Working on my 2007 Volvo S60R. Have a long dowel in the plug hole to watch the piston move up to TDC as I turn the crank with a wrench. When it's at TDC I attach the gauge and it reads 50 or 60% which is pretty bad, but if I keep turning the engine past TDC, the reading gets better, like down to 20 or 15%. I thought the valves are all closed at TDC and that would give the best results. Is that incorrect? And why not test at BDC?
Trying to understand a cylinder leakdown test.
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The test is for air leaking down past rings and such. Performing the test with cylinder at TDC ensures all valves are closed, so any air escaping the cylinder is doing it by getting past rings...or an improperly seated valve.
How are you getting the compressed air into the cylinder? What are you using?
How are you getting the compressed air into the cylinder? What are you using?
The rings wear more at TDC from cylinder firing there. That's why there's a ridge in the cylinder at the top. As you take the piston down in the bore the ring gap gets tighter since the bore is not as worn resulting in less leakdown.
The most cylinder wear occurs at TDC. If we carefully measure a very worn engine (older engines were prone to this) we might find 0.0005 inches (near nil) of cylinder wear at the very bottom of the bore, where rings don't touch, and 0.020 inches at the top ring position. (or more) . This creates a cylinder ridge so deep, it becomes difficult to remove the piston as the rings catch on the ridge. Hence the need for a "ridge reamer" to get the pistons out.
Just a guess, but possibly your cylinder has a slight ridge, under which is enough wear to prevent good sealing, or the ridge itself touches the top ring and pushes on it a little, causing a loss of seal.
But that's just a guess. It may have nothing to do with wear, and everything to do with how the rings sit in the ring-lands.
In aircraft, we use a differential compression tool that does much the same thing as you are doing. We often have to move the prop a bit back and forth, to get the rings to seal up.
Start at about the 20 to 1:30 second mark to see him move the prop to get a good seal.
Just a guess, but possibly your cylinder has a slight ridge, under which is enough wear to prevent good sealing, or the ridge itself touches the top ring and pushes on it a little, causing a loss of seal.
But that's just a guess. It may have nothing to do with wear, and everything to do with how the rings sit in the ring-lands.
In aircraft, we use a differential compression tool that does much the same thing as you are doing. We often have to move the prop a bit back and forth, to get the rings to seal up.
Start at about the 20 to 1:30 second mark to see him move the prop to get a good seal.
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Originally Posted by The_Nuke
The test is for air leaking down past rings and such. Performing the test with cylinder at TDC ensures all valves are closed, so any air escaping the cylinder is doing it by getting past rings...or an improperly seated valve.
How are you getting the compressed air into the cylinder? What are you using?
But don't modern engines have overlap where both valve are open briefly? Hence why at TDC the gauge reads 50% leakdown but if I turn the engine another 10 or 20 degrees it may read 10-20%. I'm using a leakdown tester.
The test is for air leaking down past rings and such. Performing the test with cylinder at TDC ensures all valves are closed, so any air escaping the cylinder is doing it by getting past rings...or an improperly seated valve.
How are you getting the compressed air into the cylinder? What are you using?
But don't modern engines have overlap where both valve are open briefly? Hence why at TDC the gauge reads 50% leakdown but if I turn the engine another 10 or 20 degrees it may read 10-20%. I'm using a leakdown tester.
Originally Posted by atikovi
Originally Posted by The_Nuke
The test is for air leaking down past rings and such. Performing the test with cylinder at TDC ensures all valves are closed, so any air escaping the cylinder is doing it by getting past rings...or an improperly seated valve.
How are you getting the compressed air into the cylinder? What are you using?
But don't modern engines have overlap where both valve are open briefly? Hence why at TDC the gauge reads 50% leakdown but if I turn the engine another 10 or 20 degrees it may read 10-20%. I'm using a leakdown tester.
There is no valve overlap on TDC on when the spark plug fires. There may be some valve overlap at TDC at the beginning of the intake stroke.
Originally Posted by The_Nuke
The test is for air leaking down past rings and such. Performing the test with cylinder at TDC ensures all valves are closed, so any air escaping the cylinder is doing it by getting past rings...or an improperly seated valve.
How are you getting the compressed air into the cylinder? What are you using?
But don't modern engines have overlap where both valve are open briefly? Hence why at TDC the gauge reads 50% leakdown but if I turn the engine another 10 or 20 degrees it may read 10-20%. I'm using a leakdown tester.
There is no valve overlap on TDC on when the spark plug fires. There may be some valve overlap at TDC at the beginning of the intake stroke.
OK, this is what I get with a now cold engine. Will redo once more later.
Cylinder # Compression psi Leakdown %
1 115 55
2 115 70
3 67 80
4 165 74
5 125 55
Cracked cylinder liners are common on these and it's using coolant so I suspect that. These almost never have head gasket problems and the temp gauge never goes over the half way mark.
Cylinder # Compression psi Leakdown %
1 115 55
2 115 70
3 67 80
4 165 74
5 125 55
Cracked cylinder liners are common on these and it's using coolant so I suspect that. These almost never have head gasket problems and the temp gauge never goes over the half way mark.
Leak-down tests can be tricky. One problem is ring style. Dykes, Keystone or L shaped rings, or pressure backed rings will not seal with just air pressure. You have have the cylinder pressurized AND you have to move the ring pack slightly to get them to snap out into running position.
Is that why the 55% drops to 15 or 20% when I turn the crank a little off tdc?
You use the timing mark on the crank pulley to determine the correct position that the piston is at the highest point with the valves closed. You can verify this by looking at the cam marks, doing it with the dowel you will see the cam marks do not line up and the crank is one to one and a half a tooth off.
Some Volvo engines use a pin in the side of the block to stop the crank at the correct spot.
Edit: IIRC I read there is something about the VVT causing this issue you are having also. Do a little research.
Some Volvo engines use a pin in the side of the block to stop the crank at the correct spot.
Edit: IIRC I read there is something about the VVT causing this issue you are having also. Do a little research.
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Could you try it warm with a bit of light oil or kero in there?
Port injection engines compress with wet liners effecting a better dynamic seal.
Make sure to have the brake on and the thing chained or blocked well.
I've had them move when the air pushes on the piston with a lot of force.
Port injection engines compress with wet liners effecting a better dynamic seal.
Make sure to have the brake on and the thing chained or blocked well.
I've had them move when the air pushes on the piston with a lot of force.
I thought I was the only one that ever heard of those. That's an old school airplane engine tester. You want to have that prop locked nice and tight with someone holding on tight . Same with using it on a car or boat you need to lock up that crank with a nice long breaker bar and not let it get loose on you or it will toss that bar across the garage like a bullet. I always try to have a helper holding the crank, less dangerous to nearby property
With a light plane you pressure it to 80 and read the other number from the second gauge. They express the values as x/80 and you never get 80/80 . The chief benefit from one of those is finding what's leaking whereby listening at the oil cap = rings , carb = intake valve, exhaust pipe = exhaust valve. Not so easy to use on a car due to the long pipes but a mechanics stethoscope helps a lot. Old school still comes in handy and still works.
With a light plane you pressure it to 80 and read the other number from the second gauge. They express the values as x/80 and you never get 80/80 . The chief benefit from one of those is finding what's leaking whereby listening at the oil cap = rings , carb = intake valve, exhaust pipe = exhaust valve. Not so easy to use on a car due to the long pipes but a mechanics stethoscope helps a lot. Old school still comes in handy and still works.
FWIW, I don't know how critical this is, but I was taught to do leakdown at TDC on the compression stroke. If you are at TDC on the exhaust stroke, there's no guarantee that the exhaust valve has closed completely, while all valves should be completely closed on both the compression and power stroke.
That is correct. one of the easiest way to find the compression stroke on some of these deep well plug engines is jam a piece of 3/8 fuel line in the plug hole with a deflated balloon on top of the hose. As the cylinder builds compression the balloon with inflate slightly.
After you get it on compression stroke then air cylinder. Once you have it sealed walk around to rear of vehicle feel for air put tailpipe then go to front remove radiator cap, air filter tube then oil cap. With leakage you should be able to narrow it down pretty quickly.
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