Questionable leak down test results

[timmay5]

Hello all. I wanted to get your opinion on my leak down test results. I am having an issue with a small misfire. I have gone through the fuel and electrical systems. Wanted to follow through with a compression and leak down test.

Compression test came out good showing 150 psi average across all the cylinders with no more than 5 psi difference between the highest and lowest readings. I went ahead and did the leak down test and the results seem too good to be true.

I got 0% on all cylinders. So naturally, I am a little skeptical of this. I am using a harbor freight leak down tester, which makes me question the accuracy of the tool. My procedure was I warmed up the engine and starting at cylinder number 1 I turned the engine over to top dead center. Removed the spark plug, inserted the hose adapter into the spark plug hole, attached the airline to the gauge and attach the gauge to the plug adapter. I aired all cylinders up to 80 psi. I listened for any air escaping through either the exhaust or the intake which I could not hear on any cylinder. As I went up in pressure and approached 80 PSI, I could hear some air escaping through the crank case via the oil dipstick/filler tube on all cylinders. After testing several cylinders and getting zero leakage I became a little curious. On the number 3 cylinder after performing the test at top dead center, I turned the crank pulley about halfway between cylinder 3 and 5. I turned the air up to 80 and at that time could hear a significant amount of air escaping through the exhaust and the leakage gauge was reading 15 psi.

With this, I assume the gauges are working correctly. Even if they were off by a margin of five psi or so all of the cylinders would be in very good condition and are consistent. I would be ecstatic if those were the correct numbers, but I want make sure I’m not missing something.

Edit: the car is a ‘74 911 base

[timmay5]