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I have a slow oil leak on the miata, I thought it was the cas o-ring but I replaced that along with the valve cover gasket when I did my timing belt/water pump this spring. I just wiped it up again in order to attempt to find it fresh and locate the source. I gave it a smell and it smells vaguely like gear oil but the rag is old so I'm not sure that means much. It did get me thinking that it could be something other than motor oil. Is there any definitive way to tell the difference other than sending in the worlds smallest oil analysis sample? The weep hole in the transmission is not the source and the only other dampness I can find is a very light film around the dipstick (which is what made me think it was the cas o-ring or valve cover gasket).

Any suggestions? Next step may be some UV reactive dye.

If you are not looking for definitive...

I would try to clean the area around the leak as well as I can, let it leak, take a sample on a clean paper towel and then draw samples from both the engine (via dipstick) and transmission (via fill plug?) and then compare color, smell, (is anyone looking... ?even taste?... Do I have to say tip of tongue and not mouthful and to not swallow?! ) Anyhow, I would assume the motor oil is going to be visually much dirtier than the transmission oil and that it "should" be obvious which is which.

Richard

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Richard Casto
1972 Porsche 914
2013 Honda Fit Sport
2015 Honda Fit EX
http://motorsport.zyyz.com
Money can't buy happiness, but somehow it's more comfortable to cry in a Porsche than a Kia.

FWIW, Pennzoil Synchromesh comes out of the bottle looking like used motor oil...

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Mike Whitney
whit32@gmail.com, 919-454-5445
V10, V8, V8t, I6, I6, V6, F4t, I4, I4, I4, I4, I2, 1, 1

It depends upon the transmission oil, but some does look like dirty motor oil. The standard gear oil I use for rebuild older Porsche transmissions is like that, but I also notice that newer lower viscosity Honda MTF is very clear. So my method would only work if you can tell a difference between new MTF and old motor oil.

Richard

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Richard Casto
1972 Porsche 914
2013 Honda Fit Sport
2015 Honda Fit EX
http://motorsport.zyyz.com
Money can't buy happiness, but somehow it's more comfortable to cry in a Porsche than a Kia.

Alec, It can be tough to keep them from leaking. As far as determining which is leaking, gear oil has a distinctive smell. If you are sure the leak is from the rear of the engine or front of transmission area, then it could be CAS o ring, which you changed. The cam cover gasket can be tricky, make sure you clean the cam cap crevices free of old silicone and put a dab of silicone sealant in each before re-assembly. The torque sequence is also important for the cam cover. If you determine oil is dripping from between the engine and transmission, then it is either the rear main seal, or the input shaft seal for the transmission. In my experience, the rear main is more likely to leak, but given the age of our cars now, both are suspect.

The light film around the dipstick could mean the dipstick tube o ring is leaking. It's easy place to start. Remove one 10 mm nut and some wiggling should get it out.

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92 Miata 1.9L naturally aspirated. SSM 21

I finally have some new oil accumulation and the dip stick o-ring is definitely a source (and it appears to be the only leak so far). Thanks for the idea Jordan, It's great that this doesn't appear to be the rear main or anything else labor intensive to get at.

Cool! It couldn't have been any simpler a fix than that!

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92 Miata 1.9L naturally aspirated. SSM 21

one is thicker typically. tranny fluid i believe is much heavier than motor oil ,and in many cases smells different.Tranny fluid in many of our vehicles that i know. is red.oil in a car motor is amber .

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