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Another Briggs V-twin burping oil...

Posted by dlkelley713 GA (My Page) on
Fri, Mar 4, 11 at 9:08

Hello, I've read many posts on my issue, but unfortunately I haven't found my problem. I have a 24 hp Briggs V-twin with 74 hours (Model 445777 0168 E1). It's coughing oil up the breather, mostly under load. It was like that when I bought the tractor... thought it would be a simple fix...wrong! I've done the obvious things (new breather, new head gaskets (although the old ones looked fine), checked and changed o-rings in the oil filler tube and filler cap). Compression is okay (153 & 145), cylinder walls look good under heads. Exhaust valves are a bit carboned, but otherwise valves look to seal okay. Where do I go from here? I'm planning to get a manual for the engine, but don't have one now. Am I going to have to pull the engine from the tractor in order to pull the crankcase? Thanks for any ideas! Dan


Follow-Up Postings:

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RE: Another Briggs V-twin burping oil...

Dan,
I am familiar with your engine, since I have a Model 446777, which is the 26hp version.

Send me a PM and I will email you an engine service manual (pdf) that covers your engine.

Yes, you do have to remove the engine to pull the sump (crankcase cover). The manual details how to R&R the sump.

Does your engine actually burn oil and show smoke from the exhaust or does it just 'cough oil up the brather'?


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RE: Another Briggs V-twin burping oil...

Thanks Galaxieman,
I found the the PDF online before I saw your response. The oil is definitely coming up the breather. This morning I tore it down. Unfortunately I couldn't find anything suspect inside. It looks like a very healthy motor. The sump / crankcase gasket tore up pretty bad upon separation, so I can't guarantee what it's state was beforehand, but it didn't look like it had blown out. The crankshaft seals rubber was pretty firm, but no cracks or tears. The mating surfaces on the heads and covers are good and true (no warping). The cylinder walls still show the honing cross hatch pattern all the way around each cylinder, so there's no apparent wear there. The pistons and rings look good. The exaust valves and bores have a bit of carbon, but they seemed to be functioning okay. The intake valves and bores look great. There has to be a limited number of places the air pressure can get into the sump, but darned if I can find it! Am I missing something? I don't fully understand the "normal" state in the crankcase / sump during operation, but it seems to me that as the pistons move up and down, they would cause alternating positive and negative pressure in the crankcase. I don't understand why an external leak other than the breather (like an open dipstick) would cause an increase in crankcase pressure that would cause oil to exit the breather. If anything, it seems to me that it would relieve the pressure and stop it. If someone can enlighten me, I'd much appreciate it!
Thanks for any ideas!
Dan


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RE: Another Briggs V-twin burping oil...

***"seems to me that as the pistons move up and down, they would cause alternating positive and negative pressure in the crankcase"***
That is exactly the "normal state" of the crankcase pressures, and the reason for it.
It is the alternating "+pressure/-pressure" that has been exploited to operate the pulse powered diaphragm fuel pumps featured on many of today's engines.

***"I don't understand why an external leak other than the breather (like an open dipstick) would cause an increase in crankcase pressure that would cause oil to exit the breather. If anything, it seems to me that it would relieve the pressure and stop it. If someone can enlighten me, I'd much appreciate it."***
I believe the reason for that is because the "design parameters" intend that the crankcase be "partially evacuated" by means of the breather system to keep the INCREASE of + pressure when the piston(s) move toward BDC to a minimum. This is accomplished by venting the crankcase to the intake manifold and thus into the combustion chamber.
Ideally, the only gases present in the crankcase are what escape past the piston rings (unavoidable minimal leakage) during normal operation.
Under normal conditions, the crankcase breather system is able to accomplish the task of keeping the + & - pulses to a low difference of pressure swings.
BUT.........if there is leak somewhere (not as large as an open dipstick) allowing additional gases (atmospheric air) to be drawn into the crankcase when the piston(s) move toward TDC, those gases......plus any normal blowby gases...must be vented out through the crankcase breather system, and that means a greater flow of gases/oil mist through the breather assembly. If it gets great enough, the oil separator may be overwhelmed and pass liquid oil and mist into the intake passages.
Something as big as an open dipstick tube probably decrease the + pressure level of the crankcase, but likely you would have oil mist transported out the open tube.
A large opening such as the dipstick tube could likely reduce the pulse fuel pump delivery rate, or stop it altogether.


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RE: Another Briggs V-twin burping oil...

Thanks Mownie for the explanation - that makes sense. So the reed valve in the breather prevents air from coming from the breather into the crankcase during negative crankcase pressure. So if there is a bad air seal somewhere else, air will be drawn in there during negative pressure. As the piston drives down to BDC, the blowby gases combined with the crankcase volume decrease causes positive pressure that exceeds the normal amount. So I guess the breather is able to evacuate the extra air, but since there's a lot of oil vapor in the exiting air, the breather fills up until it 'burps' liquid oil into the intake, and cycle repeats. This seems logical to me. This problem usually shows itself under load. I would assume this is due to an increase in blowby gasses past the rings while under load? On more thing... I wonder what would happen if one were to seal the breather hole in the crankcase... would the air leak show itself by way of exiting oil since there would be pretty constant positive crankcase pressure? Or would the engine self-destruct? Just something to think about... Anyway, should I fool with new rings given my compression above? If I can minimize my blowby, it's bound to help somewhat I guess. I'd appreciate any other ideas you or anyone else may have. I apologize for the long post, but it forces me to think it all through...
Thanks,
Dan


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RE: Another Briggs V-twin burping oil...

Dan,

Here is some more information for you. It may not apply to your specific problem, but it may be food for thought.

I have a similar problem I am trying to isolate. Plenty of blue smoke. Heads good. Compression good. Leakdown good.

In order to eliminate the crankcase breather, altogether, I took an old breather, opened it up and removed the reed valve so that it was totally 'open'. I reassembled the breather and put it back in place. However, I did not connect the rubber hose to the carb horn. I inserted a plastic hose into the rubber hose and routed it to the 'outside' air. I also covered the hole left in the carb horn. This eliminated the possibility that 'vaporized' oil was getting back into the carb and being burned and then showing up as blue exhaust smoke.

The engine ran fine like this, even though the crankcase was vented through the empty breather to the outside. However, the same blue smoke continued. It seems, the only thing left, for my problem, is the rings and/or cylinder walls.


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RE: Another Briggs V-twin burping oil...

I would never suggest that one "seal the breather hole in the crankcase". You would run the risk of turning some oil seals inside out. There are a couple of ways to evaluate the "health" of piston rings prior to doing any "fooling".
The best way is to perform a cylinder leakdown test. In this test, the cylinder being tested is held firmly at its TDC Compression stroke while compressed air is applied to cylinder using a "Leakdown tester gauge" via the spark plug hole. The leakdown gauge tells how much loss of air pressure and volume (as a percentage) is occurring on the cylinder being tested. It still requires some sleuthing to determine if the loss is occurring due to ring wear/damage, poorly seating valves (in combustion chamber) or if it's due to a leaking head gasket.

Another way to tell if the rings are losing the pressure is to perform the ole "dry & wet" compression test.
This test is done by performing a proper compression test, proper meaning "ENGINE WARM and THROTTLE BLOCKED WIDE OPEN, MINIMUM of 4 compression stroke registering on the gauge.
Perform the "dry" test first, recording the pressures observed on a scratch pad or note pad.
Then add about 2 teaspoons of clean motor oil to the cylinder through the spark plug hole. Cautiously (don't be in front of the spark plug hole))spin the engine a couple of turns to distribute the oil and expel the excess.
Now perform the compression test again. Record these results beside the dry readings.
If you see an increase of compression that amounts to 10 to 15 MORE PSI than the dry readings, your rings may be worn out, have lost their "tension" due to overheat, or they may be stuck in the grooves and failing to inflate properly under compression...........or a combination of conditions.
Typically, the wet test will not show an increase of PSI if the loss is from poorly seating valves, or a blown head gasket.
Of course one can always do a tear down and inspection, but aside from finding actual broken rings, or rings that are worn badly on one end area and not the other (would indicate the lightly worn ring was "stuck" to the piston) just looking at the parts post mortem probably won't toss an answer in front of you.


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RE: Another Briggs V-twin burping oil...

Thanks guys.
Galaxieman, thanks for the experience. I'm curious though - was that engine also pushing oil out of the breather? I disconnected the breather tube from the intake and added a section of clear tubing so I could watch while running the mower. Big blobs of oil came up from the breather (and onto my yard), but I had no smoke at that point.

Mownie,
I wish I'd gotten your advise before I pulled the motor :). I did attempt a leakdown test, but my POS tester was from Harbor Freight and has more air leaks than my crankcase! I ended up just doing a cold 'ear' test by skipping the gauges and pumping air straight into the cylinder and listening & feeling for the air leaks. At TDC on the compression stroke I had just a small flow of air coming out of the breather tube. I could put my finger on it and hold it for 10 to 15 seconds before losing the seal. It didn't seem like a lot of air, but I'm no expert... Regarding the 'wet' compression test, I wish I'd done that, but it's too late now... lesson for later:)

One more thing please. The manual says to remove and discard the valve stem seals... Does that include the steel insert or just the intake seal with the rubber lip? The steel insert didn't want to move with reasonable pliar force... If I'm supposed to remove those, how does one get them out?

Thanks again,
Dan


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RE: Another Briggs V-twin burping oil...

"Big blobs of oil came up from the breather (and onto my yard), but I had no smoke at that point."

With my experiment...no blobs of oil...just a fast puffing at wide open throttle. Virtually no oil in the 'puffs'

"The steel insert didn't want to move with reasonable pliar force... If I'm supposed to remove those, how does one get them out?"

My God...DON'T pull on those steel inserts. Do not harm them! They are the valve guides. Briggs does not sell them (they tell you to buy a whole new head for about $100). Treat them with care.

The seal on the intake valve guide can be removed and replaced with the one that comes with the engine gasket kit.



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RE: Another Briggs V-twin burping oil...

Thanks for the info on the valve guides! I thought they looked awfully permanent, but the manual diagram looked like the pliars were gripping the insert, not the seal!

I'll keep y'all posted on my reinstall. I haven't ordered my gasket kit since I wasn't sure what else I might find in there.

On final question for now: The rings look good on the ends (and elsewhere) and move freely in the pistons. Given my compression numbers above (153 and 145), and the fact that my motor didn't smoke when the breather was disconnected from the intake, would you guys recommend anything other than a new gasket kit and reassembly? I hate to put it back together without knowing what was wrong, but I can't think of anything else to do...
Thanks,
Dan


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RE: Another Briggs V-twin burping oil...

I would at least include a new set of piston rings, seeing as how you already have it down AND you are chasing a "smoke and oil consumption" problem.
I suggest you "mike" the pistons AND the cylinder bore to check for wear and distortion. If this engine was ever subjected to an overheat event, it's possible the cylinders are a bit oval shaped. These things are not typically what you might find, but if you have the means to check, don't neglect to do so.


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RE: Another Briggs V-twin burping oil...

Thanks Mownie,
I'll post after my reinstall and let y'all know how things went.

Dan


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RE: Another Briggs V-twin burping oil...

I have a single cylinder Briggs 14 hp with this same condition. Replaced the rings (one was cracked) and still a lot of air rushing up through the breather tube (at bottom dead center). I do have a leaky exhaust valve, replaced the valve and still leaky so have to machine the head seat or get a new head. I used thin foam sheets instead of a head gasket to ensure good air seal when pressurizing the system (I don't want to buy another head gasket until I get the problems all fixed, head on-off of there so many times now, and with air compressor instead of combustion it's ok for now). So something is still leaking severely passed the piston rings. Even when good and wet.


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RE: Another Briggs V-twin burping oil...

At bottom dead center?
Why are you placing the piston at bottom dead center for this test?
Piston should be at TDC of COMPRESSION stroke and the flywheel locked or held firmly at that position (so it can't turn from the air pressure).
Is this a Briggs overhead valve engine? 14 horse dose not disclose that fact (but the model number and type number from the engine sure would).
When you had the rings out of their shipping package and before you put them on the piston...........did you insert the compression rings into the cylinder to check the ring end gap?
A broken ring(s) can indicate one or more things have taken place:
Cylinder and piston worn over bore and undersize.
Engine has suffered one or more overheat events.
Improper original installation of rings.
Attempting to start a reluctant engine by using ether starting fluid.

Also, if the cylinder walls are scored or scratched, you may not be able to trap enough air to actually inflate the rings and make them seal to the cylinder walls and the piston ring lands (of the ring grooves).


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