compression check
Postmaster
Joined: Jun 2014
Posts: 3,206
Likes: 3
From: Oakhust NJ Jersey Shore
What it should be is no more than 20% of the highest reading, If you have a low one squirt some oil in that cylinder and do the test again if it goes up rings are shot. Make sure you have a battery charger on the truck, you want the same rpm's when turning, all the plugs out, throttle wide open, and count the same number of strokes, so you know there right. Good Luck
Hotshot
Joined: Apr 2009
Posts: 14,333
Likes: 246
From: Stanley, VA
Ford doesn't publish actual compression figures. There are too many variables to give one definite figure. All Ford says is the high and low cylinders should be within 25% of each other. I agree with David7.3 though, I would rather see them within 20%.
To run an accurate compression test, first, pull all the spark plugs. Next, block the throttle wide open. Third, make sure the battery is fully charged and hook up a battery charger to it. Then, going cylinder to cylinder, run the compression test. Spin the engine until the reading stabilizes, usually about 4-5 "hits". Record the readings.
If there is any big discrepancy, run the test again doing a wet test. Everything is the same as the dry test except that you squirt a small amount of oil into the cylinder before you do the test.
If there is a noticeable increase in compression, then that cylinder probably has worn rings. If there's not much of an increase then either there are valve problems or possibly broken rings.
Your hi/low figures are out of specs. At 25% (Ford) the low cylinder shouldn't be less than 127 PSI. At 20% the low cylinder shouldn't be less than 136 PSI.
Before you jump the gun, do additional troubleshooting. Unless the compression is uniformly real low, that's usually not the cause of lack of power. Uneven compression is more a cause of rough idle rather than lack of power. It could be something as simple as retarded timing.
A compression test is a good start, but don't stop there on your troubleshooting. Keep going.
To run an accurate compression test, first, pull all the spark plugs. Next, block the throttle wide open. Third, make sure the battery is fully charged and hook up a battery charger to it. Then, going cylinder to cylinder, run the compression test. Spin the engine until the reading stabilizes, usually about 4-5 "hits". Record the readings.
If there is any big discrepancy, run the test again doing a wet test. Everything is the same as the dry test except that you squirt a small amount of oil into the cylinder before you do the test.
If there is a noticeable increase in compression, then that cylinder probably has worn rings. If there's not much of an increase then either there are valve problems or possibly broken rings.
Your hi/low figures are out of specs. At 25% (Ford) the low cylinder shouldn't be less than 127 PSI. At 20% the low cylinder shouldn't be less than 136 PSI.
Before you jump the gun, do additional troubleshooting. Unless the compression is uniformly real low, that's usually not the cause of lack of power. Uneven compression is more a cause of rough idle rather than lack of power. It could be something as simple as retarded timing.
A compression test is a good start, but don't stop there on your troubleshooting. Keep going.
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