Briggs Leaking Oil

[Mr-Mom]

I have a Craftsman walk behind mower with a Briggs model 128T05-5268-B1 engine. Recently the machine seemed to be using quite a lot of oil. It was working hard and was hot and a bit smokey. I checked the oil and it was off the stick. I topped it up and after ten minutes or so, it was smoking again. I checked the oil and it was low again and again I topped it up. This went on for a while and I decided to dig into the thing and see if I could find an obvious problem. I was unable to find anything obvious but looking around at the parts diagrams I noted the oil breather assembly and decided to have a look at it.

Once the engine was apart, there was no obvious problem with the oil breather but I tore the gasket getting the part off. A new gasket is $3 and the whole vent with a gasket is $7. I got a new vent assembly and installed it. I also replaced the spark plug, the air filter and I drained the oil and topped it up with the specified 18 ounces of straight 30 weigh oil.

I started and ran the machine. It ran OK with no more smoke than normal if I worked it hard. However, I noticed quite a bit of oil on the mower deck right under the oil vent. I checked the oil level and it actually seemed a bit high - I'm not confident that 18 ounces is the correct spec - I think 14 would be better. In any event, I checked the oil breather bolts and they took a bit of tightening and I ran the machine again. Once the machine was good and hot it started leaking again at the same place. I tore the machine down again and completely removed the oil breather. I checked the oil galleries, which were nice and clean. I did notice a bit of old gasket on the engine surface. I cleaned this off and then put Permatex FormAGasket on both sides of the gasket before reassembling and let the machine sit for two days before running it again.

My efforts made no difference. Once the engine was hot, it started dripping oil on the mower deck, right at the same place below the oil breather. There is no external orifice on the oil breather, so the oil is not coming from there. I'm quite confident the gasket is not leaking. The oil comes out around the bottom of the cylinder, but not at the head gasket. It comes out at the body of the motor. I have been assuming that the oil breather vents to the carburation system, but I don't know how. It is possible this just comes out of the engine on some vent I'm unaware of?

This only seems to happen when the engine is fully hot and working hard. An suggestions as to what is happening here?

Post Script:

Writing these things down always helps me think through them. I found a better parts diagram for this engine at https://www.m-and-d.com/pdfs/briggs/12U800-ms0989-0904.pdf and I noticed that there is an "oil breather passage", part number 697734. This appears to run under the motor and over to the carburetor side. I suspect this is leaking; possibly a bad gasket or just loose bolts. An suggestions on how to get at this component without removing the motor from mower chassis?

 

[Born2Mow]

You are correct. The engine breather is typically connected to the carb or air filter so that engine fumes will be burned in the combustion chamber. It's important to have the engine able to "breathe" or water and other vapors will be trapped inside the engine. The breather hose may have been removed or may have become stopped up. Trace the breather system and find the 2 connection points. Clean and re-attach the hose. If not, dust and debris may be sucked inside the engine.

An older engine may simply make more "fumes" when it gets hot.

 

[Mr-Mom]

It appears that the breather is attached to the carb by this "oil breather passage", which is a casting attached to the machine by four bolts. They look difficult to get at, but maybe I can reach in there with a small wrench and check them for tightness. I may also have bad rings. My compression gage did not show a lot of compression, though the machine runs fine (and the compression gage is old and of suspect quality.) When hot, the machine may be pushing a lot of oil, which if it weren't leaking, would probably smoke heavily. I'll get in there and have a look.

 

[Scrubcadet10]

18oz is the correct capacity,
actually the gasket (697734) and breather passage is on top of the engine, under the flywheel and right behind the ignition coil.
you also won't get an accurate reading of compression as the camshaft has a compression relief device on it that makes it start easier (Pull easier). Leakdown tests are better for engines with a Comp. release. but around 90PSI is usually what i get on these engines.

 

[Mr-Mom]

18oz is the correct capacity,
actually the gasket (697734) and breather passage is on top of the engine, under the flywheel and right behind the ignition coil.
you also won't get an accurate reading of compression as the camshaft has a compression relief device on it that makes it start easier (Pull easier). Leakdown tests are better for engines with a Comp. release. but around 90PSI is usually what i get on these engines.

Well, that explains the crazy low compression reading in an engine that runs! Thank you.

So if the "oil breather passage" is on the top of the engine, and the oil is not coming out of the breather gasket, where else, on the bottom, valve side, of the engine could it be coming from?

 

[Mr-Mom]

Studying this nice parts diagram closely, I see that the crankcase is split horizontally and that there is a gasket that goes between the cylinder block and the sump. I can't see in there well enough to see exactly where the leak is, but it is in that area, just below the valves and oil breather. Is this a common failure in these motors?

If this is the source of the leak, would it be typical that the leak would get worse as the motor heats up? I don't seem to get any leaking until I work the thing hard and it's getting fully hot.

 

[Scrubcadet10]

i wouldn't call it a common problem, but it's not impossible to happen.
i would also check if there is any oil ontop of the blade and crankshaft. just in case the crankshaft seal is leaking.. remove the spark plug wire if you move plan on rotating the blade.

 

[Mr-Mom]

I just watched a couple of YouTube videos showing this repair and it does not look difficult. Worst part is that I just filled the fuel tank and the machine has oil. A bit messy but not difficult. I'll pick up some good gasket goo and take the pan off. I think I've been chasing the wrong issue with the oil breather. Thanks for your help.

This is my first time with this forum and it seems to be a good one!

 

[Scrubcadet10]

Let's not get the cart ahead of the horse. next thing i would do is CLEAN the engine and mower deck good enough to eat off of, then run it, or wait for it to leak, it will be easier to PIN POINT the leak.

 

[Mr-Mom]

I've got it quite clean now. I even used some carb cleaner to blow out the suspect area, but I can't really see where the oil originates from, just that it is below the oil breather, then down onto the mower deck. I can't find any other place that it could be coming from other than the pan gasket. I don't know much about splash lubrication, so I don't know how leaks in this type of motor manifest themselves. A leak at the pan gasket checks a lot of boxes though; worse when the oil is full, worse when the engine is hot, no significant exhaust smoke. I guess I could check it a little by seeing if it gets worse when the mower is tipped to the left. I can have this engine stripped and off the deck in about a half hour (I'm well practiced at the tear down - done it three times!). From there it's just seven bolts to see the gasket.