Pressure and Vacuum Testing Basics

[JoeMay]

I've done the tests a couple of times just winging it and have been lucky to identify and isolate problems. This testing is very useful. So I want to be sure my set up and methods are correct and consistent. I would much appreciate responses to these basic questions.

Pressure Testing

Is 7 to 10 PSI a safe and reliable reference range for the test?

What position should the piston be in?

Vacuum Testing

Is 7 to 10 inches Hg a safe and reliable reference range for the test?

What position should the piston be in?

General Questions

Should the pressure/vacuum line be connected to the intake track, to the spark plug hole or will either location work?

Besides crank seals, what critical engine components and characteristics are being evaluated with these tests?

 

[nckcalifornia]

Plug off your exhaust of course then testing is done best from the intake. Gets tricky, but if you can mock up a piece of something that can stand in for your carb and take on the test connection, you’re set. If you test from the spark plug you just need to make sure your intake is plugged off and your piston position leaves the transfers open. Someone on here advised to turn the crank while testing which seems like good advice.

 

[nckcalifornia]

You’re testing everything between your air seals. Intake boot/impulse, crank seals, case/base gaskets, spark plug, decomp. Your pressure/vac levels are around what I would use. You got this.

 

[hotshot]

My favorite old post below, but there are dozens if you use the search function. Also, author ‘Lakeside53’ was a great asset on leak checking, but he has since retired.

https://www.arboristsite.com/showthread.php?t=42768&highlight=vacuum+testing
Your vacuum range is low, remember that one inch Hg is half a psi. So 7 psi pressure is about 14” Hg vacuum…

 

[J D]

Pressure/vac I go a bit higher... ~12psi/15 in hg. Less drop the better but shouldn't be more than about a third over a minute.
Turn the engine over when doing both tests, reading should fluctuate slightly - this tells you that you're testing the whole system (not just the tube up to the kink hiding under the saw )
Can connect to any part of the saw that leads into the crankcase.
Done properly it should test every seal the engine has between the exhaust & carburettor (case & cylinder gaskets, decomp, spark plug, crank seals, inlet block, impulse, etc)
Easiest way to seal inlet/exhaust is with a piece of inner tube between the engine & carb/muffler.
If you make a plate up for one & put a hole in it you can use the part of the inner tube with the valve in it as your seal & connection point (have to remove the actual valve)

 

[hotshot]

No need for that, likely to get failures unless you’re using pressure seals.

Standard nitrile oil seals in saws are designed for 7 psig.

See page 26 of the Dichtomatik seal catalog linked below & Stihls service manuals.

http://trywhisler.com/wp-content/uploads/2019/02/CAT-Dichtomatik-Oil-Seals.pdf

 

[J D]

I test mine at 0.6 - 0.8 bar... I'm fairly sure that's the Husky spec.
I go on the low side for vac but I didn't think about that when I converted to in Hg for the previous post... have amended it accordingly

 

[stihltech]

All good info. Do vacuum first. Pressure first may actually seal a leak temporarily. I find many more vacuum leaks than pressure unless a rubber component is ripped , cracked, or torn.

 

[Vintage Engine Repairs]

 

[JoeMay]

Wow, what a great response. So much knowledge and information. I thank you all very much. Biggest problems are the vintage saws that have just that little hole, where carb mates to cylinder, that is bored and channelled to carry impulse from the crankcase to the carb. Test pulsing thru the spark plug hole seems the best option for old saws that have this set up for crankcase impulse flow. I have a metal piston stop that I don’t like using as a piston stop. I will use my drill press to bore a hole through the center of it. Modifying it to accept a testing feed line should be easy. So that is what I plan to use to subject a vintage saw to vacuum or pressure. I’ll share a photo if it turns out Ok and it actually works.

 

[heimannm]

Another option to connect your pressure/vacuum tester is to knock the insulator out of a spark plug and install a barb fitting (braze, solder, thread, JB Weld).

I keep a spray bottle with a little dish soap and water handy to spray around the seals, crankcase seams, etc. when pressure testing to identify leaks.

Mark

 

[a. palmer jr.]

On some saws you can block off the intake manifold with a rubber cork.

 

[JoeMay]

Good idea. I gotta justify my carless purchase of that metal piston stop. This plan to use it for P-V testing vintage saws will hopefully help me save some face.

 

[Mad Professor]

Tom's tools are cleaner than my dishes. He does things right, not quick.

All his posts/vids are worth watching.

 

[Vintage Engine Repairs]

Tom's tools are cleaner than my dishes. He does things right, not quick.

All his posts/vids are worth watching.

Thanks mate I really appreciate the kind words!

 

[Mad Professor]

Thanks mate I really appreciate the kind words!

The credit is due You Tom. Excellent work.

 

[J D]

The credit is due You Tom. Excellent work.

+1

 

[a. palmer jr.]

Thanks mate I really appreciate the kind words!

My wife does our dishes so I can't say but his tools are cleaner than my tools..