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Old 12-10-2008, 01:45 AM   #1
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Default Oil Leak Diagnosis Help

Hey all, I just bought a 91 miata. It has an oil leak I would like to take care. When I got it yesterday and left it overnight there were some really dark oil. I didn't drive it today, and put some cardboard under. There was much more oil and it's fresh oil. I haven't checked to see where exactly it's coming from yet.

I've read that the CAS o ring is a common culprit, it might make sense because the car just had timing belt, water pump, head gasket, valve job(from overheating) fixed very recently. If that is the case where does it usually leak, and does it still leak when it's parked(that doesn't make sense to me).

Like I said, I haven't taken a really close look, hope it's just the oilpan gasket or the drain plug.

TIA!
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Old 12-10-2008, 02:56 PM   #2
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Read the articles in the miata.net garage about the CAS. You need to get your car in the air and look around, there are a bunch of possible culprits. I for one am thinking that I forgot to seal the bridge over the CAS when I rebuilt my engine so I am pretty sure that is where my oil leak is coming from.

And for the record, you really do NOT want an oil pan gasket leak.
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Old 12-10-2008, 03:04 PM   #3
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One thing that's leading me to believe it's not the CAS o-ring is that it continues to leak even when the car has been sitting there for more than 24 hours without starting. Is it still possible for it to be the CAS o-ring in this case?

yeah I talked to a pretty knowledgeable mechanic today who knew right away about the CAS O-ring, which really gives me some confidence about him. Many mechanics have no clue what that is.

He said he's never seen a leak in a miata from oil pan because it's sealed with high temp silicone. I got a quote from him for the what the rear main replacement would cost as reference for cost of his work and he is more than reasonable in price, so I'll take it to him. I hate dealing with wet parts and seals/gaskets, cuz I've not had good luck, dry stuff I've had better luck on when I work on it myself.

I'll probably do the CAS o-ring myself though, cuz he wants $100
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Old 12-11-2008, 12:16 PM   #4
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He has not seen my Miata's oil pan then. They do leak sometimes.....
I have just put more "Puky" on it for now until the 99 bottom goes in.
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Old 12-11-2008, 12:39 PM   #5
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Ok, so I went to the mechanic yesterday, there is probably a small leak out of the CAS O-ring, and most of the oil is seems to be coming from the drain plug...they cleaned it up a bit, and I am gonna go back today.

Haha, since I just got the car I haven't even looked up jack points for the car yet...
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Old 12-11-2008, 01:18 PM   #6
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Originally Posted by lazybummm View Post
Haha, since I just got the car I haven't even looked up jack points for the car yet...

Yet you want us to magically tell you where the oil leak it?


Start by using these two spherical things mounted within your head. Some people would call them eyeballs. They are great for examining sources of oil leaks. I believe most point them at various locations around the entire engine block and find possible exit points of oil. I'm sure this tool is what your mechanic used, yet he charged you money for the use of analysis of the data.
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Old 12-11-2008, 01:32 PM   #7
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+1

Unfortunately, there are many possible culprits for an oil leak on the backside of the Miata engine. The CAS is one potential contender, but bear in mind that it's in a location which does not receive any forced oiling, and relatively little sling. IOW, you can get some seepage here, but not large amount of oil.

The valve cover gasket is another possible candidate, though again there's no pressure working against it.

You've already discussed the rear main seal. This will definately put a lot of oil out the bottom of the motor.

In interests me that you recently had the head gasket replaced, due to overheating. I bring this up because that wound up being the source of my relatively major oil leak earlier this year. There's a pressurized oil feed passage right at the back corner of that thing, and after 17 years, mine finally gave up.

While the head was off, did you check both it and the block deck for straightness? Overheating, depending upon the severity, can potentially warp the head and cause this sort of leak even with a new gasket. Likewise, were both mating surfaces cleaned after the old gasket came off? Those things tend to leave a lot of crud suck behind when you peel 'em away, and I can potentially see if a chunk had come off and stuck to either side that it could be causing the new gasket not to seat properly.
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Old 12-11-2008, 03:13 PM   #8
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Quote:
Originally Posted by Joe Perez View Post
+1

Unfortunately, there are many possible culprits for an oil leak on the backside of the Miata engine. The CAS is one potential contender, but bear in mind that it's in a location which does not receive any forced oiling, and relatively little sling. IOW, you can get some seepage here, but not large amount of oil.

The valve cover gasket is another possible candidate, though again there's no pressure working against it.

You've already discussed the rear main seal. This will definately put a lot of oil out the bottom of the motor.

In interests me that you recently had the head gasket replaced, due to overheating. I bring this up because that wound up being the source of my relatively major oil leak earlier this year. There's a pressurized oil feed passage right at the back corner of that thing, and after 17 years, mine finally gave up.

While the head was off, did you check both it and the block deck for straightness? Overheating, depending upon the severity, can potentially warp the head and cause this sort of leak even with a new gasket. Likewise, were both mating surfaces cleaned after the old gasket came off? Those things tend to leave a lot of crud suck behind when you peel 'em away, and I can potentially see if a chunk had come off and stuck to either side that it could be causing the new gasket not to seat properly.
Thanks for the reply. I just wanted to find some common leak points in addition to the CAS o-ring with this post. Unfortunately the previous owner took it to a mechanic to have the everything done from the overheating, so i am not sure what was done. The receipt said valve job for $240, I'm not sure what that entailed, I hope it included some machining. But yeah, so far it would seem that, drain plug gasket might be the main culprit, I'll have that changed out and the do the CAS o-ring as well and see if the leak goes away. If so, then I will be thrilled.

Thanks again for everyone's help and inputs
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Old 12-11-2008, 05:33 PM   #9
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Yet you want us to magically tell you where the oil leak it?
His SN is lazybum.
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Old 12-12-2008, 01:15 AM   #10
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real problem uncovered...drain plug. there is already a sleeve in it...and it's still leaking, any ideas? Tried some silicone and see if that is gonna hold up

Thanks
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