Having a senior's moment.... I am working on a 1980 International Harvester Scout II with a V8 engine (I think it is the 305 cubic inch one). I am trying to determine if the compression rings are just "okay". The test of removing the PCV valve, put a vacuum gauge on the oil dipstick tube, block of the fresh air intake (coming off the bottom of the air filter housing), and then start the engine and run it is for a very short period of time until negative pressure builds on the vacuum gauge will tell me if my compression rings are "okay"? In constrast, if I have little to no vacuum then I have unnecessary pressure in the crankcase?
Is the above good or bad logic - please advise. As always, thanks in advance for any thoughts and advise given.
If I understand you correctly, with the crankcase blocked off, a negative pressure would tend to suggest an intake leak to the crankcase. That's the only way I can think of that the pressure would go into the negative. Known good engines will produce positive pressure in the crankcase under the same conditions.
IMO, it has less to do with the piston rings, and more to do with crankcase and intake sealing.
I have never seen negative pressure in a crankcase with the PCV removed but id can't be bad. but it can't be thing to worry over.
Thre are half a dozen of ways of checking the compresion rings one no special tools way is to remove all the spark plugs except and turn the engine by hand through a couple of compression strokes useing a wrench on the crankshaft pulley bolt, the feel of turning it through compression should be equal for all cylinders.
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
With this post, I now understand why we check for what kind of pressure (i.e. positive or negative) we have in the crankcase and what it means. Before this post, I did not. Thanks for that clarity.
On nearly multi-cylinder engine the up and down stokes balamce each other out the exceptions are singe engines or three cylinder 4 stroke motorcycle (2 up 1 down) engines odd fire V6.
The PCV effectively acts as a vacuum pump , apart from scavenging blow-by combustion gas that has leaked past the rings a partial vacuum in the crankcase enables the the rings to seal better and prevents leak past the oil seals.
The main force pressing the piston ring outwards against the surface of the cylinder bore is gas pressure from the compression and power stroke which gets into the gap between the piston and the piston ring forcing the piston ring downwards and outwards a partial vaccuum in the crankcase asissts this,
" We're trying to plug a hole in the universe, what are you doing ?. "
(Walter Bishop Fringe TV show)
