The idea behind the Engine Compression Test (in the case of a Misfire Condition or Rough Idle Condition) is to find out if any one engine cylinder is not contributing 100% of its power to the overall engine output. If only one cylinder has lower than normal compression, your GM 4.3L (or 5.0L, 5.7L) equipped vehicle will run rough or cause a Misfire Code to set and turn on the Check Engine Light.
So then, to wrap things up, the individual cylinder compression readings of each engine cylinder can not vary more than 15%... and this is how you can find out:
Grab a calculator and multiply the highest compression reading that you recorded by .15.
So, let’s say that cylinder #4 gave you the highest reading of 170 PSI. Well 170 X .15 gives you 26 (25.5 rounded off).
Now, the next step is to subtract 26 from 170... which gives us 144 PSI.
So then, 144 PSI is the lowest possible compression reading that any one of the rest of the engine cylinders can have. Any compression reading below this.. and that engine cylinder will misfire.
Now, so that this calculation can make more sense to you... let’s say that my 4.3L C1500 Pick Up (or Astro or Blazer, etc.) gave me the following compression readings:
The next step is to do the math: 175 x .15= 26, 175-26= 149. So, now I know that Cylinder #4 is the one causing the Misfire!!
So, you've found the ‘dead’ cylinder or cylinders... what next? The next step is to do a ‘Wet’ Compression Test and find out if the low compression value (or 0 compression value you got) is due to bad cylinder head valves or bad piston compression rings.
This involves adding a few drops (2 tablespoons) of Engine Oil to the cylinders with the low Engine compression result and repeating the Compression Test.
The results you obtain from this second ‘We’ Compression Test will help you determine if the low Compression you recorded in the ‘Dry’ Compression Test are caused by worn piston rings or worn Cylinder Head Valves.
OK, this is what you need to do:
Add a small amount of Engine Oil to the Cylinder that reported low compression or no compression in the ‘Dry’ Compression Test
Install the Compression Tester onto the Cylinder.
When all is set up, have your helper crank the Engine.
You'll get one of two results, either the compression value will go up (from the one you recorded before) or it will stay the same.
CASE 1: The Compression value shot up.. This tells you that the Piston Compression Rings are worn out and thus the problem is in the bottom end (block) of the Engine in your GM 4.3L, 5.0L, or 5.7L equipped vehicle.
Here's why: The engine oil helped the piston rings seal better, thus bringing up the compression value almost back to normal. If the problem were in the cylinder head valves... the engine oil you just added wouldn't make a difference at all (on the compression value).
CASE 2: The Compression Value stayed the same.. This confirms that the problem is in the Cylinder Head Valves.
Here's why: If the cylinder head valves and their seats are worn out (or maybe even bent from a broken timing belt), no amount of engine oil is gonna help seal the compression in, in the cylinder. So, if the compression value, for the specific cylinder you're testing did not go up (after you added oil to it)... then this is a dead giveaway that you've got cylinder head valve damage.
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