Motor heads: Leak down question.

What is the difference that you have seen on a cold vs. warm leak down test?

Discuss.

Thanks!

I've never done both. It seems when you get around to doing it the car has been in the shop for days.

15% cold after sitting for several days and then warmed up to hot
Let us know your results
Try rocking the crankshaft a tiny bit each way with pressure on it both cold an hot No more than 2 to 4 degree from TDC, be careful while moving IT!

I bar the engine around while doing a leak down.

Pull all the plugs, plug the tester into a hole which will likely push the piston down the bore if the valves are closed. Quickly rotate the engine around with a long bar until the valves close and pressure starts to increase. Continue to try to rotate the engine against the pressure until you can't go any further. Give it a few seconds to normalize and take a reading.

Consistent from hole to hole. Doesn't require any disassembly beyond pulling the plugs. Been doing it like this for years.

That is the way A&P (aircraft) mechanics do their leak down tests using the props, usually they would say that cylinder is fine before I got it up tp 90 BTDC Depending on the A&P some said that anything better than 30% was good on air cooled opposed motors, the bigger round air cool motors where a different matter

I always do my leak tests cold. I am sure a warm engine will leak better than a cold one, but the engine is cooling off the whole time you are doing the test, so will the first cylinder really leak as good as the last one, even if all the cylinders are good? I leak my engines on the engine stand after they are built, just to make sure I did not screw up. I write down the numbers, then leak it after a few passes and I am sure the rings are broken in. I usually see the leak tests improve about 10% Then I have a baseline to monitor the engine to see when its time to freshen the engine. Just like cranking compression tests can be altered by the charge of the battery, if you have the throttle open or closed, or the number of pulses when you record the compression. When testing remove as many variables as possible.

What are some good (%) numbers?

I don't have much faith in leak down tests based on what I've seen, its just another number really but does give you some insight.

I would not try to hold or move an engine with 100 psi in the cylinder, it is pretty difficult in my experience. In fact my car with a 4 speed and 11 inch drums, adjusted very well, will eventually push through the E brake with 100 PSI in any cylinder and the trans in gear.

If you get the cylinder you're testing at TDC it will stay there when its pressurized. I really don't see how a leak down test anywhere other than TDC is giving you useful info.

The only time I have ever gotten "hero" 2-4% leak down results were on engines that were in the 120-130 PSI range when perfect would be 160. They had worn rings, and were at the bottom of the compression test spec, but they were small engines with relatively tight ring gaps. If you gap your rings at .024 I don't think ever going to see results like that. I'm starting to think that the results are easily influenced by ring gap and bore size.

My personal opinion is that anything below 15% is perfectly acceptable, 10% is good. If you've got 20% or more its high, and it will show up in a compression test, but you will still make power and the plugs will look OK. There is other info out there that will certainly disagree. A fresh engine with decent machine work really should be around 10% or less though.

your experience agrees with mine. For many years the OEM said up to 40% was OK, a few years ago they changed that to 20% as being the acceptable limit. I seen a guy almost lose a finger by trying to hold an engine with a breaker bar when the air was hooked up, could not hold it, and got between a stainless steel fan blade. Zero gap rings will fool you as well when doing a leak test. They always test good unless you got bad damage. Engine started filling vapor tank with an air pump every run, leak tested good with zero gap rings, but in pulling engine apart, rings were just worn out.

I wish I could find that youtube video of Big Daddy leaking down his engine in the pits. In probably less than 2 minutes, while giving an interview with a TV reporter. It was amazing to watch considering how long it takes us normal folks to do it.

I like to see no more than 15% on race motors when warm, after that is time to figure what is not sealing up properly
I've seen cold motor leak down really well due to oil leaking down the exhaust valves into the cylinders when the engine builder hadn't install seals on them
Started up those motors and they leak worst warmed up with the cylinder walls dry with very little oil on them

I always do leak down and compression cold. Its not the proper way but if there is a broken ring, or leaking valve it will most likely show cold or hot. When you do it hot you start at one temp and finish at another, unless you are doing it with the engine on a block heater your temp is going to change in the process.

My last two engines have leaked down in the 5% range cold.