Need help interpreting leak down sound

[jwakil]

I did a leak down test on my engine, and while three of the cylinders were solid (2-5% leakage), I got three others with 15-25% leakage. The bad cylinders were 2, 4, & 5, so one on the left side of engine and two on the right side. For all three of these bad cylinders I heard kind of a pulsing, gurgling sound coming from the upper, LEFT side of the engine. Hard to make out but close to the top valve covers on the left side. No significant sound from anywhere else. What could this mean?

[Discseven]

JW... typically one listens to the exhaust pipe (for leaking exhaust valves), intake (for leaking intake valves), and case breather (for leaking rings.) A pulse and gurgle... that sounds organic!

[jwakil]

I don't think pulse/gurgle is significant, the point is the sound is coming from one place, which is near the top left of the engine, near top valve covers but below the intake manifold. I wasn't sure exactly where in intake valve leak would be heard.

[Joe Bob]

Have you checked the head studs?

[jwakil]

Head studs were recently replaced as part of a top end rebuild. What part of the intake would one listen to for leaking intake valves? I haven't adjusted the valves since the rebuild and thinking I might have valve leakage but don't know where the air would escape if the throttle is closed?

[BK911]

Intake valve can be heard through the intake, try near the air cleaner.
Exhaust valve through the exhaust, put your ear at the muffler tail pipe.
Rings through the oil fill.
Head studs usually through the alt fan.

[turbo owner]

Originally Posted by jwakil

Head studs were recently replaced as part of a top end rebuild. What part of the intake would one listen to for leaking intake valves? I haven't adjusted the valves since the rebuild and thinking I might have valve leakage but don't know where the air would escape if the throttle is closed?

Who did the work? You or a shop? If it was done by a shop let them figure it out, if it was you, your out of luck.

If this was just done, did you rering it and hone the cylinders or replace them?

You should re-torque the head studs and adjust the valves too, then check the leak down again.

[jwakil]

Originally Posted by turbo owner

Who did the work? You or a shop? If it was done by a shop let them figure it out, if it was you, your out of luck.

If this was just done, did you rering it and hone the cylinders or replace them?

You should re-torque the head studs and adjust the valves too, then check the leak down again.

I had a reputable shop do it and will be taking it back to them but wanted to get as much info as possible and possibly avoid a trip down there.

[jwakil]

Originally Posted by ClickClickBoom

How many miles/hours driven in the weeks preceding the leakdown. If not done hot, I mean smoking hot the results are a huge crap shoot. I've done thousands of compression tests over the years on aircraft engines and the results can be amazingly variable, outside of valve(s) leakage, a low cylinder or 2 will come up to snuff after the manufacturers recommended 10 hour rerun. Think of leakdown testing as more a trend monitoring tool than one shot diagnostic cureall, unless it is valve related, then its a 2 or 3 time test to eliminate valve seat contamination. If you did the leakdown cold, well, try again hot.

Every set of instructions I've read says leak down test is supposed to be on a COLD engine.

[jwakil]

I could have sworn Wayne's 101 book said leak down should be done on a cold engine but maybe that was for the valve adjustment. I'll look again. The logic is that the clearance specs are set for cold, not hot.

[turbo owner]

You have 3 good and 3 no so good, you have a problem. Hot or cold you should have similar readings across the board.

[turbo owner]

Originally Posted by ClickClickBoom

Nope,
It really is random, from ring position to crud on the valve seat. His problem is not following standardized procedures. If anyone looks hard enough they can find someone suggesting a leak down be done with a garden hose and Yoohoo on the interwebz. My experience comes from 25+ years of aircraft A&P experience, yours?

45 years of having fun in Motorsport

[juanbenae]

since 911's don't have wings id consider that the tried & true method of leakdown with a cold motor should be the choice. I realize a hot motor should make for higher numbers due to components expanding, but those may be skewed if previous efforts were done cold.

im not an aircraft mechanic, or a Porsche mechanic, but do spend a bunch of time a local shops as I have no life.

[afterburn 549]

There is a big difference between hot and cold.
I like it hot.
Next- drive it around the block and try it again.
Make sure you are at TDC or close as possible
An AC engine only ask about 1/2 HP per a cubic inch. ( Way derated )
Plus -
They are loose and wore out day one by auto standards.

[dad911]

Originally Posted by jwakil

I could have sworn Wayne's 101 book said leak down should be done on a cold engine but maybe that was for the valve adjustment. I'll look again. The logic is that the clearance specs are set for cold, not hot.

Originally Posted by jwakil

Every set of instructions I've read says leak down test is supposed to be on a COLD engine.

Originally Posted by turbo owner

45 years of having fun in Motorsport

Originally Posted by car 311

since 911's don't have wings id consider that the tried & true method of leakdown with a cold motor should be the choice. I realize a hot motor should make for higher numbers due to components expanding, but those may be skewed if previous efforts were done cold.

im not an aircraft mechanic, or a Porsche mechanic, but do spend a bunch of time a local shops as I have no life.

I don't know where you guys are getting it should be done on a cold engine. Directions I have seen say 'warm engine' and as to Wayne, he says:
However, if the leak-down test is performed on an engine that isn’t warmed up, then the test may not give accurate results.
Tech Article by Wayne: Pelican Technical Article: Time to Rebuild? - Part II

[Ronnie's.930]

Originally Posted by car 311

since 911's don't have wings id consider that the tried & true method of leakdown with a cold motor should be the choice.

Like Mr. Boom noted, a leak down test on a cold engine is an inaccurate way to measure cylinder pressure loss regardless of the vehicle the engine operates in. You can get a wildassed guess measurement that way, but if something doesn't seem right, then the next thing you do is repeat the test warm, so might as well do it correctly in the first place.

[juanbenae]

I may have only made it partially clear that however you set your baseline numbers, cold, warm or smoking hot that commits you to that method moving forward. if you base a healthy motor on a warm reading it should be stated as to how warm. most shops do it cold so they do not have to burn knuckles or explain where exactly the number are derived. a cold motor may not tell you what you want to hear, but it's a good, solid basis to set an even playing field, without numbers skewed by a warm, hot, or somewhere in between motor when tested.

[tharbert]

Originally Posted by car 311

... numbers skewed by a warm, hot, or somewhere in between motor when tested.

You have this backwards. If you want skewed numbers, start with a cold engine. Then, progressively warm the engine to get less skewed numbers. The least skewed and most predictive will be those test results when the engine is at operating temp.

[turbo owner]

Originally Posted by ClickClickBoom

I take it you aren't a mechanic......

And you are a legend in your own mind, I want to see you do a HOT engine leak down on a 930 engine instead of your B&S lawnmower engine or a Lycoming Cessna engine with flip up cowls.

[jwakil]

My bad, after going back to my references I realize the cold engine requirement is for valve adjustment, not the leak down test. I will go back and redo the leak down on a warm engine and see what happens.