally just checking all the cubbies & crevices like the rear tire well, on either side of the rear hatch area, glovebox, under the front seats, in the hidden compartments under the middle row of seats (there's a cool flip-out cupholder and tray in the passenger's side 2nd row seat that my old '06 didn't have!) hoping someone left an old bottle of oil somewhere and it started to leak or something, but found nothing. I'm planning on installing a trailer hitch and some wiring, so I'll be pulling back some of the carpet to run power wires for that soon. I've also got a new cabin air filter to replace and will check inside the HVAC system while I'm there tomorrow.
 
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FC, FE, Mazda5
Well, everything's back together now - new PCV, thermostat, coils, intake & cabin air filters, fresh oil and coolant, and things seem about the same. I can't tell about the oil smell yet since I basically fogged the neighborhood out on first start-up with all the oil poured into the cylinders. Compression after getting everything warm was still low 150's, and with a bit of oil in the cylinders it rose to 180-190 psi, so there's definitely some gas escaping around the rings. The strong pulsation out the oil fill cap is still there - it may be slightly weaker now but it's hard to tell.

For the moment, I'm just going to plan on driving her and seeing how things progress, and giving her at least a redline-a-day to see if that loosens anything up.

The bores had also looked OK, with the worst being some more prominent vertical lines in cyl 3 - they appeared to have gone through the cross-hatching but were still reflective and didn't seem to have much depth.
ACtC-3eOhBucUIBwyYp17gGJBT0h2iqlqnMK0ifMM76y0ehx2sgzl5igyLlUoywoVfqLUL-gAq4dx3gwaJTN2RcBMC-9nTy9fNDnn4VmWpAsiJMh8y4Vu1WTZ9UhvGjOQ09Y40Udug0rdPUBj0N_pL3tvTOa=w640-h480-no
 
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2020 CX-5 AWD
... Compression after getting everything warm was still low 150's, and with a bit of oil in the cylinders it rose to 180-190 psi, so there's definitely some gas escaping around the rings. ...
That's confirmation of a ring problem, but of course you can't tell which one. The lack of major scoring on the cylinder walls would suggest that they haven't been damaged, but that's just a guess at this point.

I don't know what your next step is (if it doesn't improve over time), but I recommend trying to free up the rings by soaking the cylinders with one of the power cleaners, before getting into one of the big repairs that you mentioned earlier.

It would mean also needing to do another oil/filter change afterwards, but I think that's a small price to pay, in exchange for the possibility of avoiding spending lots of time and $$ on a major engine repair. JMO, and as always your vehicle, your choice.
 
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FC, FE, Mazda5
Yea, that's my assessment now too... no harm in trying some more aggressive cleaners now, and driving it like I stole it.

After doing some errands today, it definitely still smells of oil especially when everything's nice and hot like coming off the highway. I'll keep looking for external leaks too but all the usual suspects (valve cover gaskets, crank main seals etc) look dry
 
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FC, FE, Mazda5
Well, after all that, some sea-foaming, interior cleaning, and repeated aggressive driving once it's warmed up, the oil smell seems to be going away and not returning. Compression seems to have increased by about 5 psi across the board when warm, so that's a start, but I haven't been able to do much driving of her lately since one of the front tires developed a bulge and I just picked up the summer wheels that'll be getting new tires.