# Crankcase pressure/vacuum



## Rentahusband (Aug 25, 2008)

Paul, could you explain how to test crankcase pressure/vacuum. I would like to add to my notes for reference.
Thanks


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## paulr44 (Oct 14, 2008)

I will send you a service manual that demonstrates it, respond to my email with your home email address so I can send the attachment.

The basics though, are that you have to:

seal the exhaust port
seal the intake where the carburetor mounts
seal the impulse line
seal the decompression valve port if equipped

STIHL sells inexpensive flat rubber block-offs in wedge shapes. You could use old inner tube sections too, but I've found the stihl ones are so cheap and well made they're a must-have. Part numbers are 0000 855 8106 and 0000 855 8107.

The idea is to seal the crankcase completely. You don't want to remove the manifold and seal the block there, as you'd be excluding the manifold from the test and it may be a source of a leak. I've seen decompression valves leak excessively too, but you'd have to test with and without it to do a proper test.

In one of the port block-offs, you'll need a typical fuel line barbed fitting (NPT) threaded into the plate or fixture. Into this fitting you'll attach your mityvac and do a pressure and vacuum test.

Pressure test, it should hold about 7.5 PSI, or .5 bar for about 20 seconds. If it creeps SLOWLY, that's okay as long as it doesn't lose more than 2 PSI in that 20 seconds (.13 bar). BASICALLY though it should HOLD the reading for 20 seconds.

IF IT LOSES pressure, spray soapy water around the potential leak points to find the leak:
Crankshaft seals
Intake manifold
Transfer ports (if bolt on, such as Strato)
Cylinder base gasket
Impulse hose
Cylinder head (as in old Tecumseh AV/AH engines with bolt-on heads)
Deko valve area (as in old Tecumseh...see above, they were external (boy, 
I'm OLD))
Test with and without Deko valve(you'll need a plug)...they shouldn't leak dramatically.

One problem you may run into, is that the intake area of chain saws may be confined and require a special OEM adapter. Now, in the case of the newest STIHL MS201T chain saw, they wanted *WAAAAY* too much money for the adapter (like $50), and I made one in less than 10 minutes from an old carburetor. The intake manifold has two brass bosses that protrude into carb. as it's bolt bosses are counterbored for them, as a receiver. I simply took an old carb, drilled the bolt bosses open appropriately and use a rubber block-off behind it.

Other models, such as STIHL's MS362 and MS441 have adapters that are more resonably priced. I can't speak for the Husqvarna's out there, but the the STIHL kit and homemade plate's I've made we get along just fine.

As for vacuum test, one OEM says no more than a .3 bar (4.5 PSI) loss in 20 seconds should exist. I would say that's excessive for most standards, but you have to play by their rules when doing failure diagnostics.

Now, if you're wondering what a bar is, it's basically equivalent to 15 PSI. In any case the basic conversion is simple, multiply bar times 15 to get PSI. EG .5 bar x 15 = 7.5 PSI. 1 bar x 15 = 15 PSI. Get it?

If you really get into engine testing and diagnosis, you need to learn mercury inches (as in setting bike carbs), water column inches (as in testing crankcase vacuum) and the like. You don't need to understand each, just the conversion if applicable and the application for each. Crankcase vacuum testing on 4-stroke while running is a good test for blow-by or a leak for instance.

http://en.wikipedia.org/wiki/Bar_(unit)

Okay, I've been wordy enough...


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## chuck_thehammer (Oct 24, 2007)

I use a blood pressure arm cuff setup (analog type).. from a drug store. discard the cuff.
I make block off plates from aluminum plate. (thin is OK)

remove electrical stuff, pressurize crankcase, place engine into bucket of water. any bubbles is the leak.

I do this to most 2 cycle (stroke) engines. works for me.

I use a mini-hand-vac (mini-vac) for the vacuum test.


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