Cylinder Leak Down Test

cv300g

cv300g

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On a recently rebuilt engine it started to blow smoke out of the oil breather cap. I did a Compression Test and No. 2 cylinder is 30 PSI and No. 5 is 40 PSI. All the other cylinders are from 120 to 135 PSI. I want to do a Cylinder Leakdown Test. I understand that you go from one cylinder to the next being sure that the cylinder you are testing is at " Top Dead Center of the Compression Stroke. Isn't the main purpose of TDC on the Compression Stroke to assure that both valves in that cylinder are closed ?

Why can't I just remove the rocker arms ( the one valve cover is already off ) ? That way all the valves should be closed and you just go to each cylinder . Will this work and if not why not ? I was going to get a reading on all 8 clyinders.
 
Check the torque on all the head bolts? Just a thought, might need a re-torque after the rebuild.
 
cv300g said:
On a recently rebuilt engine it started to blow smoke out of the oil breather cap. I did a Compression Test and No. 2 cylinder is 30 PSI and No. 5 is 40 PSI. All the other cylinders are from 120 to 135 PSI. I want to do a Cylinder Leakdown Test. I understand that you go from one cylinder to the next being sure that the cylinder you are testing is at " Top Dead Center of the Compression Stroke. Isn't the main purpose of TDC on the Compression Stroke to assure that both valves in that cylinder are closed ?

Why can't I just remove the rocker arms ( the one valve cover is already off ) ? That way all the valves should be closed and you just go to each cylinder . Will this work and if not why not ? I was going to get a reading on all 8 clyinders.
Click to expand...

That won't work because you will not get any intake air to gain compression. If you are blowing smoke out of the breather cap, you most likely have broken rings or a bad piston. A valve issue would leave one or more cylinders with low compression, but that will not normally cause vapors to enter the crankcase. You possibly could get some crankcase vapor IF you had a broken exhaust push rod as when the cylinder fires, the exhaust has to go some place. In that instance some exhaust gas might be vented past the rings into the crank case. You would also note a big drop in vacuum pressure if that is what is happening and possibly farting back thru the carb. Remove both valve covers and be sure all rockers are going up and down. If you compression readings are accurate, you have low compression on opposite sides of the engine, so you problem is not likely to be a bad head gasket.

Are you running a high lift cam? If so, you may have an interference problem that has holed a piston, very likely if it is a solid lifter cam. How long did the engine run before you noticed a problem?

Dave
 
All the cyl leakdown test will do, other than generating "numbers" is tell you where the air is escaping (crankcase, tail pipe, carb). I concur that as the "smoke" is coming out the breather, then it's coming from the crankcase as blow-by past the piston rings or from a hole in a piston(s).

How long has this condition existed? Immediately after the rebuilt engine was installed and fired? Started later? How long has the engine been in the car and running? How much was the block bored in the rebuild process?

In doing a compression test or leak-down test, what your testing is the quality of the seal between the compression (top) ring and the cylinder wall, mainly, plus the compression ring end gap, placement of the ring end gaps in relation to each other, and how well the piston ring fits into the appropriate groove in the piston. ONE possibility is that when the piston/ring/rod assembly was installed into the cylinder bore (hopefully a fresh bore), IF the rings were not compressed tightly enough, the rings would not be compressed enough to slide into the bore as they should. This can result in cracked ring lands on the piston, which means a new piston will be needed, plus the rings could have been broken/twisted in the process, too, meaning they will need to be replaced, too.

END result is that the engine will need to be removed and the issues found and addressed. Knowing which cylinders have the low compression will tell you where to look. If you want to do a leakdown test, that's up to you, but it's probably NOT going to yield anything different than what you already know. Best to use that time to get the engine removed and fixed.

Please let us know what you find when you get the engine apart.

CBODY67
 
Wow that sucks.
Maybe you should pull the other valve cover off, then make sure the intake valves are opening fully.
Then if they are opening fully, get the suspect cylinder on the firing stroke, and put some air to the hole.
If the valves were done really badly, and you have a couple bad guides, pressure could be coming back through the valve covers.
If you're "lucky"? With that much compression loss you will hear air going somewhere through a guide.
I believe the reason the test is done at TDC, is because when the piston takes the force of compression, it's most prone to rocking and wear.
A case could be made that putting high PSI to a cylinder and let it push the piston to the bottom, could point to an out of round bore, or cracked cylinder wall,
If you have a good leakdown tester that you can put 50 psi or more , use that. A Harbor Freight 15 psi LDT is next to useless.
If you don't have a good LDT, just take your compression tester and screw portion, and quick connect it to your compressor hose.
It would be frustrating to pull the whole engine when it is a head issue.
Hopefully it isn't catastrophic.
 
cv300g said:
On a recently rebuilt engine it started to blow smoke out of the oil breather cap. I did a Compression Test and No. 2 cylinder is 30 PSI and No. 5 is 40 PSI. All the other cylinders are from 120 to 135 PSI. I want to do a Cylinder Leakdown Test. I understand that you go from one cylinder to the next being sure that the cylinder you are testing is at " Top Dead Center of the Compression Stroke. Isn't the main purpose of TDC on the Compression Stroke to assure that both valves in that cylinder are closed ?

Why can't I just remove the rocker arms ( the one valve cover is already off ) ? That way all the valves should be closed and you just go to each cylinder . Will this work and if not why not ? I was going to get a reading on all 8 clyinders.
Click to expand...
You probably won't get any new information from the test, as pointed out by others. To answer your question, you can do the test with the rockers off, but you should have the pistons at TDC when doing so. If you get the piston at the perfect top of it's stroke, it will stay when you apply air... but if the rod has kicked even a little one way or the other, the engine will turn.

TDC is where the pressure occurs... there wouldn't be much at BDC... so any readings there are generally considered useless... in your case, if all you want is comfort of knowing its blowing past the pistons... fine... but it always blows past the pistons during this test... you can use the percentage gauge to tell how much and you can verify the valves are sealing and the cooling system isn't being pressurized, regardless of piston position, with the rockers off.
 
Thanks to everyone that replied.
In the fall of 2014 I had a blown head gasket and the heads were rebuilt , by a shop, that winter. When the pan gasket was off I found a piece of piston skirt in the pan and some copperish sludge(bearings). At that time I just put the heads on and ran the car in 2015, maybe 1,000 miles, knowing that in the winter of 2015 I would remove it for a complete rebuild.
I had it rebuilt over the winter and fired it up in May of 2016. Bored 30 over, new pistons, mild cam, bearings and rings,lifters, new oil pump. I installed the heads that were done the year earlier. The car ran fine in 2016 and maybe did another 1,000 miles. In 2017 it ran fine and later in the year it started blowing smoke out the breather cap. A new PCV and oil breather cap was installed and it still blew smoke out the oil breather cap.
 
I did pit a borescope camera in each cylinder and all the pistons look good. From what I could see the cylinder walls seemed normal. All spark plugs look normal and inentical. I am waiting for warmer weather to do more research / diagnosis. The leakdown tester that I will be using is the Snap-On MT 324.
 
Broken rings, probably forced on install. I had a 305 crate engine in a Chevy truck that got worse and worse it had 3 broken rings in it.
 
That's my Red 1962 300 Sport Convertible with the 413 with the problem.

I am restoring my Black 1961 300G. Soon ready to install the engine and transmission but that will be on hold until I get the other problem corrected.
100_0951.JPG
100_1029.JPG
 
70bigblockdodge said:
Broken rings, probably forced on install. I had a 305 crate engine in a Chevy truck that got worse and worse it had 3 broken rings in it.
Click to expand...
Wouldn't broken rings show up as oil on the plugs ? All 8 plugs look normal and identical and not oil fouled.
 
cv300g said:
Wouldn't broken rings show up as oil on the plugs ? All 8 plugs look normal and identical and not oil fouled.
Click to expand...

Broken oil rings would cause the car to smoke out the tailpipes. Broken compression rings will blow combustion waste out of the oil breather.

Dave
 
Only the top ring is a compression ring. The 2nd ring is by design an oil scraper. So you can break the top rings and not the 2nd. If the top ring is broken (my impression is not from force but something else) it may not use oil because the 2nd ring is still working. The end result is the engine needs to come out and apart. Simple as that.
 
moper said:
Only the top ring is a compression ring. The 2nd ring is by design an oil scraper. So you can break the top rings and not the 2nd. If the top ring is broken (my impression is not from force but something else) it may not use oil because the 2nd ring is still working. The end result is the engine needs to come out and apart. Simple as that.
Click to expand...

The other thing that could have happened is that the top rings might have been installed upside down by the re-builder. The compression rings have a beveled cut on the upper side. This causes the ring to press tightly against the cylinder wall on the firing cycle. If the beveled cut is facing down, the high compression of the firing cycle will compress the upper ring and blow past it.

Dave
 
I have seen the oil ring and compression ring reversed. i.e. put the oil ring in the compression ring groove.
 
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