Engine Crankcase Pressure and Engine Oil Leaks

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*Always follow the instructions in your repair manual when doing repair or maintenance work on Outdoor Power Equipment. Manuals can be found at the manufacturers website.*
An abnormally high crankcase pressure can cause oil leaks. The engine crankcase is designed to operate below atmospheric pressure, and the breather valve or pcv (positive crankcase ventilation) valve is the device used to keep crankcase pressure below atmospheric pressure. Excessive blow-by, a blown head gasket, a faulty breather valve, or a plugged breather hose can cause crankcase pressure to go beyond the working parameters of the breather valve, and cause crankcase pressure to exceed atmospheric pressure.
Crankcase pressure is constantly changing, and the primary reason for this is crankcase volume is costantly changing. The bottom of the engine piston and cylinder walls is part of the crankcase area. When the piston moves from BTC bottom dead center to TDC top dead center the bottom of the piston exposes more of the cylinder wall. This increases the crankcase area and lowers the crankcase pressure. When the piston moves from TDC to BDC the bottom of the piston exposes less of the cylinder wall, and this lowers the crankcase area and increases crankcase pressure. Another factor effecting crankcase pressure is Blow-by. Blow-by is when combustion gases leak past the seal created by the piston, piston rings and cyliner wall. These blow-by gases move past the seal and into the crankcase area, and this will increase the crankcase pressure. Blow-by is normal, but becomes worse when the engine is cold and when the engine is worn. Excessive blow-by can overcome the breather valve and crankcase pressure will rise above atmospheric pressure.
The area of the crankcase extends up through the engine casing and underneath the valve cover on a OHV engine. If the part of the head gasket that seals the combustion chamber from the crankcase area blows, than the combustion gases will escape past the gasket and into the crankcase, and will raise crankcase pressure above the atmospheric pressure.
A leak down test can be used to test for a worn top end and leaking head gasket.
Check your engine service for how to test the breather valve on your engine, or you can view my KZbin video about testing breather valves if it relates to your engine:
• How a Small Engine Cra...
Thanks for watching!

[@openwingshomestead5798]

This is exactly the video I needed! I probably need an engine rebuild, but now I can actually understand what is happening with it. THANK YOU SO MUCH!!!!!

[@ServetheWORD]

Ditto

[@melfyre4543]

Excellent video, straight to the point and comprehensive. Thank you.

[@fcmcfcmc]

Very helpful, clear calm and concise. Small point, volume not area. Subscribed. Looking forward to more.

[@ginoasci2876]

You should send this video to all the motorcycle dealers all over the globe so they will learn something. They tell me I don’t have a leak when I’m looking at the oil drip. They sent me a video that proves it’s not leaking but the engine is at idle and in first gear. They also pissed on my leg and told me it’s raining. Idiots. Thank you very much for this video.

[@frankpotter1982]

Great explanation 👍

[@keithshreve6736]

Super video, great explaination Thank you.

[@penguinistas]

Good information and well presented. Thank you.

[@Brian-ds5gq]

excellent video the first thing I check when I run into an issue oil leaks not so much on this style engine but including this one is the seal around the oil filler cap whether it be a gasket or o ring if it leaks a small amount of oil it'll suck a large amount of air these engines actually do run on a vacuum and if it's mounted on a mower make sure that the person did not just recently tilt the mower to clean out the from the blade or something of that nature A good rule of thumb is spark plug disconnected and muffler or spark plug pointed up always if you have to tilt the engine

[@doublewide6]

This video is very informative. I learned a whole lot of new things. Thank you for taking time to share this information.

[@smallengineshop]

+doublewide6 Thanks for watching!

[@eldigitom9680]

interesting video, thx for the knowledge!

[@aleblanc3547]

GREAT explanation! Thanks for sharing.

[@smallengineshop]

Thanks

[@bobbydhal2199]

Awesome video !!! Your the best Your the best !! Thank You

[@MrTebi7]

Great vid thanks for sharing!

[@g.king.]

very nice learned a lot

[@alderete74]

superb video

[@ericsaur9791]

Great. Thx. So clear.

[@hristuppiteitinu]

That was great, learned a lot. Thank you!

[@smallengineshop]

+hristuppiteitinu Thanks for Watching!

[@chlln1591]

Great vid

[@jdimeo1966]

Very informative. Thanks!

[@smallengineshop]

Thanks for watching

[@superhands8041]

Great vid. I learnt a lot here. Found it because I have lots of oil in the airbox of my single cylinder bike. Is this a big worry? Im hoping it was just over filled at service. Have cleaned it out and changed filter is there anything else I can check?

[@TheMeishi]

Learnt a lot from your vid. Thank you

[@smallengineshop]

Good to hear! Thanks for watching!

[@bojk13]

well done

[@wyattoneable]

You communicate very well. I appreciate the insight and your experience in this field.

[@smallengineshop]

Thanks

[@AbhishekRajput-zn7ns]

thank for share this video.

[@ihateemael]

one side of the pcv valve is open to engine fumes, in a car via the oil mist separator in the cylinder head. Under high intake vacuum conditions these fumes are sucked into the intake manifold to be burnt, restricted by the pcv-valve. To prevent a vacuum in the crankcase the opposite end of the cylinder head is vented usually via the airbox. A draft is sucked through the engine and fumes are prevented from polluting our air.

[@awrosenoff]

You really helped me out with this video. I couldn't figure out what was wrong with my scooter. The crankcase vent tube was kinked and wouldn't vent. I also couldn't add any oil. All fixed!

[@ajithanand1312]

how

[@JOEZEP54]

Nice clear information. .👍 993 Stay safe, Joe Z

[@samuelmengsteab5540]

Thank's for the video I got oil coming out on blowby diessel engin about 1/4 small water bottle every week they told me to rebuild the engine. Did I need rebuild? What's ur advice Thank's

[@geoffhess2267]

Excellent explanation! I’m doing an engine rebuild on an old Honda Harmony lawnmower that I replaced the crankcase gasket last year but started to leak some oil way too prematurely. Given the age of the engine, blow-by due to worn rings might be causing excessive pressure in the crankcase. Thanks!!

[@philliphall5198]

Might install governor arm seal while your at it, bad about leaking on deck

[@michaelherbert3237]

Very good info.thank you.

[@smallengineshop]

Thanks for watching!

[@staceystarling15]

Important question, I have 1/4 or MORE play in my crankshaft and opposite side on flywheel. Can get the play by pushing/pulling on either side. Any advice on problems or solutions. Engine is a 5hp tecumseh yr 95-99

[@isaacchua0405]

Thank you so much important detail in an article i read....who else is here also because of Mercedes F1

[@PHAYSo]

Excellent video thanks mate

[@philliphall5198]

I’ve had very few head gasket issues with the GX Honda engines

[@jsmcguireIII]

so a new oil leak on the top edge of my kodiak 400 crankcase might be caused by blowby from a bad ring, or a failing PCV valve? I was thinking it was just an old gasket that needs replacing. I'll now check the PCV valve/hose and replace and seal the gasket - and see if that fixes things otherwise it could be $$$$

[@bottomknockinbaja6092]

Would you be able to remove the crankcase cover with an engine like this in the frame or does it have to be laying flat on its side out of the machine? I have a similar engine and leaky gasket in a golf cart

[@simdogg8412]

Hi, I was looking at an old vespa the other day and i noticed that there was oil dripping from the exhaust at idle. Is this oil leaking from the crankcase into the cylinder

[@seanwalker5157]

can a partially sheared flywheel key cause excessive pressure, I have a honda gx 390 hard start pullback and oil over the crankcase cover.. low on oil after a mud motor long-distance run .tks

[@shaneburton7813]

what about 81 kz550 when you put old filters on carbs?

[@peterhayes3040]

I have an old Onan E140 H engine that sucks the case cover “in” at the same place. I read that it’s a common problem...suggestions??

[@biddser81]

great video, informative

[@geojor]

thanks for sharing...

[@sheilaclaxton187]

pei broker my briggs generator oil fill plug on back side fell out while running now acts like there is no compression didnt know it had two fill plugs until fell out blowing oil everywhere

[@marywoods5460]

What happened ifur crank case freeze up and crack what all dose the cooler run throw cause I have one that got water in oil guy said he winter raised it but dose Look like it please get back to me thank you jeff

[@adeyemibabatunde1814]

@smallengineshop If the crankcase oil is affecting the performance of a carburetor negatively, where else can the crankcase oil be directed to safely. Crankcase oil becomes thickened and deposited in the carburetor and make it performing poorly(piston not rising, throttle plate becomes stuck and not moving freely)

[@tinashechikosi7960]

I have a hatz 1d81z it leaked oil from exhaust manifold and l changed the stamp seals.after putting new seals the problem started again leaking oil from exhaust and l thought maybe the engine head was damaged and l changed the engine head with everything new but still it's leaking oil through the exhaust what could be the problem?

[@workwithnature]

Cheers for the video ;) One quick question. I have liquid in my air filter (there is a round ring at the bottom and it has prob. oil in it, about 100ml) what may be the cause of that?

[@notsolrflare1079]

Same

[@DRECK813]

So I have a oil leak only when I'm riding full throttle. Can u help with that

[@nonqabalungile6703]

If the engine case pressure has a crack, is it dangerous? do I have to replace my car or how can I fix the crack?

[@MadazMazdas]

there's oil leaking out of my exhaust on my 6.5hp clone, the engine refused to start so I figured it was flooded so I turned off fuel and it seemed very easy to pull very smooth which is odd, it feltlike it had compression but it felt like less than usual. I took off the air filter and noticed oil when I put my finger down the hole, so I removed the assembly and it was oily so I drained the fuel bowl and it was full of oil. Not only is there oil coming out the exhaust but it's also leaking into the intake... how do I tell what kind of oil leak this is and how do I prevent it after I replace what ever I need to? someone pls help

[@mikewestcott7180]

Thanks for the vid. Do you know also, if crankcase pressure can be caused by back-pressure from the exhaust system? Either due to blocked exhaust/mufflers etc. or sticky exhaust valves? I guess that might just be considered part of the blow-by if it does?

[@jonienglish3231]

Good Video i just rebuilt my Briggs 250cc OHV Snow Engine 15c107 , 2 Times !!! Similar to the Clone Engine in this Video . Now Starting to Leak oil !!! Engine Runs Great but after running oil leaking ?

[@battifortesempre1432]

Thanks for video. I have same problem. Oil in air filter from crankcase, the engine is chinese copy of honda 4 stroke engine, it's new and i want solve problem.

[@midnightrun7]

Good Video. Does anyone out there know how to properly clean out the crankcase of a Harley twin cam?? I have the top end apart and rebuilding. I want to get some of the sludge and carbon particles that may have dropped in the crank, out of there. Thanks in advance!

[@darrinsiberia]

Could "stop oil leak" additive to oil ... cause oil leak out of crank pulley seal.

[@GNX157]

A couple things. To have positive crankcase ventilation, there has to be constant suction of the crankcase to bring it slightly below atmospheric. That happens only with a pcv valve connected to a vacuum source, usually the intake manifold after the carb throttle plate and before the intake valve. A pcv valve is basically a calibrated vacuum leak, which the engine maker has designed to account for by its carb/engine calibrations. This is important because you should never use a different pcv valve than what is called for as it may leak (flow) a different amount. Proper crankcase ventilation also improves cylinder ring sealing. On some high performance engines such as pro stock, you could find them using a belt, electric, or gear driven vacuum pump that would create very high pos ventilation pressures to make the piston rings seal really well because pro stock uses low tension/friction oil control rings and maybe even one compression ring so they can rev to higher rpms, less drag.

[@paulamoore290]

M8 how would go about fixing this

[@quocducnguyen]

Nice video. Question: Why do you need a one way valve for the breather? Don't you want air to get back into the crankcase when the piston go toward top-dead-center to prevent a suction pressure in the crankcase? Instead of breather hose, can you just have a balloon like device where the balloon will expand at piston's bottom-dead-center and deflate at piston's top-dead-center?

[@Tomee62538]

You want to relieve the positive pressure.

[@tomgallier0103]

I've got a 3hp tecumseh. . flathead. . I'm getting oil , coming thru where the value springs are on the side..I thought it was normal so the springs and valves didn't seize up ?

[@hereb4theend]

Gotta keep it clean every now and then 🤗

[@carlitoswaytorres5309]

who can help me out if anyone was a video to rebild a manual transmicio for a 03 suzuki aereo

[@clementheldsinger3482]

Cam anyone tell why my engine dipstick keeps popping out

[@richardrisch]

question I have a 9hp Tecumseh snow blower engine with correct oil levels, but when running oil blows out breather tube- any ideas?? ricker

[@kennethcollier9791]

ricker bear junk it

[@hereb4theend]

Your PCV could be clogged 😜

[@telorino9209]

Bad head gasket

[@tengranido6188]

its a blueby