brad-cam

I pretty know everything I need to know about leakdown tests under normal circumstances - ie. on a warmed up, broken in motor. Question: I have a newly rebuilt motor on the stand, and I figured I'd do a leakdown to see what it shows. Well, I was surprised. Leakdown figures varied from 25-45% depending on the cylinder. It seemed like the leaks were on all of the exhaust ports, intake ports and crankcase - arghh! Then I thought, the engine is cold, and it is not broken in. The rings are not seated, the valves are not seated, etc. I assume that after a proper running break-in, these things will seat and seal better ?

So question is: are the leakdown numbers basically meaningless on a fresh motor that hasn't been run, with new rings and valves ?

special tool

What means did you use to ensure that the valves were closed?

944kid

I'd say so. Take it for a nice drive. See how it performs. Then test it. Besides, seat of the pants tests are often a good start. Things need to get used to their new surroundings, in the engine I mean.

My guess is wait for the 500 mile first oil change. I'm doing these kinds of tests in BOCES right now, so I hope I'm acurate.

Happy Motoring,

GlenL

The valves should be sealing well if they were ground and lapped correctly.

I wouldn't worry about the rings for 1000 miles.

brad-cam

The valves were not reground or lapped but the head was off a good running engine with good compression. I did take the valves out and cleaned everything up and put the valves back in the same cylinders they came out of. I'm going to take the cam box off to close up all the valves tight and try it again. This is easy when the engine is out of the car on a stand :-)

inactiveuser92616

if you reallllly wanted to make sure the valves are closed you could pull off the cam carrier and then do a leakdown....

special tool

Yeah - I guess he ignored my question.
A 100 psi leakdown test will roll a warm 951 engine.

progenitor04

did u make sure the number 1 cylinder is at top dead center?

brad-cam

Guys, I pulled the cam box off last night (as I stated I would do in my last message) to make sure all valves are closed. All tests were with the pistons at TDC for the cylinder under test. The leakdown readings varied between 40% and 55%. Since this is a fresh rebuild with new rings, smooth cylinder walls (no scoring) and the valves are not in the exact same orientation as original (they are in the same cylinder they came out of, but no guarantee they haven't been turned relative to the seat), I'm going to assume the engine needs to be broken in before I can trust any leakdown figures. This makes sense to me. There is little to no information out there on this as I don't think anyone does leakdown tests on pre-broken-in engines it appears...

And no, the air pressure was not turning the engine off TDC during the test...

jonnybgood

On the 356 and VW engines the rocker arms contacted the valve stems off center to apply a rotational force to the valves. This prevented the valves from wearing in the same place on the guides. I don't know if this is common practice in all engines but it seems like it would be a good idea. I would say that valve orientation on the seat would not be the same at all times and should not affect your tests.

What about the trick of spraying oil around the rings to give a little extra seal? On a warm engine there would be some oil around to help seal as opposed to a dry ring and cylinder wall during a rebuild.

GlenL

Brad,

What pressure were you using? Are you saying that the engine didn't turn and didn't need to be held?

If it was my engine I'd pull the head and have it checked. A leak-down test doesn't identify where the air is going with any precision. If some pistons hiss at the intake or exhaust more than others, then there is likely a problem. Might be the rings and end up fine.

Question for the forum:

I know that some engines are designed to rotate the valves. This promotes even wear. Is it true for the 944?

brad-cam

I was using 100 psi from the compressor, but the pressure gauge on the leakdown tool only registered 35 psi when I zeroed the leakdown gauge with the regulator control on the tool (it is one of those 2-dial leakdown tools). The engine didn't turn over and didn't need to be held.

I think maybe I need to have my head checked...in more ways than one :-)

Zero10

My 951 showed leakdown of ~20% until it broke in, and all came down to less than 1%.
That said, leaking valves won't get any better. If they are allowing air by, the seats need to be ground, then the end of the valve shimmed to set the height correctly. A good machine shop should be able to do this for a reasonable price.