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]

[11BC2]

He got a couple of choices here:
1. Be blissfully excited that your Engine is in such good health.
2. Find a calibration lab in your area, and have those gauges calibrated.
3. Buy some high-end pre-calibrated pressure gauges and replace the ones that you have. Redo the test.

Are you setting each piston at top dead center or the way I prefer to do it, bottom, dead center?

By placing the piston at bottom, dead center, you can fill the cylinder and put your ear to the intake, the exhaust port, and the oil tank, to hear, which one is allowing more air to bypass without fighting the engine, turning over on you

[timmay5]

All were done at TDC. Yeah, I was hesitant to buy the harbor Freight gauges. But I hate to spend a ton of money on a tool I’m likely only going to ever use once. I think my plan is I’m going to redo the test with the engine stone cold. With that, I would expect some amount of leakage. And assuming that the readings are equal across all cylinders at that point, I would feel confident enough to say that the first test was accurate.

[PeteKz]

How do you do a leak down test at BDC? The intake or exhaust valve should already be opening at BDC. No?

[11BC2]

Originally Posted by PeteKz

How do you do a leak down test at BDC? The intake or exhaust valve should already be opening at BDC. No?

Full disclosure:
I have never performed a leak down test on a Porsche engine.

I have performed leaked down tests on a multitude of other engines, and the issue that has me can/will encounter is that at top dead center, when you supply 80 to 100 psi of air into the combustion chamber, the engine will want to rotate.
Placing the piston at bottom, dead center prevents the engine from wanting to rotate

[PeteKz]

Yes, I know the engine wants to rotate down from TDC, which is why it takes several tries (for me at least) to get it at exactly TDC, so it doesn't rotate.

I will wait to see if anyone else reports whether they do it at BDC, for the aforementioned reasons.

[11BC2]

Originally Posted by PeteKz

Yes, I know the engine wants to rotate down from TDC, which is why it takes several tries (for me at least) to get it at exactly TDC, so it doesn't rotate.

I will wait to see if anyone else reports whether they do it at BDC, for the aforementioned reasons.

That’s a good idea.
But of all the different types of engines I’ve done this test on at bottom, dead center. I’ve never had to not struggle on the engines at top dead center.
These are cooled, wonder mints are certainly magical, so I might be missing something. Once I get my engine reassembled, I will do some testing to confirm/deny my previous experiences.

[PeteKz]

Here are the cam specs for the SC cam (also used in the Carrera engines). I used the 1982 SC to formulate the question for ChatGPT.

U.S./Canada (engine code 930/16):

Intake lift at TDC overlap (with 0.10 mm valve lash): 1.4–1.7 mm
Timing @ 1 mm valve lift (crank degrees): Intake 7° BTDC open / 47° ABDC close; Exhaust 49° BBDC open / 8° BTDC close
Cam IDs (stamped on ends): LH 930.147.08 / RH 930.148.08.
LSA: 113°. Duration: Intake 236°, exhaust 224°.

This indicates to me that the exhaust valve opens before BDC after the power stroke, and the intake closes after BDC on the compression stroke. Thus, neither of the BDC points will hold pressure for a leak down test.