Leak down test questions

[sizod]

My 73 sIII has been burning a little oil lately. I noticed it when the the truck is warm and idling or when pulling away I get a puff of blue then it clear until the next idle. I decided to conduct a leak down test, and the results are somewhat inconclusive. I started at cylinder 1, rotated the engine until the piston was TDC attached the airline which was set to 100psi pressure dropped to 20psi. The same happen to cylinders 2 and 4, but 3 was solid no pressure drop. With cylinders 1,2 and 4 I could hear an feel the air escaping through the exhaust. So it should indicate exhaust valve issue, right? However the pressure dropped seemed extreme for my blue smoke symptoms. So I retested again got the same results, however..... The cylinders were definitely at TDC but if when the pressure test was engage I rotated the engine ever so slightly the pressure would go back up and I could feel it against the wrench. I then disconnected the air rotated the engine slightly and reattached, gauge would the go up to 100psi then fluctuate and return to original 20psi reading that I was getting.

So I wondering its been 15 years since I last did a LD Test on an engine. Too much pressure? are the tappets are off slightly so that they aren't closed at TDC?

Am I'm on the right lines before I rip off and rebuild the head. thanks.

[antichrist]

Should be TDC on compression stroke.
There are a number of videos on youtube on doing a leak down test.

[d90dan]

So I wondering its been 15 years since I last did a LD Test on an engine. Too much pressure? are the tappets are off slightly so that they aren't closed at TDC?

Am I'm on the right lines before I rip off and rebuild the head. thanks.


You can pull the rocker shaft and test again. If it still leaks, you've got a start on tearing it down.

[o2batsea]

These readings are wet or dry? If I am not mistaken, part of doing a leak down test is to inject oil into the spark plug hole and test it when the cylinder is wet with oil. This helps seal up the ring gap on a cold engine. This you compare to the dry reading to tell you whether the leakage is valves or rings.

[sizod]

Should be TDC on compression stroke.
There are a number of videos on youtube on doing a leak down test.


I used both a vacuum/pressure gauge and the wooden stick method for finding TDC.

[sizod]

These readings are wet or dry? If I am not mistaken, part of doing a leak down test is to inject oil into the spark plug hole and test it when the cylinder is wet with oil. This helps seal up the ring gap on a cold engine. This you compare to the dry reading to tell you whether the leakage is valves or rings.

Dry, the wet/dry method you are talking about is for a compression test.

[sizod]

You can pull the rocker shaft and test again. If it still leaks, you've got a start on tearing it down.

good point. I'll try that.