Compression check
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Compression check So my 98 Wrangler with a LS6 swap has bit of a miss. I let it run for a bit and using my infrared temp gun the left rear header tube is about 90 degrees cooler than the other cylinders. No knocks or other bad noises. I have a scan tool and no codes detected. Engine was rebuilt about 17 years ago. New forged rods and pistons. About 20-25K miles on it.
So I replaced the coil pack on that cylinder. Had all the fuel injectors cleaned and flowed. They checked out just fine. Replaced the plug wire. Pulled the spark plug and it looked fine. Still a bit of a miss and lower temp on that cylinder.
So I'm thinking compression test on that cylinder.? Do I have to pull all of the spark plugs to get an accurate reading? Or can I just pull one of the other plugs and compare PSI wise?
How much is typical compression PSI wise? Never did a compression test on a LS engine.
Many many moons ago we were taught on carbed SBC engines you pulled all the plugs and held the throttle blade open as you crank the engine..
So I replaced the coil pack on that cylinder. Had all the fuel injectors cleaned and flowed. They checked out just fine. Replaced the plug wire. Pulled the spark plug and it looked fine. Still a bit of a miss and lower temp on that cylinder.
So I'm thinking compression test on that cylinder.? Do I have to pull all of the spark plugs to get an accurate reading? Or can I just pull one of the other plugs and compare PSI wise?
How much is typical compression PSI wise? Never did a compression test on a LS engine.
Many many moons ago we were taught on carbed SBC engines you pulled all the plugs and held the throttle blade open as you crank the engine..
You can do it either way, as long as it’s a consistent test. I pull all plugs personally. It will spin over easier and quicker usually….and won’t try to start without having to pull fuses. It will fuel all the cylinders evenly so it should be consistent across all 8, in that regard.
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Yeah you wanna pull em all so the engine spins as fast as possible. Ideally the same speed for all of em. Keep a battery charger on it while you're doing it, to take one more variable out of the whole equation.
In a compression test you're usually looking more for consistency among cyls than some specific # (which is why you want it to spin at the same RPM for all cyls... consistency). Any number of things can affect the absolute #; temperature, humidity, the cam you're using, whether it's advanced or retarded compared to other installations, valve adjustment or hydraulic lifter status, and so on. For a gross rule of thumb, 5% consistency or so among all 8 is ideal. 10% or greater variation is an indication of possible fornication and calls for further diagnosis.
A pump gas engine should show no more than about 200 psi, usually. 180 - 200 is pretty typical. A way high # compared to the others on only 1 cyl, is a classic sign of a cam lobe down, usually exh. A longer duration cam can drop it ALOT; sometimes as low as 150. 2 cyls next to each other with low #s is a blown head gasket (they most often blow where they're the weakest / narrowest and the pressure differential is highest, which is between adjacent cyls, NOT between a cyl and the coolant jacket). And so on.
Doesn't too much matter what kind of engine. Air is air, fuel is fuel. That's what you're really working with, not the brand of the castings, as far as what the #s should be. Fuel doesn't care whose castings they are, or how many cyls are in it, or any of the rest of that; it detonates at pretty much the same pressure in anybody's.
In a compression test you're usually looking more for consistency among cyls than some specific # (which is why you want it to spin at the same RPM for all cyls... consistency). Any number of things can affect the absolute #; temperature, humidity, the cam you're using, whether it's advanced or retarded compared to other installations, valve adjustment or hydraulic lifter status, and so on. For a gross rule of thumb, 5% consistency or so among all 8 is ideal. 10% or greater variation is an indication of possible fornication and calls for further diagnosis.
A pump gas engine should show no more than about 200 psi, usually. 180 - 200 is pretty typical. A way high # compared to the others on only 1 cyl, is a classic sign of a cam lobe down, usually exh. A longer duration cam can drop it ALOT; sometimes as low as 150. 2 cyls next to each other with low #s is a blown head gasket (they most often blow where they're the weakest / narrowest and the pressure differential is highest, which is between adjacent cyls, NOT between a cyl and the coolant jacket). And so on.
Doesn't too much matter what kind of engine. Air is air, fuel is fuel. That's what you're really working with, not the brand of the castings, as far as what the #s should be. Fuel doesn't care whose castings they are, or how many cyls are in it, or any of the rest of that; it detonates at pretty much the same pressure in anybody's.
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From: Carnation, WA
Personally I have disconnected the injectors when I run a compression test so that fuel doesn’t wash down the cylinder walls and impact the ring seal. I learned about this back in the 80s. Maybe it’s an old wives tale, but I do it … seems plausible.
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disconnected the injectors when I run a compression test
And of course unplug the big connector(s) to the coils for kinda the same reason.
Both of these are universal common procedure for this testing.
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Engine was rebuilt about 17 years ago. New forged rods and pistons. About 20-25K miles on it.
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Launching!
Joined: Jan 2011
Posts: 203
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From: chicago
Pull the fuel relay and save time and possibly damaging a wire or plug when it s not needed.JMO
