About our pistons and oil rings...
Smoke on cold start
Bumping the thread back, with some good/bad news...
Well, the good news is: I found the problem of cold start smoke.
Bad news: Well, it's the rings...
Worse news: The rings are fine, not worn out, not scored, not broken...So, what is it then?
Ok, here's the details:
Mazda's pistons are crap! So f#"$%&ng bad, because they have one design flaw. The flaw that mazda jerks did not put any improvement in the pistons dept in 20 years!
The oil ring channel on the piston, has just 4 (four) tiny oil holes. Theese holes are used for the oil that was scraped off the cylinder walls while the piston was moving down to escape to the sump.
Naturally, more holes, bigger holes, the easier it is for the oil to sink down...
Ok, so, during time, oil and gasoline carbon deposits gradually plug these tiny holes up, preventing the oil from flowing down, and cooling the pistons, and bla bla...
The pistons overheat slightly, causing the oil that gets caught in the oil rings retainer to get sticky and guey and carbonised, turning it self into that hard -impossible to clean- resin / gunk / lacquer that sticks to your kitchen fry pans...
So, at the end you are left with the oil rings that are so badly stuck, glued, burned into the oil channel of the piston, that the oil rings do not even touch the cylinder walls.
The compression rings are a whole different story, and they are doing their job pretty good.
To make things worse, oil rings have sooooo weak initial tension, compared to the compression rings, that it is almost natural to have such a result.
Take a look in this picture. Look at the gap between ends of the compression rings, and compare to the gap between oil rings... You will get the idea..
Ok, so why does it only smoke on cold starts...
Well, here's why:
Once you start up the cold engine, there is increased idle rpm, creating a huge vaccum in the intake manifold, as well as in the cylinders, as the pistons are trying to suck the gases in, but the throttle fully closed prevent thet from happening...
So, as the oil ring doesn't seal the oil, the vacuum sucks the oil past the compression rings ** and into the combustion chambers, and.... tadaaa! Bluse smoke. Now, the oil is fresh and cold, hence, there is no pale blue smoke, but thick white smoke, clearly smelling like oil.
As the engine runs, 10, 20, 30 seconds later, the piston warms up, the compression rings warm up, expand a little, close the gap, so does the crappy oil ring, that finally starts to touch the cylinder walls, and the oil stops sipping up onto the piston.
There is stil some run by, but not so noticable, so the oil consumption is pretty high.
The solution:
Rering the sucker, but drill some extra drain holes in the piston oil channel, and make the existing ones a bit bigger.
** Compression rings are designed to hold the pressure, but not to scrape the oil of the flat surface. it is like your rain wipers on the windshield. They have to be thin, almost blade-like, to be able to scrape.
That is the reason why the compression rings do a perfect job of sealing the compression but leak oil past them like swiss cheese. And why oil rings are thin, blade-like!