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Will leakdown test show false positive on non interference engine?
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Thanks for looking into my thread.
Non interference engine is designed with enough clearance between pistons and valves to allow the crankshaft to rotate (pistons still moving) while the camshaft stays in one position (Several valves fully open). If this condition happen normally, no internal engine damage will result. It will not bent valves , when timing belt failure occur.
If you perform a leak down test on Non interference engine that had timing belt failure and show air coming out of tail pipe.
It could. It's unlikely to give a false positive though, unless it's other valvetrain damage - like broken springs.
The cams don't have enough holding strength to keep the springs compressed without the extra rotational mass of the crank. The cams will rotate to a rest position and snap all the valves closed if it can. When the timing belt or chain breaks, that's what happens.
You could put a scope into the cylinder. Otherwise, you've got to pull the head.
Loosen rockers on exhaust valves, turn cam gear/pulley. You might get some air by the valve if the valves are gunked up enough, but that would cause other issues. Knowing what engine it is will help.
I don't see how a 'leak down" test yields a "false positive".
If you're pressurizing the cylinder at TDC with the cam rotated to a position where the valves should be closed ...
and enough air volume is coming through the exhaust that it's audible ... then air is coming past the exhaust valve.
You could back up the test with a visual inspection of the exhaust valve, but my bet is that you'll be needing a cylinder head removal to investigate and resolve the problem.
I don't think a cam belt breakage event in a non-interference engine will have any effect on the valves, that's what "non-interference" means. As Sun notes, though, regardless of how it got that way, if you have the engine in the correct position for the cylinder you are testing, and you can hear the hiss of air coming out the tail pipe - yeah, that exhaust valve is leaking. Unless the engine is pretty badly sludged up, in which case some sort of flush product *might* possibly free things up such that the exhaust valve closes fully (and that is pretty doubtful, I have never seen it happen in real life, but I guess it's theoretically possible) - you will need to pull the head to find out what's wrong, and to fix the problem. You could probably see this as whatever mechanism works that exhaust valve won't come all the way up to the low spot on the cam, compared to other valves that are working correctly, by pulling just the cam cover off.
If your car has an aluminum head and iron block, and plenty of miles, you will have to replace the head gasket at some point anyway, at least in my experience. So while this is a fairly expensive repair in a shop, or going to eat up a couple of weekends if you DIY, you were headed for a head gasket R&R anyway, with the head off you can have a machine shop install the valve guide seals which will reduce oil consumption at least some, and you can deal with what very likely is a burnt valve.
Put it back together with a new water pump and cam belt, and you should be good for quite a few miles.
If the designers/ manufacturers state that their engine is 'non-interference' it means that no matter at what point the belt breaks, there will be no contact between the valves and the pistons.
When my engine belt broke [12,000 miles before recommended change] I was told that it was an 'interference' engine and 'we have to remove the head to check things' After being told the cost, I decided to do it myself. I removed the plugs, turned the cam and crank and didn't feel any resistance. I installed a belt and water pump for $80 and the engine runs great. Either my engine wasn't 'interference' or I was very lucky.
If the designers/ manufacturers state that their engine is 'non-interference' it means that no matter at what point the belt breaks, there will be no contact between the valves and the pistons.
When my engine belt broke [12,000 miles before recommended change] I was told that it was an 'interference' engine and 'we have to remove the head to check things' After being told the cost, I decided to do it myself. I removed the plugs, turned the cam and crank and didn't feel any resistance. I installed a belt and water pump for $80 and the engine runs great. Either my engine wasn't 'interference' or I was very lucky.
As a shop owner I can tell you there are a significant number of engines that are listed as interference which, in actual fact are not and routinely can be repaired with full compression and cylinder balance simply by installing a timing belt package.
Shouldn't the combustion-chamber be sealed anyways on a good engine......interference or
non-interference?
of course, an internal combustion chamber engine must be sealed for the expanding gases to push down on the piston.
but it doesn't have to be 100% perfectly sealed for the engine to run properly. many engines will run within specs with modest leaks revealed by the leak down test.
however, the OP's leak-down test has revealed a leaking exhaust valve to the extent that he can hear the airflow in the exhaust system. That's a pretty big leak.
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