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Old 04-13-2017, 07:19 AM
anix anix is offline
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Default Cylinder leak down test - need input

Newbie to the forum. I have been trying to run down a smoking issue on an engine that came in an El camino that I bought a few weeks ago. I bought the car and knew it had a broke push rod so I did not get to hear or see how it ran. The owner claimed it had low miles on a rebuild but you know how that goes. BTW its a 402 with .060 396 pistons in it. they are TRW L2287F (read the numbers with a scope) Anyway I am down to trying a leak down test. I ran across something that seems strange to me. All cylinders are running around 4-8%. The strange part is that I observed that on several cylinders as I was cranking down on the pressure regulator gauge number two would trail gauge number one by 20-25# and I could hear more blow by untill it hit say around 50#s and then I would hear a slight bump or sharp tap and then gauge number two would come to within 5-6# of Gauge one and stay there till I stopped at 100#s. My guess is the piston moved down enough to take any slack that might be in the ring lands. I have never done a leak down test before and have nothing to compare it too. Would love to hear from some of you that have more experience with them. Is this normal or do you think the lands are worn out. Thanks
P.S. Great forum guys I love it.
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Old 04-13-2017, 07:24 AM
Ol' D Ol' D is offline
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Hi Al and welcome from So Cal. Should be able to find answers here.
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Old 04-13-2017, 07:34 AM
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Thanks. Sometimes I find my own answers by hashing out with others. Just trying to build a game plane and not over look something obvious.
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Old 04-13-2017, 08:06 AM
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..welcome to the site, I've done that ,but the engine is on the stand. Ifound one cyclnder was low (110 PSI )and what I did was hook up an air line to that cyclnder and put about 35 LBS of air pressure. then, I got some WD40 and sprayed into the ports ( Exhaust and intake).on the exhaust valve it was blowing the WD-40 right back out,so, I knew right there,was a bad valve.so, I pulled the head off and took it to the shop and had it fix, so, now it's back up to 150 PSI of compression.I would think, if it's in the truck, leave the radaidor cap off so you can see air bubbles if it's a blown head gasket.if you hear air coming up through the intake manifold ( or carbuator.) you should hear it and the same with the exhaust listen through the tail pipe..
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Old 04-13-2017, 08:23 AM
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welcome Al

Quote:
My guess is the piston moved down enough to take any slack that might be in the ring lands.
or maybe the ring was off the land and it moved down and sealed on the land??

are you doing the leak down test with the piston up on TDC or down on BDC??
how did a compression test go??

4-8% leak down at 100psi is good..

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Old 04-13-2017, 08:44 AM
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The ring or the piston moving is my first thoughts. I am using TDC. It was 16-18 until I move the crank so the piston drop a tiny bit then the needle went to 4-8%. Hey that might be normal to see that. just not sure since I have done a leak down before. And no bubbles in the radiator or here nothing coming by the valves so I think they are ok and the compression test showed pretty much showed 195-210 across the board. Oil smoke coming from somewhere though.
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Old 04-13-2017, 04:00 PM
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Quote:
Oil smoke coming from somewhere though.
maybe valve seals??
under the intake leak?? re-tork the intake, see if any bolts are loose..
does the PCV hav a baffle under the valve cover??

the rings can be tight in the groves/lands on a new build,
just hope its not the ring gap is too small..

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Old 04-13-2017, 04:00 PM
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Does it do the same if you release the compression, rotate the engine several revolutions and re test?
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Old 04-13-2017, 05:09 PM
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JJLT1 I have had the intake off a couple of times and it has not made any difference plus I checked for internal vac leaks and showed nothing. Valves have a right new looking set of positive lock stem seals. PCV VALVE nor the snap in breather has a baffle under them. Just noticed that today. It smokes some when you first start it up like bad seals but if you get in it hard going through the gears it leaves a pretty good cloud behind you. Do you think it would suck that much oil past the PVC Valve.

Paul I have not tried spinning it over several timing for another try. What would that show?
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Old 04-13-2017, 05:27 PM
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Paul I did notice that moving the piston one way or the other could effect the outcome. I still wonder if it is the ring lands. I can't trust anything I was told about the engine. It was one of those 1000 mile rebuilds and the first thing you find is a set of roller rockers that have not been manufactured since 1975. If those L2287F060 piston are TRWs and not speed pro they may be 40yrs old.
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Old 04-13-2017, 06:35 PM
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I was thinking it had something to do with the ring lands.....clicking and all. Smoking indicates there's something amiss.....the initial low compression on the leak down test says something. What kind of readings do you get with a regular compression gauge? Hot, cold, with some oil in the cylinders?
Regarding rebuilt engines.....when buying a vehicle with what is supposed to be rebuilt, I like 10,000 miles on that rebuild. 1000 miles or so means the seller doesn't have much faith in his work. Personally, I'd rather have a junk yard engine than something some cheapskate put together in order to sell the car.
Never a fan of leak down test, airplane mechanics love it so......

Last edited by paulj; 04-13-2017 at 06:40 PM.
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Old 04-13-2017, 08:33 PM
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I use a differential compression tester on airplanes, same principle, and listen for valve leaks at the exhaust, the intake (carb) and at the oil filler cap for piston ring blow by.
When I move the prop around TDC, sometimes the rings will move in the piston, and the number will come up higher, which seems mostly normal on these air cooled engines, with any amount of hours on them.....
Bad valve seals seem to be a contributor to exhaust smoke, mostly on start up, in my experience, on auto engines....
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Old 04-13-2017, 10:33 PM
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Are you doing this test with the engine at operating temp? If not that's a problem. Your supposed to test each cylinder with the piston up, in the middle and bottom to get the full answer how good each cylinder really is. If you test at the top only you may get a good reading and miss the problem. Only take out one spark plug at a time not all of them or the crank can move and give you a bad reading.

Just did mine a couple of weeks ago.

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Old 04-14-2017, 06:44 AM
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Quote:
PCV VALVE nor the snap in breather has a baffle under them.
yes that will make a big difference on some motors, depending on some valve covers
and PCV location..
.
.
Quote:
Paul I did notice that moving the piston one way or the other could effect the outcome.
the ring only really seals under compression, on the up stroke,
when the ring is on the land and pressure on top of and behind the ring..
if you move the piston down the the ring looses seal on the land untill the ring
snaps down on the land again and seals from the pressure..

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Old 04-14-2017, 08:45 AM
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Yeah that makes sense. I made a baffle plate to go under the PCV valve. I will stick it back together and see if that helps. Thanks for the feed back
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