Engine Briggs 18 hp Vanguard engine

I have A 18 hp Vanguard engine on a 616 Grasshopper mower that I would know what the compression should be. I have checked the compression and have found that on the left side facing the flywheel I have 135 lbs. on the right I have 120 lbs. I would like to know if the right could be putting pressure in the lower end and building pressure and blowing out the seal on the dip stick tube. Thank you for you time.

Melvin1942

Your compression readings sounds within reason for that engine. Not likely causing the seal problem. make sure the breather isn't clogged allowing crankcase pressure to buildup. There could be a head gasket issue but not likely with those readings.

Something else to check would be to make sure it is running on both cylinders. the vanguard has a happen of randomly bending and dropping push rods, causing a single cylinder run situation and will force oil consumption to go up. Also a non sealing breather will cause oil consumption and excess oil in the breather, but not likely causing your blown dipstick tube seal. Make sure the engine isn't running excessive hot, because overly hot oil will cause hardening of the o ring leading to failure.

I'm going to ask you a few questions: Are you using full synthetic 5w-30 or 10w-30 oil? Is oil filter B&S brand oil filter? Have you adjusted valves with engine cold? Have you ever removed blower housing { top cover for engine } & cleaned topside of engine? Have you checked air filter for replacement? Please post model xxxxxx, type xxxx, trim xx from engine data plate, thanks.

Re: Briggs 18 hp Vanguard engine with crankcase pressure issue

18HP Briggs V-twin on a Grasshopper 718. Engine model #=350447 Type= 1195E1

My issue(s): Running poorly but not so bad that I think it is on one cylinder only. There is oil coming up through the carb rebreather tube, there is also oil coming UP through the top edge of the seal between the crankcase/sump cover and the block. When I check the compression I get 125-130 on the "drivers" side (#1 or #2 ?) and 150 on the other side. Obviously there is excessive pressure in the crankcase/sump area. My understanding is this oil COULD be caused by head gasket blown or sump breather valve. Does the fact that I have decent compression on both sides (although unequal) exclude either of these, i.e. head gasket? Could I have a ring/piston/cylinder problem with those compression readings? Would a leaky valve guide create this problem? Just so I can understand further, what does the sump breather valve do, i.e. does it prevent air flow from sump to carb or the other way around? Any help would be greatly appreciated, and as I start to follow this thread, I will try to help anyone else that might have a question that I know the answer to.

Easy bit first.
Breather has a sort of one way valave which in theory causes a partial vaccuum to be created in the crankcase while the engine is running.
They get oiled up and particularly if the engine gets too hot go gummy and stick allowing excessine pressure to build up in the crank & it has to go some where.
Because crank case fume is oily it gets dumped into the engine to be burned rather than venting as oily fume.

Inlets suck & exhausts blow so a failed exhaust guide will cause crankcase pressure to build up
A failed inlet guide will suck oily vapour into the engine and cause the engine to blow smoke .

A head gasket that is starting to fail can cause excessive crankcase presure, again gas will go where ever it is easy.

Actual numbers from compression gauges are somewhat irrevelant as 20 mechanics all reading the same engine will get 30 different numbers if they are using the same tester and 60 different numbers if using different testers.
As a rule of thumb 10-20% dfference is an indication that something might be wrong and further investigation is needed.

Also as a rule, white smoke on acceleration indicated worn rings
White smoke on over run indicates bad valve seals.
Only the inlet guide has a seal.

Thanks Bert, "easy bit" might be overstating my understanding. Be that as it may, I have some questions that may indicate how deep my ignorance is;

I assume, and correct me if I am wrong, that a failing head gasket and/or a failing valve guide would, to some extent, affect the performance from the cylinder that had one or both of those conditions. If one were fortunate enough to have BOTH of those conditions occur in the SAME cylinder (rather than one condition in each cylinder), then would pulling the plug wire on one cylinder at a time (and running the engine) give one a reasonably good clue as to which cylinder was so affected? That is, pulling the plug wire on the cylinder that is NOT affected would allow the engine to run "better" than pulling the plug wire on the good side.

Would the so called "compression run off test" reliably confirm which side is fouling things up?

If the valve guide or head gasket has failed on the side that "powers" the fuel pump, would the fuel pump keep on working?

As others have suggested elsewhere, if I suck and blow on the breather tube that leads to the carb from the crankcase, will that reliably confirm or deny that the breather valve is operational? Common sense leads me to believe that for the breather valve to be a cause for pressure to build up in the crankcase, it would have to be stuck in the closed position, but if it was stuck closed, wouldn't that mean I would NOT see oil coming out of the breather tube into the carb?

Thanks for your time and have a "Goo'day Mate" (hope that isn't offensive)

J R Clark

You mentioned that there is oiling coming up between the block and the side cover, is that correct. That is a weird place for the gasket to fail. But if part of the gasket is missing in that area it won't leak much oil since it is facing up and doesn't have direct exposure to the oil. what it will cause is loss of crankcase vacuum, and could cause the other problems with oil getting into the breather tube.

ILEngine, Yes, I just went out and started the thing up, waited about a minute or two and it started bubbling up oil out of the top seam between the engine block and the cover over the end of the engine opposite the flywheel end (?sump cover), at the same time there is copious oil coming out of the breather tube into the top of the carb.
If I pull off the plug wire on the drivers side cylinder, it runs pretty much the same but if I pull the plug wire on the passenger side (and reconnect the drivers side) it won't start. So I would assume if it is a valve guide/head gasket problem, then it must be on the drivers side.
Seems if there is oil coming out of that seam between the sump cover and the block at the top, and a lot of oil coming out the breather tube, there must be a lot of pressure build up in the case. Also, if I blow into the breather tube at the carb end, I can easily blow air into the crankcase, so maybe that means the breather valve is shot as well.

Thanks for the time you spent answering me.

J R Clark, Keene, KY

Sounds like we are back to one of my originals comments about are we running on both cylinders, and make sure it hadn't dropped the push rod. Running on one cylinder will cause excessive blowby and oil consumption. Couples with the leaking side cover gasket may have damaged the breather reed assembly.

If the push rods are fine. you may need to unplug the wire between the ignition modules, since they have diode that will go back causing a short between the modules. If it will run on both sides with kill wires disconnected then need to check the diode. If if not then need to check for spark on the bad cylinder, will the kill wire disconnected and no spark, replace the module.