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2 cylinders @ 35% leak down

So I have an '81 3.0 SC engine that has driven fine the past several years. Cylinders 4/5/6 are fine with 167-171 compression (warm test) and less than 10% leak down (cold test @ 100psi). Cylinder 3 is at 190 compression and 3% leak down. However cylinders 1 and 2 are at 155 compression and 30% and 40% leak down respectively. I don't hear any air escaping through the intake or the crank case. It sounds like its coming out the bottom of the engine but I don't think (not really sure) it's the exhaust.

Valves are adjusted and head studs are snug. Also cylinders 3-6 are good on spark plugs but 1 and 2 are dry fouled. Same Weber settings for all 6.

Is there really anything else I can try to figure out the cause? I'm assuming it's exhaust valves or blowing out between the cylinder heads and case.

And finally - if it is one of those 2 things or similar how much do you think I should expect to pay somebody to fix? A range...of course not knowing exactly what the problem is.
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Old 10-10-2018, 08:03 PM
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If you are sure you don't have any broken head studs, that leaves the exhaust valves as the problem. It is common for the exhaust valves to contact the pistons during an over rev bending them.

However. It is also very common for some carbon debris to get caught on the valve face from removing the plug.

Why did you do a leak down?

I would remove the lower valve cover and while the cylinder is pressurized, tap the adjuster with a mallet a few times and see if the leak down numbers improve.
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Old 10-10-2018, 08:27 PM
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Blast it around for a bit and redo those two cylinders. I never take the first high leakdowns too seriously.
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Old 10-10-2018, 08:53 PM
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Head studs are definitely solid - I double checked them all. The rocker arm had play when at TDC. I loosened and tapped - no change. I think air is coming both on top and bottom of engine. See picture in spots marked X - I notice black marks on the 2 cylinders that the other 4 cylinders are clean in these spots. Also in spots with the arrow under the metal braces - there is also sludge and when I place the stethoscope I can hear air movement. But I also hear it when I put it on the exhaust. I'm really not sure where the air is going.

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Old 10-11-2018, 04:03 PM
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Local mechanic stopped by and confirmed - the air is coming around the heads. Will have to figure out what I want to do next!
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Old 10-11-2018, 06:49 PM
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Does car run fine? Does it idle nice? Does it have power? I would have another mechanic double check. Spray some kB blaster on the valve adjustment areas.
Old 10-11-2018, 07:58 PM
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Sounds like a/some stud(s) has let go? If you are leaking top and bottom of a cylinder. You will find them if you look in the stud holes with covers off. Magnetic pick up if necessary.
Alan
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Old 10-11-2018, 08:12 PM
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Do John's advice and don't rip into it until you know for sure that's the actual leakdown numbers.
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Old 10-12-2018, 06:56 AM
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In addition to doing as John suggested, I've used a nice trick to localize compression/exhaust leaks. While the cylinder in question is pressurized, take a length of fuel hose with one end close to your ear (not too close though), and probe around where you think the leak is. When you get close to it, you will hear it very clearly through the hose.

Scott
Old 10-12-2018, 07:42 AM
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+1 ^^^
I find it hard to believe that much air, if any, could be leaking from between the cylinder and head if the head studs are tight. But anything is possible.

You would surely hear it when the motor was running?
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Last edited by Trackrash; 10-12-2018 at 10:11 AM..
Old 10-12-2018, 10:06 AM
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you could spray the suspected area with soapy water.

leaking between cylinder and block should not happen as there is no pressure there. even if air leaks past the rings it can escape easily.

check head bolts as said
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Old 10-12-2018, 11:44 AM
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