leak down test

How do I do a leak down test on my 08 yzf450?

You need a specific tool, a leak down tester, actually, and a source of high pressure air.

The tester consists of a metal tube with an orifice in the center, and a pressure gauge on each side of that. One side connects to a hose screwed into the spark plug hole, and the other to the air source. The engine must be held at TDC on compression so that the valves are closed. Regulated air pressure is fed to the outer gauge (100 psi is often used because it makes the math simple) and the inner gauge is read as a percentage of the outer. For example if you use 100 psi as the test pressure, and the inner gauge shows 95 psi, you have a 5% loss. Not bad, usually.

Leak down tests always don't mean anything all by themselves. Obviously, if you have a major pressure leak, and a source can be found for it, that's one thing. But the 5% in the above example could be 2% or 10% with another tester, because there's no standard way to build the tester. So, either the tech doing the test has to be able to compare the reading he gets on your bike to other engines about the same bore size, or you have to have used the same tester on your bike before to see if it's better or worse than it was.

One thing about it is that with the leak down test, you can pinpoint the source of any leak very certainly. All rings will leak because they all have gaps. If there is a significant loss, you will hear air leaking into the exhaust if the ex valve leaks, or into the carb if the intakes leak, or just into the crankcase by listening at the timing plug.

I had an idea for one, tried to implement it the other day but it didn't work out.

Basically I have an air compressor with gauges that give you a readout of the available tank pressure. It was my crazed idea that I would use a compression tester hose with adapter for the spark plug, and fit it to the end of my air compressor adapter. The compression tester had the same fitting for the adapter.

I would hook it all up and thread the spark plug adapter into the head of the bike, find TDC and start up the pump bringing it to 100psi. I could then watch the gauge and time it for a pressure drop and calculate the loss etc.

Everything went to shit when I realized that the spark plug adapter was much too big, and second the fitting for the air compressor was also too big... lol.

Could this have worked?

Not very well, no.

lol, yes I suppose..

I suppose there's no point in doing it if you can't get accurate results.

You need a specific tool, a leak down tester, actually, and a source of high pressure air.

The tester consists of a metal tube with an orifice in the center, and a pressure gauge on each side of that. One side connects to a hose screwed into the spark plug hole, and the other to the air source. The engine must be held at TDC on compression so that the valves are closed. Regulated air pressure is fed to the outer gauge (100 psi is often used because it makes the math simple) and the inner gauge is read as a percentage of the outer. For example if you use 100 psi as the test pressure, and the inner gauge shows 95 psi, you have a 5% loss. Not bad, usually.

Leak down tests always don't mean anything all by themselves. Obviously, if you have a major pressure leak, and a source can be found for it, that's one thing. But the 5% in the above example could be 2% or 10% with another tester, because there's no standard way to build the tester. So, either the tech doing the test has to be able to compare the reading he gets on your bike to other engines about the same bore size, or you have to have used the same tester on your bike before to see if it's better or worse than it was.

One thing about it is that with the leak down test, you can pinpoint the source of any leak very certainly. All rings will leak because they all have gaps. If there is a significant loss, you will hear air leaking into the exhaust if the ex valve leaks, or into the carb if the intakes leak, or just into the crankcase by listening at the timing plug.

and my personal favorite, if it's a head gasket, you will get bubbles in the coolant.

I suppose there's no point in doing it if you can't get accurate results.

After a rebuild and break in, you can do a baseline test to assess what "the standard" is, then compare to that as things age.

and my personal favorite, if it's a head gasket, you will get bubbles in the coolant.
:prof: True story. A customer brought a Chevy Diesel truck into the shop with symptoms that sounded like they could point to a head gasket. I opened the radiator cap and had him start it. When it immediately shot a 4 foot column of water into the air, I told him that I thought I knew what the problem was :smirk::smirk:
:prof: True story. A customer brought a Chevy Diesel truck into the shop with symptoms that sounded like they could point to a head gasket. I opened the radiator cap and had him start it. When it immediately shot a 4 foot column of water into the air, I told him that I thought I knew what the problem was :smirk::smirk:

too funny.

So I've tried to do leak down tests in the past and one of the problems is that pumping 100 psi into an engine at TDC creates quite a lot of force. What's the best strategy for keeping the engine at TDC?

That can be difficult. On mine, I have used an 18" ratchet with a socket on the crank, with a nylon web strap coming down from the handle bar, and the engine rocked very slightly forward of TDC so I know which way it will want to turn. I hold the socket end of the wrench square so it doesn't want to slip off, and I have a helper apply the air pressure gracefully. :smirk:

One thing about this is that you are testing the seal at only one spot in the sweep of the rings, and if there's a bore problem, it could be elsewhere. Sometimes it can be helpful to use less pressure and roll the engine up into the compression stroke from the bottom where the intake valves close to get a more complete picture of the condition of the bore.

But you bring up a good point relating to safety, too. There's quite a bit of power in compressed air. Watch out for that wrench.

I use a breaker bar and "sneak" up on TDC with everything hooked up and pressurized.....but alot of times the force will loosen the flywheel nut....so I go backwards.

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