496HO Blowby
 

Starting this year, one of my 200 hour 496 HOs is pushing oil out the dipstick tube. When the boat was put away at the end of last season, I had no issues, but since the boat has been put back into service this year it has been a huge problem.

I have pulled the fill caps off both motors to compare vacuum. With a paper towel held over the fill tubes, the good motor has a slight vacuum, creating a consistent dish shape in the towel. The motor that is pushing oil will cause the towel to fluctuate between vacuum and pressure.

These are bone stock, naturally asperated motors with the only change between last year and this year being the oil change and motor fog. Do these motors have a history of sticking or broken rings? I have begun removing the interior in preparation for a leak down test later this afternoon.

Any suggestions would be greatly welcomed.

do a leak down and see which cyl. is causing the problem.
hopefully it is just a stuck ring and not something more serious.

Thanks, kinda what I am hoping for...any magical suggestions for correcting a stuck ring/rings other than surgury?

if it's the ring, you could try filling the cyl. with a product called PB Blaster, get it at wally world, spray it in and let it sit, hopefully it will free it up.

Got out to the marina last night and pulled out all the plugs. All looked pretty much the same with a golden brown color over the insulator except cylinder #1; it was damp and oily. We started with the cylinders on that bank doing compression tests:
#7 = 160 psi
#5 = 165 psi
#3 = 165 psi
#1 = 0 psi :(

This wasn't good...stuck ring would at least get me something I would think.

Before prepping the engine for a pull, we quickly pulled the valve cover on that side to see if there might be a bent valve or other tell tail....nothing. Everything on the upper looked good.

The only thing that makes sense now is a burnt hole in the piston. If anything had broke or hit, I would think I would have heard some noise when it is running or the spark plug would have shown signs of debris in the combustion chamber.

Having the motor pulled today and hopefully by this weekend will have some good new to report. Anyone heard of these cast pistons burning from normal asperated usage?

There could be a number of things that could cause a piston to burn even on a N/A engine. Could have been sucking air in from somewhere or could be an injector issue! I would think you would get something on the compression test if just a stuck ring.

well I guess my idea is out, hopefully damage is not to bad, good luck on your quest.
Larry

I'm leaning (no phun intended) toward a fuel problem as well, but if this is the case aren't all these (496s) ticking time bombs then?

Well again a number of things could cause an injector to not work properly. Does not mean all 496 are going to blow up.

I was just thinking out loud that this would pose a potentially small margin of safety if it turns out to be a burned piston from a faulty injector...however, i won't know anything until the heads are off later this week.