Compression Readings on a 302
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Senior User
Joined: Feb 2010
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Compression Readings on a 302
Ever since I bought my truck a few months ago I have been chasing a rough idle/miss. I pretty much did everything under the sun the book said to do and then some with the help of the members on this forum and my final troubleshooting step was a compression check. Well it turns out that #2 was the worse, and #5, #6, #8 weren't to far off! Here are the numbers -
#1 142psi
#2 83psi
#3 148psi
#4 150psi
#5 130psi
#6 124psi
#7 138psi
#8 130psi
So with the low reading in #2 I redid the test in that cylinder last night and got 85 this time, I then decided to use some oil, however, I think I used to much the first time because the reading was off the gauge! So I blew some oil out and the readings ranged from 150 down to 122 as I let more oil out. Here is my questions, did I use to much oil and if so how much do I need to use? And I understand that if the reading goes up then the low compression is caused by the rings, right, and could the oil also so help the valve seal better as well. I am having a hard time believing its rings. I do not have any oil consumption, smoke, and no soot on the plugs or pipe.
#1 142psi
#2 83psi
#3 148psi
#4 150psi
#5 130psi
#6 124psi
#7 138psi
#8 130psi
So with the low reading in #2 I redid the test in that cylinder last night and got 85 this time, I then decided to use some oil, however, I think I used to much the first time because the reading was off the gauge! So I blew some oil out and the readings ranged from 150 down to 122 as I let more oil out. Here is my questions, did I use to much oil and if so how much do I need to use? And I understand that if the reading goes up then the low compression is caused by the rings, right, and could the oil also so help the valve seal better as well. I am having a hard time believing its rings. I do not have any oil consumption, smoke, and no soot on the plugs or pipe.
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Joined: Nov 2010
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Well I would have to say that to have a range from 120-150 psi isnt too alarming to me. Most of the time to try to find a more specific range is hard. If you look in a book it may say that proper compression range is somewhere between those numbers. But to have 7 cylinders within that range and then one that drops clear into the 80s, does not sound like a good thing. I would think there has to be something going on to cause that extreme drop in compression.
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Senior User
Joined: Feb 2010
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Yeah but to much oil is going to give a false reading correct? How much oil should I use to determine if it the rings or not. Also it's been about 15years since I tore a motor apart but if my rings are bad is that a complete rebuild or can I just re-ring it?
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Senior User
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Got it, I used more than that! Someone told me to fill the cylinder enough to cover the piston with at least 1/8" coverage!
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OK so I re-accomplished the compression check and used a cap full of oil, much better I think. Dry the pressure was 82 and wet it was 91. The hobby shop on base does not have the tools to perform a leak-down test, however when a friend of mine shot air into the cylinder and I could barely hear a hissing sound coming from the tail pipe. but didn't really hear it anywhere else. I will have to pull the heads regardless if it is the bottom end or top end!
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Joe Kucinski302, 351, a302, compresion, compression, dry, fod, ford, problem, psi, readings, rings, troubleshoot, troubleshooting, wet
