Low Leak down

[Catorce]

Originally Posted by Emo993

Catorce.....I did the leak down because I'm a 70 year old sports car buff still trying to learn, your comment is well appreciated. Every thing I've read about 911 engines is having leak down done. So, also knowing it should be done warm/hot was not possible. The part new to me (thanks to your experience) is the difference regarding a rebuilt engine verses previous running engines. I've owned this 914 for 40 years, lots of auto-cross's and track time (Road America, Blackhawk Farms) but its been apart for 25 year. Two year ago, final body work and paint. Suspension upgrades done, gear box with short gears and new synchros etc. Now just need a running motor. Best, Mark

Good, because while your numbers will be totally useless, you STILL learned how to do a leakdown. Now, when you run the motor, you will be able to do an accurate one with a warm engine, then you can compare your numbers.

The exercise itself is not wasted time, even though the numbers won't be accurate.

[Neil Harvey]

There is nothing wrong with doing a leak down test on an engine when cold. We do it all the time. We do have a coalition between hot and cold though.

Every time time we rebuild a cylinder head we always vacuum test each valve seal. No different in doing a leak down to test the valve sealing. When testing the valve sealing, cold or hot it wants to be the same. If this engine sat for along time, its quite possible that the valve margins and seats have got some dirt on them. Most of the valves would have remained open while the engine sat.

Although the rings have only their tension and 100 PSI pushing them against the cylinder walls, the leakage past them is normally low. No where near 50%. Typically around 2% - 3%.

Rings can stick in their groove from sitting especially if assembled with engine oil. This is not so sticky and can drain down to the bottom of the piston.

What are your choices. Run it and hope it all sorts itself out or pull it apart and check. Best to eliminate the valve seating. This can be done with the engine assembled and cold.

To check the valve seating, someone already has suggested tapping on the valves. Make sure both rockers on each cylinder have lash, (piston some where close to TDC) and with a plastic drift
and the leak down tester still connected, give the rocker adjuster a smart smack with a dead blow hammer. Don't go nuts here. What you should hear is a loud pop as the valve is lifted of its seat. If the leak down number go down, you are essentially removing any dirt from the seat. If no change, you have a seat issue that probably will not get any better. Remember here, any leakage past the rings will remain constant. What you are trying to do is to seal or see if the valves are seating correctly.

This is all based on your initial findings of some leakage past the valves.

[boosted79]

"The comparison you are depicting is also meaningless. I don't assemble motors with oval cylinders so..."

We're not talking about an engine you assembled, the history on it is unknown, thus the LD test.

It is entirely possible to obtain cold LD of 10% or less on a fresh engine. 40% is not good.

He has an indication that there MAY be a problem.

Run it and then check.

[Emo993]

Thanks Neil again for the explanation. Did the leak down again...TDC, both valves with lash. Popped valves a few times, also put some oil into cylinder... Did two cylinders, both around 10% leakage slowly going up to 15 or so. From every thing I see, motor has never been run. Thanks for the help. Mark