I've got a 89 Mazda MPV 6 Cyl Auto trans that smokes from the rear left side of the engine after it warms up. I've changed the head gaskets and washed down the motor in that area, but can't trace the leak. I believe it's a leak because I can find a very tiny bit of oil drip under the engine just in front of the oil pan on that little plate below the frame. The oil pan itself is nice and dry. Could a rear engine seal or a front engine seal allow smoke to get on the exhaust manifold? The oil level doesn't appear to be dropping at all, so if it's a leak, it's a very tiny one, but enough to make the car smoke and stink when it gets hot.
And what is that little plate on the left upper rear side of the engine (perhaps it's on the automatic transmission). I believe it's some sort of breather cap that has a bolt holding it down. The smoke is coming from that area or just behind it. I've changed all the transmission fluid and it seems to be doing fine too. By the way, I had a bad trany slip until I replaced the Oxygen sensor and now no more slip.
Any help would be great.

 

If it is trans fluid, it should be Red in color. If Engine oil, light brown.

Can you take a pic of the area you are talking about?

 

On the backside of each cylinder head, centered and about 1.5 inches down from the valve cover gasket surface is what is called a camshaft drill plug that are very common to leak. The plugs are round and 2 inches in diameter. When the camshaft bore was drilled during construction of the heads these holes were made. At the rear of the head, the hole is sealed with this plug. At the front of the head. The camshaft protruded thru the hole to allow attachment of the timing belt pulley and is sealed with a front camshaft seal. You should be able to get a hand down to these easily and feel if they are wet with oil.