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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.

mysticheadlice said:

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

Click to expand...

You have to remember we are talking about a very basic engine engine here, not a 100Hp / liter top fuel drag engine.
In many cases the engine will run and cut grass quite fine with a broken con rod thus only running on one cylinder and a lot come into the workshop just like this with the owners stating " it seems to bog down in long / wet grass "

You will not notice much till the gasket has totally failed and the cylinder has no compression at all.
Usually the first indiction is smoke on start up, followed by smoke for 10 minutes after start up, progressing to hard to start & smokes all the time followed by won't start at all and this can span several years.
No one ever noices the drop off in power because most mowers are overpowered by a facor of 2 to 3. Except for bagging 4 Hp per 1" of cut is generally all the power required. Most modern mowers run near 2Hp / 1" ( GT mowers).

Pulling plug leads is a standard test and yes if it runs better ( or no worse ) then that is the problem side just remember to earth out the idle plug as some ignition modules can not cope with wunning full spped open circuit and they are expensive little buggers to replace.

The fuel pump is also way over capacity required but yes eventually it willnot be able to keep up however remember that the whole internal space is all connected together.
Kohler Courages can develope a crack from the top of the engine case to the bottom and most will contine to get enough pressure pulse to run till all of the oil has drained out the crack, then the don't run.

Most times the vent valve will not stick totally closed but remember it has to open & close 7200 / minute on a twin so just going slow will make a big difference.
Because of the shape of the chamber, oil will condense in the bottom and flow into the carb even if the valve is not working properly but yur original assumption stands, more likely to be failing gasket than the valve but you check the easy bits first.

Blown gaskets need to be presure tested to be definative.
You pressreize the cylinder with the piston in the firing position and then listen for escaping gasses and even then it still could be rings.
Gaskets are designed to fail early in these engines and are quite cheap and relatively easy to replace.
Do them one at a time with the piston in firing position so as not to put strain on anything when tightening down the head.
25 ft/lbs of torque is very low and even a desk jocky can exceed this using a 6" wrench so be careful when tightening the head bolts.

And no offense.
It is a soft G prounounced sort of Ger but very short & written as G'day

Well, considering that I am gainfully employed, and with Spring fast approaching, I find I have more money than time. Thusly, I have decided to go ahead and pull off that head and replace it along with the head gasket and I will replace the breather valve at the same time. It is likely I will need to pull off the sump cover as well to replace the gasket. I say "likely" because I discovered today that at least one of the bolts (I couldn't reach the others) holding the upper half of the sump cover was loose (less than hand tight). To change the sump cover it looks like I will have to remove the crankshaft pulley that is attached by belts to the PTO pulley. Do you know right off hand if I am going to need specialized tools (other than a standard kind of pulley puller) to get that off? I guess there will be some kind of oil seal in there as well. I am thinking I should remove the pulley first and then see if a little tightening of the sump cover will fix the oil leak before pulling off the sump cover to replace the gasket (unless removing the sump cover is simply a matter of removing the bolts that hold it down). Any thoughts. Realistically, should I replace both heads like they usually recommend, rather than just one? The one I am thinking about NOT changing had the higher compression reading-150 PSI). Thoughts?

With the sump cover bolts being loose, I will guarantee that there is a piece of the gasket missing. You will also need to replace the seal around the crankshaft. Unless the pulley is rusted to the crankshaft a pulley should be all that is needed to remove. May be able to remove by hand after removing the retaining bolt.

I don't really think you have a head gasket problem You have problem that is causing you to run on one cylinder. Fixing that may solve most of the problems.

Yes the engine will have to come out.
Buy a full rebuild gasket kit it is cheaper & easier than finding out you are short 1 gasket.
Not sure if the kit for this engine contains the inlet valve seal.
The flywheel is generally a lot harder to pull than the pulleys.
Undo the pulley bolt while the engine is in situ then if necessary lever the pully against the underside of the mower.
Generally a std wrecking bar will work fine as most times the pulley drop off as soon as it is undone.
Rattle guns make the job a lot earier.
Watch for a loose drive key some have them while ohers are part of the pulley and any spacers between the pulley and the crankcase.
If you have an electric PTO then you will need a deep socket to get in there.
Most are 9/16 or 5/8.
Ditto for the flywheel, a lot easier to remove with the engine firmly bolted to the mower.
Get a milk crate or similar and make a 1 &1/2 " hole in the middle to drop the engine on to when working on it or make a frame out of real 2 x 4's, again makes life easier.
Don't forget to drain the oil before you start then once out lay the engine on the oil plug side to remove the rest of the oil ( well most of it any way) .
I have some stands that fit inside 25L ( 5 gallon to some ) plastic drums cut in 1.2 lengthways.
This not only contains the rest of the oil but also traps all the fiddle bits that otherwise fall off and end up in that parrallel universe along with all of the left socks and spare keys.
Cover you work with some cloth ( I use old denim ) or in the morning it will be full of wasps, beetles and moths. They seem to love to crawl in the bottom of a deep hole & die there.
Allow a couple of weeks for the job as while the heads are off take them to a head shop and get the valves & seats done. Most will do a small job like that while you wait. If you have a real ower repair shop ( not part of a glass front retail outlet ) they will probably do the valves for you and if you go there for the gaskets the week before you can ask them.
I charge $ 10.00 a valve so a twin will cost $ 40.00 ( Aust )
Good luck but again as ILENGINE has already mentioned it is better to properly diagnose before you start wily nilly replacing perfectly good parts for no cause.

ILL Engine: Let me clarify, My first post about MY engine was #5 on this thread. It is probably my fault, but I couldn't see how to start a new thread on this forum and this one was titled "Vanguard 18 hp" so I just kind of jumped in. So the first 4 posts in this thread were about someone with a problem blowing out their oil dipstick seal, and some of that discussion may have leaked into the help you were trying to give me. My push rods and valve clearances are fine.
My problem is with the oil blowing into my carb through the breather tube in copious amounts, and by that I mean actually oil pouring out of the tube, not just an oily cloud of air. At the same time oil pours out between the seam at the top of the sump cover/block. So, something is causing a great deal of pressure build up in my crankcase. I appreciate your thoughts on all of this, but I simply don't see how these conditions could occur without something pressurizing the crankcase and I don't think running on one cylinder would do it. I think it has to be pressure coming from one of the cylinders, by way of a blown head gasket or a cracked head or a leaky exhaust valve guide, or I suppose a broken ring could do it but my compressions are 120/150. I don't think oil would leak out the top of the sump cover without significant pressure to drive it, i.e. the running engine is slinging oil all over the place inside the sump area and the pressure inside is pushing the oil out when it gets to that leaky gasket, without pressure, gravity would probably make the oil fall back down away from that leak.

This discussion, with you and the gentleman from Australia has clarified my thinking on it tremendously, i.e. I have learned a lot, and I hope I can continue to pick your brains as i proceed with this beast.

What are your thoughts on replacing just ONE head rather than two? It seems many think by introducing a new head, the previous wear and tear on the old one will create some kind of mismatch between the two sides.

Thanks, John Clark, Keene, KY

Well that explains a lot.
To start a new post you need to be o the index page of the particular forum.
That is the first page with the list of all the posts on it.
On the left side is a green button "Post new thread".
You click on it and start typing.
It is not good idea to tack onto another thread as it confuses every one and you will end up getting repair advice for the original posters problem.

Valve guides won't cause oil to get into the breather tube. You said it wouldn't start with one of the spark plugs disconnected but would on the other. Which confirms you are running on one cylinder. Even if you had a blown head gasket on the low compression cylinder, 125 psi is still plenty for it to run on. I am thinking you may not have spark on that cylinder for some reason.

The missing piece of the sump cover gasket most of the time doesn't come all the way out, It gets partially sucked in but hangs, so when the pistons go up it sucks air into the crankcase. When they go down that piece can act like a one way valve, and then the extra pressure gets pushed out of the breather. I would still remove the breather and make sure it is operating properly and that the drainback holes are not clogged. I have see cases where the read valve gets broken which can also cause oil in the breather.

Running on one cylinder can cause fuel dilution of the oil on the cylinder walls, and thin the oil. I would worry about the leaking side cover gasket and inspect the breather and if damaged then replace it. But I wouldn't worry about the head or the gasket until everything else is confirmed.

Sorry about the "interlude". As Ill knows, it suddenly got cold again and wasn't much I could do out there until it warmed back up. Well, that and having to turn my attention to wood splitting and hauling and keeping a woodstove going 24 hours a day- a bigger job than most people appreciate.
I have read these posts through again and the only update I have is that I do indeed have a good spark on both cylinders. I have spoken with a couple of shade tree small engine mechanics and all 3 of them were of a mind that the most likely cause for my problems was a blown head gasket. They agreed that there were other potential causes and that of course there could be more than one problem going on. I should mention that the head with the lower compression that won't start on its own, is a head I replaced about 4 years ago when it lost all compression and when I pulled the head I found that one of the valve seats had popped up out of its proper location and was wedged under the valve, preventing it from closing. I was told the valve seats could not reliably be replaced and that I would have to replace the head, so that is what I did- ran great after that. Since this is the same side that I think is giving me problems now, I wonder if for some reason that side of the engine is getting hotter than the other. Just food for thought. I couldn't find anyone with enough experience with this particular engine to say whether or not having problems with this particular side (drivers side= ?#2) was common with this engine.
Be that as it may, I am planning on tearing into this thing shortly. My goal is to inspect and replace the breather valve, pull off the head and inspect and replace the gasket and have a local machine shop examine and rebuild that head (of course I will inspect the cylinder wall etc while I have the head off). I'll remove what all I need to in order to replace the sump gasket and take a look inside there for stray bits of metal and signs of wear and tear etc. I don't currently see myself tearing down the lower part of the engine but depending on what I find, that could end up on the agenda.
All for now, the grass is back growing today (