Why does compression usually test so high?

[Avery4]

The expansion of air when heated makes perfect sense, thanks for the responses! I'm sure that's one of the reasons compression is normally significantly higher once the engine is warmed up than while it's cold along with the expansion of the pistons, cylinders, and rings obviously.

 

[Slow Car Sport Mode]

@Avery4 It might be so much more simple. It might be putting fuel in the cylinder as you do it. Some people don't know how to do a compression test.

Some people pull the fuel pump fuse.. easier way is depress accelerator pedal (throttle) to WOT which *should* disable fuel injection during that time, apparently some models this doesnt happen.. but you may be seeing "wet" compression test numbers (akin to a small amount of oil in the cylinder) making your numbers go higher. This is how you can get about 230psi on a compression test.. wet. Dry maybe 140-ish.. should be different every engine.

Your battery may be protesting at the end of the last cylinder. You can turn the engine with the key 5-7 times before you stop. I'd let it turn over 7 times. Some let it go 5. Engine sounds different cranking with no plugs and firing.

This isn't a Ford with "Fuel pressure relative to manifold vacuum".. you're checking the ability of that can (cylinder) to seal during that 4-cycle turning. Suck Squeeze Bang Blow..

You also want to do it cold, if possible. Most accurate reading.

First thing I think when I see higher numbers are wet tests or warm/hot engine.. and to do it again, with the proper method, cold, before speculating about carbon on the tops of pistons (which you can see if you shine a flashlight down a spark plug well, at least on engines with the plugs right there at the top, maybe an inspection camera for engine bays with them crammed all up in there.)

It takes several seconds of cranking before the pressure in the gauge peaks out. Therefor, you are reading the pressure trapped in the gauge and not the cylinder. The velocity of the air being pushed up by the piston increases the pressure.

Yep! 5 to 7 revolutions.

 

[Avery4]

@Avery4 It might be so much more simple. It might be putting fuel in the cylinder as you do it. Some people don't know how to do a compression test.

Some people pull the fuel pump fuse.. easier way is depress accelerator pedal (throttle) to WOT which *should* disable fuel injection during that time, apparently some models this doesnt happen.. but you may be seeing "wet" compression test numbers (akin to a small amount of oil in the cylinder) making your numbers go higher. This is how you can get about 230psi on a compression test.. wet. Dry maybe 140-ish.. should be different every engine.

Your battery may be protesting at the end of the last cylinder. You can turn the engine with the key 5-7 times before you stop. I'd let it turn over 7 times. Some let it go 5. Engine sounds different cranking with no plugs and firing.

This isn't a Ford with "Fuel pressure relative to manifold vacuum".. you're checking the ability of that can (cylinder) to seal during that 4-cycle turning. Suck Squeeze Bang Blow..

You also want to do it cold, if possible. Most accurate reading.

First thing I think when I see higher numbers are wet tests or warm/hot engine.. and to do it again, with the proper method, cold, before speculating about carbon on the tops of pistons (which you can see if you shine a flashlight down a spark plug well, at least on engines with the plugs right there at the top, maybe an inspection camera for engine bays with them crammed all up in there.)



Yep! 5 to 7 revolutions.

Good point. However, when I do a compression test on a Honda, I unplug the fuel injectors or pull the ECU fuse to disable fuel since they don't have clear flood mode. At least my 1997 and 2005 doesn't, I haven't tried my K24 swapped 04 or or anything newer. Maybe newer Honda's with drive by throttles do, I'm not sure.

I'm also not sure that gasoline would increase the compression, my experience with flooded engines has been that it actually decreases the compression by washing the oil film off the cylinder walls and rings, sometimes to the point of lowering the compression so much the engine won't start again unless a little oil is poured into each cylinder to bring the compression back.

 

[Slow Car Sport Mode]

Good point. However, when I do a compression test on a Honda, I unplug the fuel injectors or pull the ECU fuse to disable fuel since they don't have clear flood mode. At least my 1997 and 2005 doesn't, I haven't tried my K24 swapped 04 or or anything newer. Maybe newer Honda's with drive by throttles do, I'm not sure.

I'm also not sure that gasoline would increase the compression, my experience with flooded engines has been that it actually decreases the compression by washing the oil film off the cylinder walls and rings, sometimes to the point of lowering the compression so much the engine won't start again unless a little oil is poured into each cylinder to bring the compression back.

Well, you are correct a cylinder "wash" is no good, a flooded engine won't fire, especially if you pull a plug and it is "wet with fuel." I too have encountered vehicles that do NOT have "clear flood" mode; you'd think they all do but nope.. You'll know if you have your foot to the floor and you fire it up and the engine races up. Better hope engine was warmed up when you do that because.. not optimal. This happens the most when an engine is wanting to be started, but doesn't. Extra fuel. They also say to disable spark, but I've never done that.

The point I was trying to make was about liquid. The engine is turning, so it is still cycling, everything rotating. Just not firing.. In times I have seen either strange (or REALLY high) compression numbers where there should not be.. maybe you have some liquid in the cylinder that should not be there.. if the fuel situation has been ruled out, maybe there is coolant getting into the cylinder on the rotation during the test OR there is some kind of funky warped head/block issue.. sure, low number can be sharing compression, but head gasket leak might also be putting compression from somewhere else OR allowing liquid in.. even a small amount.. making a higher number. The smell or evidence of would be apparent when you take out the compression gauge. Essentially mimicking a wet test.

If the engine is of normal health and you KNOW that is not the case and ALL cylinders are high then my first thought would be carbon or, again, a wet test.. or even a possible timing issue although that may not make total sense. The readings are done with normal atmospheric pressure because it is always vented from the cylinder on the exhaust stroke so.... If I see "abnormally high compression" this makes me think something is wrong unless a re-test verifies normal results.

As alluded to before, on a warm engine the compression number may be higher because of not only the hot air but everything else being hot, different metals of different densities etc expanding at different rates.. still, then, I suppose consistency across your 4, 5, 6, 8, whatever you got cylinders could indicate engine health, and no dead or hurting cylinders, even in that case.

I hope this was helpful.

 

[Avery4]

Well, you are correct a cylinder "wash" is no good, a flooded engine won't fire, especially if you pull a plug and it is "wet with fuel." I too have encountered vehicles that do NOT have "clear flood" mode; you'd think they all do but nope.. You'll know if you have your foot to the floor and you fire it up and the engine races up. Better hope engine was warmed up when you do that because.. not optimal. This happens the most when an engine is wanting to be started, but doesn't. Extra fuel. They also say to disable spark, but I've never done that.

The point I was trying to make was about liquid. The engine is turning, so it is still cycling, everything rotating. Just not firing.. In times I have seen either strange (or REALLY high) compression numbers where there should not be.. maybe you have some liquid in the cylinder that should not be there.. if the fuel situation has been ruled out, maybe there is coolant getting into the cylinder on the rotation during the test OR there is some kind of funky warped head/block issue.. sure, low number can be sharing compression, but head gasket leak might also be putting compression from somewhere else OR allowing liquid in.. even a small amount.. making a higher number. The smell or evidence of would be apparent when you take out the compression gauge. Essentially mimicking a wet test.

If the engine is of normal health and you KNOW that is not the case and ALL cylinders are high then my first thought would be carbon or, again, a wet test.. or even a possible timing issue although that may not make total sense. The readings are done with normal atmospheric pressure because it is always vented from the cylinder on the exhaust stroke so.... If I see "abnormally high compression" this makes me think something is wrong unless a re-test verifies normal results.

As alluded to before, on a warm engine the compression number may be higher because of not only the hot air but everything else being hot, different metals of different densities etc expanding at different rates.. still, then, I suppose consistency across your 4, 5, 6, 8, whatever you got cylinders could indicate engine health, and no dead or hurting cylinders, even in that case.

I hope this was helpful.

Interesting, that makes sense. Never thought about the possibility of coolant in the cylinders increasing compression, but I could see that happening. Anything that takes up space inside the cylinders will increase the compression ratio. I've seen excess oil in the intake manifold and cylinders increase compression enough for fuel injected gasoline engines to diesel for 10+ seconds after taking the key out!

Perhaps that explains why I've seen engines with leaky head gaskets that still had even and normal compression across all cylinders- just enough coolant got into the affected cylinders to increase the compression ratio by just enough to offset the loss in compression through the leak.