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I have a 18v with a Judson Supercharger with about 4 lb boost. All cyliners show 150 pressure except #4 which shows 90 lb. I put oil in and it went up to 115 lb. I put it on a leak down detector and I could hear air comming out of the valve cover. When the engine was running, I also saw smoke comming out of the cover. The cyliner #4 was on compression stroke and I used a screwdriver to see that it was close to TDC. This is when I could hear the leaking from the valve cover. I cranked the engine and the leaking STOPPED. The position of the piston was now near the bottom of the stroke. This is where I am confused. At TDC both valves should be closed and I should have 0 % on the leak down and I now have 80% with air comming out of the valve cover. With the piston down I have 0% leak with no air leaking. Any thoughts as to why pressure leak at TDC which would be normal and no leaking with the piston down at near bottom of stroke

with the piston at the bottom there is no pressure!
Neil McGurk

Hi Jeff -

Based on what you say, it sounds as if your head is going to need to come off for a check.. It may be a combination of a damaged compression ring and a partially burnt valve.. though I dont understand the lack of a leak at bottom dead center you mention.

Regardless, you know you need to pull the head and pan and take a look.

Let us know what you find. Good luck

J. W. Delk

Air hissing out the valve cover means leaky piston rings blowing compression through the crankcase. The only cure is new piston rings, if the engine is not otherwise too badly worn you can do that without removing the engine from the car. Remove cylinder head and oil pan, and ridge ream the cylinder before removing the piston (to avoid damaging the ring grooves). Measure the cylinder bore carefully with a dial bore gauge.

If it is within spec's for bore size, roundness and taper, then you can hone the bore and install new piston rings. You will have to diligently wash down the bore and the crankshaft with solvent after the honing job. Do not rotate the crankshaft after honing until it has been thoroughly cleaned.

If the cylinders are badly worn it will need a rebore and a new set of pistons, also new cam bearings (which will be ruined during hot tank cleaning).

When you apply pressure to the cylinder with piston at bottom of stroke one of the valves will be open, so no pressure. With intake valve open it will blow through the air cleaner. With exhaust valve open it will blow out the tail pipe.
Barney Gaylord

This is a fresh engine with about 8000 miles on it. The boost is low at 4 lbs. The head was completley redone by Sean Brown ported and polished. I would expect to hear air leaking from either the carbs or the exhaust but most of the noise is thru the valve cover(rocker cover) filler hole where the oil goes. The engine is also redone and rebored. Is there a way to determine a broken ring. I might have two problems. I am going to do another leak down test. I might have read 100 as not leaking vs leaking a hell of a lot. The extra air might be going thru the exhaust which I did not check. Before I pull the head or the engine, I want to know what I have to do so it will come out right and so it will go



Dear Jeff. Please be so kind as to read what Barney has written. I am, also, a "guru" and concur with what he has written.

When the pistons are at the top of their travel, and the rocker arms are loose (you can feel movement when you try to move them) the valves are fully seated. Thus, any air that you might hear is caused by either the valves not sealing or by air leaking past the pistons.

My own method of determining where the air leak might be coming from is to remove the rocker arm cover (only two nuts--not difficult, nor Rocket Science), then performing the test again. Use a good stethoscope--I have two left over from my father, you can pick them up, cheaply, at "estate sales". When set to the "funnel" portion of the stethoscope, they are quite useful for determining where a noise is coming from.

You should be able to determine where, and from which cylinder, any noise is coming from.

I am sure that you are performing such tests in with absolute quality control. But, as an experiment, when the piston is near the top of its travel, and you think that both of the valves are fully seated, loosen the rocker arms so that you can move both of them up and down a noticeable amount, then perform the test again.

Once spent several days assisting a former professional mechanic with an engine rebuild. Engine would not fire. Tried to convince him to use my system of checking crankshaft position by rotating the crankshaft while watching the valves move. "Not something which professionals need to do" We spent an entire week end chasing this problem. Without success. Following weekend, as he was discussing the newspaper advertisement to sell his truck, he agreed to follow my system of watching the intake valve open, then close, to determine the compression stroke.

Then, he found that his timing was 180 degrees out and, when properly adjusted, the engine fired on the first revolution.

I have written a tech article on engine ignition timing which is available through my website, and is also linked through Barney's website.

Let us do some more investigation here. Some more reasoned investigation. Then, perhaps we can see what is taking place.

If one has a good stethoscope to assist one.

Les Bengtson

Stuck valve ?
Art Pearse

This thread was discussed between 24/04/2009 and 25/04/2009

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