Fellas,


I'm in the process of working on a customers Simplicity Coronet rear engine rider. Unit came to me with an oil leak, oil was all over the back of the engine. I cleaned it up and ran the engine while on the lift and watched. the oil was blowing out of the crankcase sump gasket.

I took the oil filler cap off and had a lot of positive pressure mixed with oil mist come out of the dipstick tube.

I connected a leakdown tester to the spark plug hole and turned engine to TDC. I pressurized to 20 PSI and I could hear and feel air coming out the dipstick tube. I couldnt build any pressure at all, due to it coming out the dipstick hole.

before I go any further am I correct in assuming rings or cylinder is wore?

should I take the head off and mic the cylinder before doing much else.

engine is a briggs, model 289707, type 0113 01.

Also, when I took the crankcase breather off I could blow and suck air in both directions on the rubber tube. shouldnt the breather act as a check valve one way ??

should I replace the breather and see what happens, regardless of the air coming out of the dipstick tube??

or do you think the positive pressure in the crankcase ruined the breather assembly??



thanks, Mark

Mark,
Sounds like you are a whole lot better equipped and trained than I am. I don't have a leak down tester...not that I wouldn't like to have one..but I'm not sure what type of pressure/% of pressure loss would be considered acceptable for single cylinder engine. That said I would probably pull the head and get a look at the condition of the cylinder walls and piston. Even if the cylinder walls don't look too scored if you can push/rock the head of the piston top to bottom and or side to side and get "excessive" play I would bet the cylinder/short block is worn excessively.

What does the plug look like? is it fouling the plug? What does the oil look like? I was just fooling with a 6.5 Hp briggs engine that would start and run just fine but it smoked like hell. I had changed the oil twice and each time it went from clean yellowish colored oil to a shiny lubriplate battle ship grey color instantly. The cylinder didn't look to be scored much at all...a well seasoned tech told me later that sometimes people run'em scored for so long they wear the score marks smooth...thats where all the silverly metalic in the oil was coming from..Anyway the piston was down right sloppy. I was able to rotate the flywheel by hand (with the head off) and watch the piston cycle and see a little pool of oil left on the bottom of the cylinder each cycle.

Spit

I would go with the rings and or the cylinder are in need of attention. I live by my leak down tester. I use it more often then my compression tester. The compression tester is really only going to to tell you that there is an internal issue . The leak down tester is going to point you in the right direction of the issue, air from the exhaust issue with exhaust valve,air from the carb issue with the intake valve,air out of the dip stick rings,cylinder etc,or air from around the head head gasket issue. [I] was taught anything over 10 % leak is a major issue that needs to be addressed.

Should an engine with good rings and cylinder be able to hold say 80 or 100 pounds of air pressure in the cylinder???

without leaking past the rings.

the one I'm working on a cant build up any air pressure because it leaks out the dipstick tube. I've turned the flywheel back and forth several times to make sure I'm at TDC.

still get air out the dipstick tube.

Hey Rotti, how long do you you let it leak for and if its leaking won't it go past 10%? I'm a little confused on how they work when air is escaping and how they won't go past 10% if there's nothing majorly wrong?

If I do any air testing, it will probably involve sticking a piece of rubber fuel line over the tip of the air gun for my compressor and blow air into the spark plug hole while cranking it by hand to see where the air goes. That would work as a general test to check for leaky and stuck valves.

Here is what I have gleaned from the internet on leak down testing which may be more than you want to know:

To perform a leakdown test, put the engine at EXACTLY TDC on the cylinder you're testing. This will test the valves and rings. you let air into the cylinder, using a compressor, and then stop. The cylinder should retain about 95% of the air, youre gonna have to do the math. For example, if you put in 100psi, it should go to 95 or so. I thing anywhere above 80 isnt too bad. If you put the cylinder at bottom dead center, it works too, and this will find deep bore scratches not evident at the top of the hole. make sure the valves are closed too. Good luck, and make sure your leakdown tester isnt the source of the leak.

ps, as much pressure as the compressor will make is safe to put in the engine, as cranking pressures sometimes exceed 180psi

It is imperative the crankshaft be securely held in place while performing a leakdown test or you may severely injure yourself. Briggs and Stratton makes a tool for securing the crankshaft. On small engines the tester is generally used to determine cylinder pressure loss. From my experience with a homemade tester you first need only to start with 30PSI. If you have cylinder pressure loss it will show up quickly at this pressure. If needed then increase the pressure.

Large pair of vicegrips on the crank. Clamp on an area where if you mar the crank it won't hurt anything.

I suppose you could block the blade.

You can't do a leak down test at bottom dead center because either the intake or the exhaust valve will be open depending on what cycle you happen to be in. There will always be some air escaping through the rings and maybe some through the valves. The idea is that a percentage of the air pressure will be retained for a specfied amount of time. Aircraft engines have a known specification that must me met. Smaller engines can only be guessed at. If you suspect that either the rings or the valves are bad the leak down tester can be employed to quickly comfirm that fact without tearing down the engine. That may be it's biggest advantage. With the cylinder pressurized a quick listen to the crankcase, intake port, or the exhaust port will quickly reveal where the problems are.

Now adjust your new leak down testers regulator knob to read 100 psi on the guage that came with it, since the female quick connector that goes to the engine cylinder hose is closed, you call this 100 psi reading "O" percent leakage. With the motor on exactly top dead center (TDC) on the compression stroke (you do each cylinder seperately) you can be sure you are on compression and not on the exhaust stroke by having the tester engine cylinder hose screwed into the spark plug hole and feel the end of the hose with your finger as you turn the engine over by hand coming up on the compression stroke, the air will blow your finger off the hose. (unless that cylinder has no compression at all).

With the piston at TDC and the valves closed when you apply air to the combustion chamber through the spark plug hole, the guage on the tester should read 100 psi or call it 100%. That would be zero percent leakage, because no air is going through the tester, but all motors leak some, even if they are in perfect condition. Any leagage above the vicinity of 8-10% (your guage will now read 90 psi) should be easy to locate by the sound of air whistling out the intake port/carb if the intake valve is not seating (worn out, no lash or bent etc) or by listening for leakage at the end of the exhaust pipe for the exhaust valves, or sometimes you can feel or hear it blowing at the crankase breather vent if the air is going past the piston rings. A faulty head gasket will leak at the cylinder and head junction. The leak down tester is a great tool to tell exactly what is going on in a motors top end, and to pin point problems that may exist. The amount of psi you read on the guage that is less than the 100 psi you originally set it at is called the same number in percent that the motor is leaking on each cylinder.

You will need a cylinder leak detector such as Snap-On #MT 324. Or a shade tree apparatus can be assembled from a few common items as follows.

First, modify a good quality compression gauge by separating the gauge from its hose and installing a female air hose coupler so the gauge can be easily separated and reconnected. Secondly, use the following parts to finish the apparatus. A 1/4" NPT tee fitting, three male air hose fittings, a 1/4" stop valve, and another female air hose coupler. Assemble the parts so the gauge connects to one tee port, the stop valve and male air hose fitting connects to another tee port, (The stop valve controls air pressure to the apparatus) and the hose from the compression gauge connects to the third tee port

Third, remove the check valve from the fitting that screws into the spark plug hole. (Use the tire valve extractor from your car tool kit) If possible, run the engine to warm it up. Remove the spark plugs. Remove the oil filler cap and the dipstick. Remove the air cleaner or air flow meter, and open the throttle. Carefully remove coolant filler cap and top up with coolant to bottom of filler neck.

To apply air to the cylinder with the valves closed, crank the engine by hand in the direction of rotation until the piston is at top dead center with the distributor rotor pointing to the same cylinder plug wire position. Now screw the apparatus into the spark plug hole. With the gauge and air supply hooked up, start opening the stop valve. Note the maximum reading of the gauge.

Listen for leakage at the following places:

* Adjacent cylinders sparkplug hole. Use a piece of small rubber vacuum hose, stick one end near the spark plug hole and the other end in your ear. Leakage, air hiss, heard here may be a blown head gasket between cylinders or it may be leakage heard through an open valve.
* Exhaust pipe. May indicate a burnt or stuck exhaust valve.
* Carburetor or throttle body. May indicate a bent or stuck intake valve.
* Oil filler or dipstick hole. May indicate broken rings or a damaged piston.
* Radiator filler cap. Bubbles here will indicate a leaking head gasket or cracked head.

.You will have to put the piston on top dead center which you did,on the compression stroke,lock the flywheel so the engine cannot turn.Screw the tester into the sparkplug hole and slowly dial about 30 to 40 pounds of air while listening where it come out.More than that can be dangerous and is not necessary.Be sure isolate the breather hose from the carburetor
,so you are not mistaken where the air is coming from.Here's some areas to listen and feel.

Muffler-leaky exhaust valve.
Carbuertor air horn-leaky intake valve
Breather hose-worn or damaged cylinder/rings
Head area-leaky head gasket
or warped head

Most small engines
nowadays have some sort of compression release. This allows air to escape through the exhaust on many of them with the piston at TDC. If it's running good, I wouldn't worry.

Just a note...even a brand new engine will leak air during a leakdown test. The issue is how fast the air is leaking, and where from.

Furthermore, to properly remove the compression release from the equation, you need to place the piston 1/4" PAST TDC before you take any readings on the gauge. At TDC the MCR (Mechanical Compression Release) is still activated. In other words, the piston must actually be on the power stroke with the piston 1/4" down the stroke. Also, it is best to take readings with a warm engine so that all internal parts have expanded, through heat, to their proper tolerances.

Furthermore, to properly remove the compression release from the equation, you need to place the piston 1/4" PAST TDC before you take any readings on the gauge. At TDC the MCR (Mechanical Compression Release) is still activated. In other words, the piston must actually be on the power stroke with the piston 1/4" down the stroke. Also, it is best to take readings with a warm engine so that all internal parts have expanded, through heat, to their proper tolerances.

The compression release activates near the beginning of the compression stroke, not at the end. It would be useless as a compression release if you waited until the end of the compression stroke to vent the cylinder. Both valves are fully closed at TDC of the compression stroke. If they are not, then your valve timing if off.

Performing a leak down at TDC is the correct procedure.

1/4" past TDC is for adjusting the valves and only to be sure the compression release is not interfering with the adjustment, but the adjustment can be confidently set at TDC as well.

If you have a lot of pressure blowing out your dipstick. You have a problem with either the Rings, Cylinder, or Piston (or a combination). If it's an OHV you could also have a blown head gasket in the push rod area.