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Old 26th November 2009, 10:47   #1
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Default Effect of low compression in 1 cylinder on piston etc.,

I wanted to know what can be the effects of minor leak from the valves(head) which leads to minor loss of compression in one cylinder.
What if such an engine has run for 25000kms with one cylinder having slight leakage.
If there is any damage, can it be easily detected by inspecting the cylinder bore etc.,

From what I understand, if there is a major leak, its causes engine overheating etc., but minor leaks can go undetected and can only be diagnosed via a compression test.
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Old 26th November 2009, 16:00   #2
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If there is slight leakage, then no major issues, but its only when one has a major leak, we can expect damage.

Also a lot matters where is the leak from. If pistons are gone or rings have gone, then more issues can be expected from a major leak.
The leak that you have was only with the cold engine so damage will not be very high.

Some mechanics have a simple fundamental understanding :

1) There is a leak or less compression or compression loss.
2) You are not feeling loss of power.
3) You are not facing overheating issue.
4) Then dont worry.


I have read about compression ratio varying upto 3-4 units in case of Indica engine, but no severe damage was reported.
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Old 26th November 2009, 16:24   #3
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Tanveer

This should help you

CR4 - Comment: Re: 1978 Ford F150 - Low Compression in One Cylinder Only



Some excerpts from the thread

I noticed the engine surge mildly from time to time which lead me to go through the vacuum set-up and check all the hoses. I used a long hose-to-the-ear and determined the need to replace more vacuum lines.

Here is a possible answer... A small defect in the one exhaust valve or seat can ultimately lead to leakage of hot gases which gets worse with time. Just after TDC the burning gases in the center of the combustion chamber are at maximum temperature (2000 deg. C or 3600 F) and under reasonably high pressure - they will flow quickly and will progressively "blow torch" a good size hole in the exhaust valve if the leak rate is high enough. The valve material goes soft above 1200 C or 2100 F. Only material in good contact with the valve seat and cooling jacket is spared. Usually, you will see a 1/4" hole (or bigger) on the valve. The hot valve can also cause pre-ignition problems too on that one cylinder. But you still have 7 good cylinders for adequate torque to burn rubber.
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The root cause of the leak may be due to material failure (cracks in valve/ seat, flaking of stellite facing or tiny impurities which dislodge). I have seen many material-related failures with exhaust valves on a production 5.6 liter I6 turbocharged engine used for medium-duty vehicles in the 1990's.
Alternatively, you could be near end of the valve life on all valves in the engine due to normal wear and this is the first cylinder to go bad.


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Last edited by Rehaan : 27th November 2009 at 01:06. Reason: Please see the note that has been put at the end of your post by the Support Team.
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Old 26th November 2009, 19:06   #4
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Easiest approach is the compression test, typically when compression drops over 20psi(could vary from car to car) in comparison to the other cylinders it would defn make sense to do a valve leak check.
However if the car has been run for that long just re-seating the valves can still help, it could have however damaged your cat con(depends on the volume of leak) due to higher outlet temperatures.

Hey Rahulk76 thats a good linky there.
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Old 27th November 2009, 10:06   #5
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Keshav, the pressure guage was showing 20 Bar(instead of 24-25 Bar). Moreover when engine block heated up, the leak went away.
Anyways, for the said vehicle, the entire head assembly has been changed, so all issues should go away(hopefully)
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Old 27th November 2009, 10:13   #6
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Tanveer, how was this small leak detected/suspected to be present? Were there any telltale signs of the same?
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Old 27th November 2009, 10:16   #7
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Yes, on engine cold start(as in winter like scenario) the rpm kept fluctuating. In summers the problem would go away. I reckon the engine block temp needed to be below 20 degrees or so.
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Old 27th November 2009, 16:46   #8
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Quote:
Originally Posted by tsk1979 View Post
Keshav, the pressure guage was showing 20 Bar(instead of 24-25 Bar). Moreover when engine block heated up, the leak went away.
Anyways, for the said vehicle, the entire head assembly has been changed, so all issues should go away(hopefully)
Sounds so much like a weak piston ring instead of a valve seat.
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