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Discussion Starter #1 (Edited)
I have low compression in the #3 cylinder, the readings were: 153,153,111,153,154,151 when hot. What I have read so far indicates a bad exhaust valve, sound right? The idle is rougher than it used to be when I use very small plug gaps, also pointing to the exhaust valve. That cylinder has always been 10-15 psi lower than the others.

There are no bubbles or oil in coolant, no coolant loss, no coolant in oil. I put the piston on TDC, and applied 125 psi of air pressure. I could not hear any air leaking out anywhere, and it actually held 116 psi for a very long time after I turned the air off!

I also added a little oil to the cylinder, and it went up to 116 psi, but that is less than 5% difference. Something unusual is the cylinder still reads the same 111 psi now that it’s cold. I do have some overall blow-by, I get a good mist from the valve cover breather after a hard run that has been increasing more lately.

Does this sound like an exhaust valve? Anyone have any ideas for other tests to try? I don’t have a leakdown tester, and it doesn’t matter to me what the leakdown % is, I just want to know where the leak is coming from. Looks like I will at least be pulling the head off in the near future!

Thanks!
Al
 

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Seems it would be worth doing a leakdown just to see where the leak is. Probably want to check that injector too in case it's not flowing as much as the others.
 

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T6Rocket so what happened here? I was told i also have low compression in one cylinder..
 

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Discussion Starter #5
It depends on how low that one cylinder is? If it's not too low, you can drive it for a while, I did. A leakdown test will determine where the compression is leaking, and whether just the head needs removed, or the whole longblock.

Al
 

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I believe 5 of mine were at 155-160 and #5 was at 135 IIRC.
 

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Discussion Starter #7
I believe 5 of mine were at 155-160 and #5 was at 135 IIRC.
It wouldn't hurt to do a leakdown test to see where the air is leaking, but I would just keep driving it. 135 psi is not that bad. That motor has a lot of life left in it.

Al
 

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Wanted to piggyback on this. So here's the scenario. All cylinders are in the 170 range, except one at 107. Leakdown test is between 11%-23% for all cylinders except the low compression cylinder; leakdown = 67%. What's the cause of this, and what does the repair entail (how significant, and rough cost)?

TIA!
 

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Wanted to piggyback on this. So here's the scenario. All cylinders are in the 170 range, except one at 107. Leakdown test is between 11%-23% for all cylinders except the low compression cylinder; leakdown = 67%. What's the cause of this, and what does the repair entail (how significant, and rough cost)?

TIA!
Bad valve or bad rings.
Listen if the leak sounds like its at the head or lower down on the block.
 

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Wow that's high. I'm fully built with cams and read 110 to 112 consistent across all cylinders. I'm told since they're all about the same and I have cams that that's normal?
 

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^Carbon causes the higher readings. It is normal in an older engine. Normal readings are 156.

op I would ask yourself do you know what your getting into with a low compression motor? Is the car being discounted because of this? What are your plans? If you want a stock TT car avoid this. If your looking for a built single and can get this for a good price your on a nice path to doing things how you want but looking at some major repairs possibly.
 

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^Carbon causes the higher readings. It is normal in an older engine. Normal readings are 156.

op I would ask yourself do you know what your getting into with a low compression motor? Is the car being discounted because of this? What are your plans? If you want a stock TT car avoid this. If your looking for a built single and can get this for a good price your on a nice path to doing things how you want but looking at some major repairs possibly.
OP is from 2007. I bumped the thread cuz I had the same type of scenario/question, rather than start a(nother) new thread.

When you talk about major repairs, how major are we talking about (in reference to my #s in Post #8)?
 

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Discussion Starter #14
OP is from 2007. I bumped the thread cuz I had the same type of scenario/question, rather than start a(nother) new thread.

When you talk about major repairs, how major are we talking about (in reference to my #s in Post #8)?
It depends on where the leakdown is coming from. During the test, if you hear the air in the intake manifold or the exhaust pipe, it is probably from the valves. If you hear the air in the dipstick hole that is the rings.You could also squirt some oil in the low cyl to see if the compression improves.

obviously it would be cheaper to just R&R the cylinder head but any good rebuild also includes the piston rings.

Al
 
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