Compression Test Looks Funny. Any Ideas?
08-06-2014, 12:34 PM
#1
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Compression Test Looks Funny. Any Ideas? I'm looking at a 5.3 that a friend of mine has. He did a dry compression test on it, and the numbers have me scratching my head.
Passenger Driver
8 - 150 7 - 230
6 - 220 5 - 150
4 - 240 3 - 150
2 - 150 1 - 210
Now I know a 15% deviation is generally a bad thing, but I'm wondering if maybe I'm missing something. It just looks too uniform, if that makes any sense. Going by the firing order, it is high-low, high-low, high-low and high-low. I don't have any specifics about the engine, other than it is a LM7 with 706 heads. He doesn't know the miles or what it came out of. It is complete from the pan to the intake and is just missing the coils, starter, alternator, and ps pump. It has been setting on an engine stand dry for the past year. I haven't asked him to go back and do a wet test yet. Is it possible the varying numbers would be a false read because of something simple, like the lifters being dry and held up from sitting, or am I definitely looking at either rings or worn head valves???
Any insight you guys could give as to what is going on with it, would be appreciated. Thanks!
Passenger Driver
8 - 150 7 - 230
6 - 220 5 - 150
4 - 240 3 - 150
2 - 150 1 - 210
Now I know a 15% deviation is generally a bad thing, but I'm wondering if maybe I'm missing something. It just looks too uniform, if that makes any sense. Going by the firing order, it is high-low, high-low, high-low and high-low. I don't have any specifics about the engine, other than it is a LM7 with 706 heads. He doesn't know the miles or what it came out of. It is complete from the pan to the intake and is just missing the coils, starter, alternator, and ps pump. It has been setting on an engine stand dry for the past year. I haven't asked him to go back and do a wet test yet. Is it possible the varying numbers would be a false read because of something simple, like the lifters being dry and held up from sitting, or am I definitely looking at either rings or worn head valves???
Any insight you guys could give as to what is going on with it, would be appreciated. Thanks!
08-06-2014, 03:46 PM
#2
I did the same on my vette. The numbers were like that, all over the map.
So I did a leak down test. All were better than 7% which is good for a 90k mile engine.
I attributed the whacky compression test numbers to the fact that I have 400lb springs and LS7 lifters some of which bleed down pretty fast. Too fast for a compression test at cranking speed.
That's my spin on this.
So I did a leak down test. All were better than 7% which is good for a 90k mile engine.
I attributed the whacky compression test numbers to the fact that I have 400lb springs and LS7 lifters some of which bleed down pretty fast. Too fast for a compression test at cranking speed.
That's my spin on this.
08-07-2014, 12:55 PM
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If you are unsure of your results running a static test....
try a dynamic compression test.
http://www.coloradocorvetteclub.org/...ontesting.html
The dynamic compression test (motor running compression test) will tell you a lot more about the overall health of your engine. Just keep in mind the numbers are a lot lower using this method, and they should be. But it eliminates the potential of skewed results because of flat lifters, or a low battery.
try a dynamic compression test.
http://www.coloradocorvetteclub.org/...ontesting.html
The dynamic compression test (motor running compression test) will tell you a lot more about the overall health of your engine. Just keep in mind the numbers are a lot lower using this method, and they should be. But it eliminates the potential of skewed results because of flat lifters, or a low battery.