...Continued from the previous page.
OK, so you tested all 8 cylinders and you've got those readings on paper... what now? Well, those numbers will tell you if the misfire condition is due to a low compression reading in one (or several) of those 8 cylinders and that you can stop looking/testing other components or that the engine compression of each cylinder is OK and not the source of the misfire condition and/or codes (P0300, P0301, P0302, P303, P0304, P0305, P0306, P0307, P0308). Here's what you need to do to interpret the results of your compression test:
Grab a calculator and multiply the highest compression reading recorded by .15. So let's say, for the sake of this example, that the highest reading was 160 PSI (Pounds per Square Inch). Multiplying 160 by .15 gives us 24
Subtract 24 from the highest reading. In my example, the highest compression reading is 160... so subtracting 24 from this reading, I get: 136.
So then, 136 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, let me give you a more specific example: Let's say that my Ford F150 (or Crown Victoria, or Expedition, or etc.) produced the following compression readings: Cylinder #1 = 165, Cylinder #2 = 95, Cylinder #3 = 155, Cylinder #4 = 170, Cylinder #5 160, Cylinder #6 165, Cylinder #7 170, Cylinder #8 160. The next step would be to apply the formula above and I get 145 PSI as the lowest possible reading (170 x .15= 25, 170-25= 145). So, now I know that Cylinder #2 is the one causing the misfire!!
My Engine Doesn't Start: If you're testing the engine compression to find out if the engine is shot: Then you'll know right away because most, if not all, of the compression readings will be at 0 PSI.
Also, if the majority of the readings are less than 90 PSI... you're engine is shot.
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 ‘Wet’ 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 Ford 4.6L, 5.4L 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.
“A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.”