Hi,
If there is low compression in one of the cylinders (80 psi), does this basically say there is something wrong with the engine and a leak down test just tells you what?
Are there situations where after doing a leak down test, it comes out okay and nothing needs to be addressed immediately/inexpensive fix/no internal engine issues...or it will definitely be something major such as rings needing to be replaced, head gasket, valves, sleeves, etc?
There have been no misfires or codes.
Thank you.
Leak Down Test
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SuperHerman
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Leak down test can provide solid clues as to what is wrong. One forces air into the closed cylinder/valve. Generally if you can hear the air leaking it is a sign that you have a problem with the valves (either from the intake side for its valve(s) or the exhaust valve(s) for its side). My experience is listening is critical for valve leaks.
If one hears bubbling then it is usually really bad piston rings (never heard this and don't know how it is possible, but I read it somewhere - what I don't understand is the oil level shouldn't be at a point that one would get bubbles - but what do I know). The important information re the piston rings is found in how fast the compression pressure leaks out of the piston rings. Usually a leak down tester gauge will have color codes red - yellow and green with percentage markings.
If only one cylinder is showing 80 and the rest are fine - then you could have one gummed up piston ring - which you could try putting some Sea Foam or equivalent (GM makes an excellent product) to try to free it up in just that spark plug well. Some use a global Sea Foam treatment and run it with the oil.
On the valve side it could be a burnt valve or a mucked up valve seat.
One other possibility is having a "furry" camshaft which has crud build up that actually makes the lobes bigger. I had signs of this on an engine top end I rebuilt. I had no compression issues I knew of, but the build up on #5 was noticeable.
Which cylinder is causing issues - usually the problems are 4 or 5.
If one hears bubbling then it is usually really bad piston rings (never heard this and don't know how it is possible, but I read it somewhere - what I don't understand is the oil level shouldn't be at a point that one would get bubbles - but what do I know). The important information re the piston rings is found in how fast the compression pressure leaks out of the piston rings. Usually a leak down tester gauge will have color codes red - yellow and green with percentage markings.
If only one cylinder is showing 80 and the rest are fine - then you could have one gummed up piston ring - which you could try putting some Sea Foam or equivalent (GM makes an excellent product) to try to free it up in just that spark plug well. Some use a global Sea Foam treatment and run it with the oil.
On the valve side it could be a burnt valve or a mucked up valve seat.
One other possibility is having a "furry" camshaft which has crud build up that actually makes the lobes bigger. I had signs of this on an engine top end I rebuilt. I had no compression issues I knew of, but the build up on #5 was noticeable.
Which cylinder is causing issues - usually the problems are 4 or 5.
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SuperHerman
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#4 and #5 are usually the ones that develop problems with the valves - so #4 is consistent.
Do a full leak down test and focus on #4. If you don't have a leak down tester, but have an endoscope (video camera you can stick into the spark plug well). You can get USB PC based units on line for $10. Just find one that is small enough to fit into the sparkplug hole, has lights, and has a mirror attachment. With the endoscope you can look up at the valve and if you play around with the crank you can position it so the intake and then the exhaust valve is completely open. This will give you a good look at the valve seat and valve itself. Once the valve starts to have problems it gets hot and hotter and hotter until you end up with a burnt valve which could drop and take out the piston and destroy the head. One can also take off the intake and exhaust manifold and get a view from above on how the valve is sitting. Before I bought a leak down tester I would use my compression tester fitting (the one that is a tube with a cone on the end) and air compressor. This works well for bent, burnt or bad valve seats - but not really for piston ring conditions. If you have skills you could fashion a tool by drilling out a sparkplug so that air flows through it and affix a tube and connector for your air source.
The repair in the end will be to pull the head and have a valve job performed. Possibly change out the offending valve and valve guides. Always replace the valve stem seals and if you don't farm it out do a lap job. No way to know what is needed until the head is pulled. Once the head is off you can inspect the cylinder wall condition and measure to specs to evaluate your piston rings. I have changed one piston ring set that was suspect on a 2.9. Just drop the pan (good time to clean out things and replace the oil pan o-rings), a light hone and replace the piston rings for good measure. From below you can push the piston out the top.
With a fresh top end the engine should be good for another 200k.
Do a full leak down test and focus on #4. If you don't have a leak down tester, but have an endoscope (video camera you can stick into the spark plug well). You can get USB PC based units on line for $10. Just find one that is small enough to fit into the sparkplug hole, has lights, and has a mirror attachment. With the endoscope you can look up at the valve and if you play around with the crank you can position it so the intake and then the exhaust valve is completely open. This will give you a good look at the valve seat and valve itself. Once the valve starts to have problems it gets hot and hotter and hotter until you end up with a burnt valve which could drop and take out the piston and destroy the head. One can also take off the intake and exhaust manifold and get a view from above on how the valve is sitting. Before I bought a leak down tester I would use my compression tester fitting (the one that is a tube with a cone on the end) and air compressor. This works well for bent, burnt or bad valve seats - but not really for piston ring conditions. If you have skills you could fashion a tool by drilling out a sparkplug so that air flows through it and affix a tube and connector for your air source.
The repair in the end will be to pull the head and have a valve job performed. Possibly change out the offending valve and valve guides. Always replace the valve stem seals and if you don't farm it out do a lap job. No way to know what is needed until the head is pulled. Once the head is off you can inspect the cylinder wall condition and measure to specs to evaluate your piston rings. I have changed one piston ring set that was suspect on a 2.9. Just drop the pan (good time to clean out things and replace the oil pan o-rings), a light hone and replace the piston rings for good measure. From below you can push the piston out the top.
With a fresh top end the engine should be good for another 200k.
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vtl
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How many miles? At 200+k miles you may have the head pulled off, cleaned, valves relapped, new valve steam seals, new piston rings, new connecting rod bearings.
SeaFoam does nothing. The deposit are so hard that it is nearly impossible to scratch them clean with a tool - and you damage the grooves. Boiling pistons in SimpleGreen solution and washing with 2000 PSI pressure washer also takes a long time. Just no way any mild chemical solution would release the rings.
Not to tell that rings after 200k miles have thermal gap of 1/32" or more, which is way too much for a good compression.
The valve faces are also pitted, that's also contributes to a bad compression, plus increases a risk of burning a valve dramatically.
SeaFoam does nothing. The deposit are so hard that it is nearly impossible to scratch them clean with a tool - and you damage the grooves. Boiling pistons in SimpleGreen solution and washing with 2000 PSI pressure washer also takes a long time. Just no way any mild chemical solution would release the rings.
Not to tell that rings after 200k miles have thermal gap of 1/32" or more, which is way too much for a good compression.
The valve faces are also pitted, that's also contributes to a bad compression, plus increases a risk of burning a valve dramatically.
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