I've tried multiple cylinders, and the best leakdown rate I can get is a hair under 50%
This engine runs better than that, so I'm doing something incorrectly.
History if you want to read about some of what I've been concerned with https://grassrootsmotorsports.com/forum/grm/but-where-has-all-the-oil-gone/189000/page1/
Gauge or hose leaking? Do you have a good seal onto the top of the head? Undamaged O-ring on the end that screws into the spark plug socket?
Assuming it's none of those, then my guess is you're getting little bits of carbon stuck between the valve and the seat that are hanging the valve open a tiny bit. This doesn't happen when running because there's lots of gas blowing stuff around, but when you're just turning the motor over by hand there's nothing to stop it. Try releasing the pressure, then turning the engine forward a bit and back, and check it again.
Are you doing it with the engine warm (not hot) or cold?
In reply to codrus (Forum Supporter) :
Brand new kit, didn't hear any hissing from the hose at the fitting, sprayed some soapy water on the bottom. Think that's good.
Engine is ~3 years old, and while there's been oil down there I haven't seen any chunks of carbon in the intake tract. Top end was cleaned out completely before I put it together, new EGR valve and tube, exhaust manifolds sandblasted in and out to remove any carbon.
So, maybe, but I'd lean towards something else if there's another option?
I'm thinking that something is off with the valve lash, but again it runs better than I'd expect for that?
In reply to L5wolvesf :
Warmish.
Mr_Asa said:I'm thinking that something is off with the valve lash, but again it runs better than I'd expect for that?
I think if you were hanging the valve open enough to leak that much compression then it would be obvious.
I did this once on a Miata motor that didn't have the timing belt installed. With the piston at TDC I could open and close the valve a tiny bit with a wrench on the cam, and it would reliably clear the "not sealing properly" behaviour if I did it with a bit of pressure in the cylinder. I don't know if it's carbon or not, but that's the only thing I can think of that fits the behaviour I saw. That motor was not particularly heavily carboned-up, either.
I've never actually managed to get useful information from a leakdown test. Once tested an engine that had eaten enough aluminum oxide to waller out the wrist pin bearings and completely take out the rings, and it showed 100% because there was so much oil in the cylinder everything sealed up.
Actually, that's not correct. I once was doing a leakdown test on one of my old Cadillacs and heard a noise from the radiator. "What could that be," I questioned. So I opened the radiator cap. Yeah, bad head gasket. You can probably imagine what happened.
Thats a lot of air going somewhere. It can really only leave in a few places: exhaust, intake, valve cover (crankcase), or the radiator. Can you tell where the leaving is leaving the engine?
I had similar issues and ended up replacing the regulator on the gauges. Couldn't get decent readings until I did that. Couldn't hear it leaking anywhere.
In reply to wake74 :
When I'm getting those readings, the most sound I get is from the crankcase. That would likely be rings, yeah?
I've been thinking I need to Borax the engine anyways.
What does a compression test show? My FoMoCo 2.3T leaks like a sieve and I hear the air escaping through the crankcase too, although I have had leaks on the exhaust side of the head occasionally. It too seems to run too good to really be that bad. It doesn't have a PCV valve, just crankcase vent to a catch can and there is a decent amount of vapor and such that comes out of the filter on top of the can when running, so there surely is blowby. It is a turbo motor with fairly wide ring end gaps, like 0.024 top, 0.026 middle so I don't expect it to be sealed up like my old drag race engines which almost never leaked more than 5%. I check it pretty often and I have found that it leaks less when I have the piston at the bottom of the hole (timing belt off, valves closed) than it does at the top so that indicates that there is some bore wear and/or taper going on. My compression tests are more consistent and as long as they are within 10% hole to hole I don't worry too much about it.
Compression test is good. 150PSI on the #1 when the motor was hot, then I had to go inside and it cooled off completely and I got 130 on the rest.
It is possible that I damaged the rings or cylinders when I rebuilt the engine and tried to start it. Got a lot of fuel washing down the cylinders because I had the cam timing off.
Is it a 4 valve head? If so I think the springs don't have enough pressure to completely seal the valve in the seat. I've had better luck with 2 valve heads doing leak down tests.
Maybe you aren't at TDC. A different approach I just observed was to hook up the tester and crank the engine by wrench through the cycle. The leak number should decrease as you get to TDC. Thatw ay there is no issue getting exactly to tdc.
porschenut said:Maybe you aren't at TDC. A different approach I just observed was to hook up the tester and crank the engine by wrench through the cycle. The leak number should decrease as you get to TDC. Thatw ay there is no issue getting exactly to tdc.
My experience with leakdown at TDC has always been that the engine will let you know you aren't at TDC when you start pressurizing the cylinder as the piston will be pushed down and turn the crank. Even a little bit off TDC is enough if you are running the leak with pressure over 80 psi or so. I normally run it to 100 psi on the apply side so that the leak gauge directly reads percentage leak. The high pressure also lets you clearly hear where the leak is coming from.
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