T-Smac

The only time both exhaust and intake valves are closed is the compression stroke and if the piston isn't tdc, the shop air will quite possibly turn the crank. I guess If the source of air is weak then it doesn't matter. That would not be as accurate though

starchland

Technically nothing manufactured is 100% hence being oblong and all Tdc refers to is
Making sure the valve is closed completely ie no pressure from the cam.

LS What?!

Ok we are saying the same thing. All I'm saying is, if you had the piston at the bottom, with the rockers loose so as not to open valves, it would work to. Tdc also could be TOP DEAD CENTER exhaust stroke, which in case the valves would be open and test would fail. Even at tdc.

So... Agree to disagree

turbo_slug

At 80% leakage on two cylinders,forget about re-doing the test at warmed up temperature because the engine will never start w/ that much air leaking away.

Are you sure your mechanic did the test correctly? Did he make sure all the valves were closed? Actually regardless of whether the mechanic did the test properly, the seller is full of crap because doing the test on a hot motor will only yield slightly better results.




BTW, if anyone is curious what kind of leakage result to expect out of an s2k motor, here's my results from past tests i've done:

AP1 motor w/ 80,000miles (engine was warm or close to cold) - 1-1.5% leakage across all cylinders
Ap2 motor cold w/ 28,000miles - 1% leakage across all cylinders


After seeing the awesome results from my ap1 motor, i personally would not buy any s2k motor w/ more than 2% leakage.

chris_barry

Cold leakdown tests on a good motor will still be under 5%. The rings bridge the gap between the piston and bores and it wont matter if the engine is cold. The only time I've seen a dramatic difference on cold motors are when the valve clearances are approaching zero. When the engine heats up the head expands faster than the valve stems and you actually get clearance back and the leakdown improves.

Leakdown tests should be done at TDC of the firing stroke of the cylinder.

When doing the leakdown test you can tell if the leakage is through the inlet/exhaust or rings by listening to where the air is leaking out. If an engine has been sitting for a long time its not unusual for some of the carbon stamped onto the valve margin/seats to start flaking. This can cause modest leakdown around the valves ( ~10%). I find some wd40 and compressed air blow out of the valve area can improve this. 80% is pretty bad and could be a sign of bent valves. Another possible cause of leakage is head gasket failure. I've had high leakdown values and couldn't work out where the air was going till I listed to the adjacent cylinder spark plug and found air streaming out of it. In this instance, the hot engine will actually hide the head gasket leakage as expansion of the block/head pulls the head down harder on the gasket.

starchland

was a compression test done?