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High Oil Consumption in Street NB

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#1
Carl

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I realize this is not exactly a Spec Miata question, but this should be the place with the knowledge.

 

My son's 99 Miata is drinking oil at the rate of about 1 qt per 300 miles.  The PCV has been replaced.  Cold compression is 210psi in all cylinders.  Can't see any blue smoke, although we need to investigate that more closely during transient conditions. There are no external leaks. It does have the "EGR" engine code, but that does not seem like it would be related to oil consumption, since it is pushing exhaust back into the intake (the oil consumption might have caused the EGR code though).  

 

My question is whether or not it is in the realm of possibility for this much leakage from bad valve stem seals?  If not any other suggestions?

 

Thanks for any thoughts you might have.

 

Carl  



#2
Tom Hampton

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front and rear crank seals...

 

I was losing oil at about the same rate on my daily driver (1990).  I replaced both seals 2 months ago as part of a water pump replacement and a clutch/flywheel job. I just added my first quart of oil since replacing the seals.  I drive about 1500mi a month, so 3000mi give or take. 


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#3
Carl

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Do those seals leak to the outside or does the oil get sucked into the engine and out with the exhaust?



#4
Tom Hampton

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They leak to the outside. But, only when the engine is operating though, and under oil pressure.

Oil from the front seal leaks onto the crank pulley and gets flung onto the belly pan, and other surfaces in plane with the pully...where it mixes with other dirt, etc. So, it doesn't tend to drip onto floor when the car is stationary.

Similarly with the rear seal, it flings onto the interior of the bell housing. there is a weep hole in the bottom of the bell housing. So, sometimes a little oil may leak out of there and drip. But, my experience with two street miatas (one at 150k and the other at 230k) is that the leak out the rear seal has to be pretty big before you ever see a drip from the weep hole make a spot on the ground.

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#5
Carl

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Thanks Tom.  I am pretty sure that the engine is totally dry, but will check again.  We actually replaced the front seal a couple of months ago, so have kept an eye on that pretty closely.  Also pressure washed the underside of the car at that time.  



#6
FTodaro

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Carl i will let one of the motor builder chime it but you could be consuming the oil even though the you have compression, two of the rings are compression and one is an oil ring so two different jobs. some times you will not see the oil blue tinge until its under a load. Bottom line it can be difficult to detect at the tail pipe. Go on line buy one of those cheap bore scopes and see if the cyl walls are scored. 

BTW we don't mind talking about engines it keeps us off of discussing parity between the classes.


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#7
Danny Steyn

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and if you ever feel your clutch slipping on gear changes under load, say heading up a hill when you shift from 3rd to 4th, it signals that you have a pretty leak in the rear seal


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#8
Tom Hampton

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and if you ever feel your clutch slipping on gear changes under load, say heading up a hill when you shift from 3rd to 4th, it signals that you have a pretty leak in the rear seal


My first donor car had a leak that bad. Clutch would slip climbing a hill, even fully engaged. Talked the seller down on price because, "whew! Clutches are expensive and hard to change."

...

Ahem...pay no attention to the racing clutch already in my garage.

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#9
Carl

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I will take a look in the cylinders with a boroscope and see what things look like.  

 

At the rate oil is disappearing, I don't know how it could be leaking externally since there is not a drop of oil anywhere on the outside, including the weep hole on the bottom of the tranny.  The clutch is definitely not slipping either.

 

 

Carl



#10
Carl

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Kind of resurrecting from the dead.  Fixed healthy oil leak on the "cam angle sensor" plug, but did not reduce consumption.  Also tried solvent in the cylinders a couple of times, without luck.  Otherwise no external leaks.  Pulled the engine yesterday to replace oil control rings and valve stem seals.  Valve stem seals seemed hard and oversized, although by the time we could see the hole clearly the metal springs were pulled out.  The oil control rings did seem to be kind of compressed, but not sure how much.  The cylinder walls are not gouged and more than 50% of the wall surface still has hone cross-hatching.  The rest is just polished with very light vertical lines.  

 

Going to replace all the rings, but having trouble figuring out what is up with the second compression rings that we got.  There is a bevel on one side, but it is on the ID not the OD.  My understanding is that there is supposed to be a bevel on the OD and that should point down.  For reference, the top ring does not have a bevel on ID or OD, which I understand is probably OK.  The old rings, which were actually resulting in 210psi in each cylinder, had a bevel on the ID of the top ring (pointing up) and the second ring had a bevel on the OD (pointing down).

 

Do you think I can use the second rings that were provided with the bevel on the ID and labeled for the bevel to point down?

 

Any suggestions would help.

 

Thanks,

 

Carl



#11
JNJ

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Carl,

 

The second ring, if OE, is a Napier ring.  The ID bevel goes up and there  is a step cut on the OD that goes down.  This is done to create positive twist although although most 2nd rings have negative twist.   The second ring is almost exclusively for oil control.   The top ring has a barrel face and again, if it is OE, it has a bevel on the ID that goes up creating positive twist.  If you look at the top ring closely, you will see a mark, usually R (for Riken, they make most of the Japanese OE rings) and or STD or the oversize like 0.25.  These marks go up as will the ID bevel.

 

As for the crosshatch marks, the last marks left in the bore are the RVK valleys and they can be deep.  This can fool you into thinking the crosshatch is good; it is not.  RK is core roughness that is an average of RVK and RPK (that you have none of now).  The more shallow valleys is what the ring breaks in to.   It would be worth it to hone the block.

 

Most OE use perform a plateau hone and the RVK, RPK have spec.  Also, they closely control RK and RA usually about 12 -25 and 30 - 50 respectively.


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#12
Steve Scheifler

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Carl,

The second ring, if OE, is a Napier ring. ...


Nice post! But are you seeing a Napier cut or just steps & bevels on OE rings? I think for the 1.6 at least they aren’t really Napier. Splitting hairs but you got me second guessing and I no longer have anything on hand to examine.

Regarding Carl’s oil consumption, it seems that if you were losing much past the rings, and therefore burning it, you should see it on the plugs and be visible to people following you as smoke. Compression sounds good though that is somewhat gauge dependent, it’s a shame you didn’t do a leak down. Did you check the intake plumbing to see if oil is collecting in it and ultimately getting sucked in and burned that way?

Regardless, you have it apart now and freshening rings & seals will be a good thing if done right. I concur that you must have it honed or you are wasting your time. Then you need to check end gaps which may approach factory specification limits. You can get away with a fairly large gap and it can even have some initial benefit, or you can buy the first oversize for the top (and maybe middle) and trim them down. The latter option doubles your ring costs which is significant, but you can get the gap you want. It also increases tension which has both benefits and costs, at least in theory. They may seat faster, seal better and last longer but the added tension also means more friction losses at least initially. None of those are likely make or break for most people.
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#13
Carl

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Pretty sure the engine had never been apart before this.  It had been in our hands since 75k miles and only has 115k now.  The rings in there are pretty simple.  The top ring has a small, straight bevel on the top ID and the second ring has a small, straight bevel on the bottom OD.  There is also a bevel on the bottom second ring land (I think I have that terminology right).  

 

After looking at the rest of the pistons, it appears that the oil control rings were jammed in a compressed condition.  The top and/or bottom oil control ring seemed to be compressed and jammed between the wiggly ring and the lands, preventing them from moving back out.  The ring combos would not move at all in the grooves until one of the thin rings was pulled out.  

 

The plugs had a lot of stuff baked on and there was a pretty heavy carbon deposit on the top of the pistons and valves, including quite a bit on the underside of the exhaust valves.  There was definitely "smoke" coming out the back, since the rear bumper would get moderately black after a while.  You could pretty much never see anything coming out the tailpipe though.  To dilute, I guess.  

 

I appreciate the advice/recommendations.  Will report back on the results in a couple of weeks. 

 

Thanks,

 

Carl 



#14
Alberto

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Hey Carl,

Here's a link to what I learned about the rings and bevels when I rebuilt my motor: http://blog.miatarac...ions-miata.html

 

I had my 180k mile block honed at the machine shop and the OE rings were adequate on the original pistons.  Barely needed to touch them.

 

Good luck


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#15
Steve Scheifler

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Hey Carl,
Here's a link to what I learned about the rings and bevels when I rebuilt my motor: http://blog.miatarac...ions-miata.html

I had my 180k mile block honed at the machine shop and the OE rings were adequate on the original pistons. Barely needed to touch them.

Good luck


Confused. The bevel looks very clear in the pictures at the end of that rant, and there was a dot as mentioned in the Haynes manual.
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#16
Alberto

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Confused. The bevel looks very clear in the pictures at the end of that rant, and there was a dot as mentioned in the Haynes manual.

 

That's the benefit of experience vs a first time, ocd-ish amateur :)


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#17
chris haldeman

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Blog could use a don’t or 2 added as well. Anybody who reads it will be mis led
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#18
Alberto

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Would you like to contribute a don't or two?


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#19
Carl

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Seems like success here.  After a 1500 mile road trip, only about 1/2 quart of oil consumed.  Kind of hoping it will get a little better as rings break in, but it is certainly acceptable at this point, especially since it was close to a quart every 200 miles just before the repair.  We did roll the dice, based on some self imposed time constraints, and did not hone the cylinders.  Next time around I would definitely do the honing though.  I was biting my fingernails.

 

One clarifications on a previous post.  The bevel on the second ring is on the bottom of the OD as I noted, but the bevel is not straight.  It is a step.  There is also a bevel on the lower land of the second ring.  If I remember right it was also a "step".

 

Anyway, should be able to keep the car on the road for a few more years.

 

Thanks for all the advice.  Even if I didn't take all of it in the end, I used it all deciding what to do.

 

Carl






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