Crankcase Pressure

[rocko]

Hi, I have what I believe to be a problem with my Sunstar M20 engine. It seems to have an unusual amount of crankcase pressure. I noticed the crankshaft seal is starting to leak. When I started the engine with the oil dipstick removed oil vapor was blowing out of the of the tube, even worse from the oil fill tube when the cap was removed while running. I have replaced a piston and valves on one bank two seasons ago and while I was at it I replaced the crankcase breather assembly. The engine does not smoke nor is there any oil in the air cleaner. The engine seems to run fine. Is it normal for these engines to have alot of crankcase pressure that the breather doesn't relieve? How can I test the breather to see if it's working properly? Any thoughts would be greatly appreciated. Thanks, Mark

[Wishin2BMowin]

Excessive crankcase pressure is from compression leaking by the compression rings on the piston. This could be that the rings are worn out or the cylinders are scratched and worn. The oil rings could still be working fine so you won't see any smoke. I suggest doing a compression test and if it's below 100 pounds or there's a difference in the two cylinders then that's where your problem is. If you put a new piston on a cylinder that's worn it will only work for a short while then the rings wear out again. Best of luck with your searching...

[Dutch]

Run a manometer test.

[D-17_Dave]

Remember the pistons will act as a pump of sorts. Every time they come in together down stroke it will push air. If you check and watch the breather, it should be flowing the same amount of air/vapor. If it's not, check to see if it's clogged and not working properly. The ait/vapor you see comeing out the dipstick or filler is common. You just don't want there to be any appreciable amount of pressure inside the crankcase. This will lead to pushing oil out in all kinds of places. I had some warranty work done on a Magnum Lohler years back and the mech. put the reeds in upside down. It was biulding pressure inside the crankcase and pushed the crank seal out of the bloke. It didn't take me long to find the oil leak, but it took me about an hr to sort through what could be causing it to pressurise like that. They wouldn't let me do the work on the engine to start with. They said I wasn't qaulified. Made a BIG deal about how good these guy's were. I sent them a bill for 4 hrs labor from haveing to pull the engine on this zero turn to get to the oil seal and fix their mess. Of course I never got anything, but I did get to bend their ear for awhile.lol

[Salthart]

As dutch said, Run the manometer test. Your engine should pull a vacume in the crankcase and the manometer is the best way ( And the factory recomended way ) to test it. An engine that is in like new shape can send oil vapor out the oil fill, dipstick tube etc simply because oil is sprayed into the air inside. When you open the crankcase to normal atmosphere, There is no longer a chance that the CC can pull a vacume. What it does do, much like Dave said is let the movement of the pistons "move" the air around. Good or bad, you can hold your hand over the open oil fill and feel air moving in and out. Your M20 has a large enough oil fill that you should be able ( if you don't have a manometer ) to cap your hand over the fill while the engine is running and after 10 or 15 seconds you should feel suction. Remember, you may still feel the movement of the air both back and forth, Thats normal. pulling a "vacume" as we are speaking of it don't mean all the air is gone just that the pressure inside the CC should be less than the air outside. If your still not sure, Get a ballon and cut out enough to place over the fill tube and wire tie in place. (be sure it don't slip into the engine !) as you spin the engine, you will see the rubber move back and forth but once the engine starts, and even though it will still move back and forth, it will either do so outside the fill neck or somewhat pulled inside. If you do have pressure, I would check for a loose fuel pump, ( Comman ) worn or stuck breather check valves, And last Bad crank seals. Good luck

[Al]

Hi, The above posts are correct. The crankcase should run a vacuum. ANY leak that lets air in the crankcase will cause it to blow oil vapor out the crankcase breather. The dipstick not sealing thightly can cause it. As I remember I posted a message on this and how to make a water manometer to check it. If anyone knows where that post is, it should detail what you need. If it doesn't show up, I will try to do another one with a picture of an easy to make, cheap manometer. The previous post did not have a picture as I am hesitant to post pics because I can't figure out the red X problem. Some of us have a big vacuum between our ears and can't overcome some of these things. One thing on almost any air cooled engine, pulling the dipstick or removing the oil fill cap will really mess up the vacuum and many times can result in having a new yellow to black ceiling, depepending on the oil color. I have seen a number of these engines with this problem, and have some bolw the seal out of the closure plate on the PTO end of the engine. They seem to run very well otherwise. Every one we have had to put a new seal in has blown up with in a short time of the seal replacement and broken the crankcase. I would personally check the vacuum, and do a leakdown test on the cylinders. Next I would do a compression test. If there is lot of leak down, I would pull her out tear it down and mike the cylinders and crank. If not really good, would punch it out enough to clean, grind the crank if it needs it, and put new rods and OS pistons in.Al Eden