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Member

Join Date: Nov 2009
Posts: 5

Cylinder compression

11/19/2009 11:40 PM

Why would I get no compression on cylinder #5 when cold and normal compression when hot?

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Guru

Join Date: Sep 2007
Location: Defreestville, NY
Posts: 1072
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#1

Re: Cylinder compression

11/19/2009 11:55 PM

Thermal expansion, most likely the piston rings, I reckon.

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Anonymous Poster
#2
In reply to #1

Re: Cylinder compression

11/20/2009 12:04 AM
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#3
In reply to #2

Re: Cylinder compression

11/20/2009 12:11 AM

What does that mean?

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Anonymous Poster
#4
In reply to #3

Re: Cylinder compression

11/20/2009 12:40 AM

It means I found it funny.

The answer by stevem was correct, concise, polite, with a slight twist of sarcasm.

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Guru

Join Date: Oct 2008
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#5

Re: Cylinder compression

11/20/2009 7:22 AM

No compression means stuck valve to me. Even with worn rings, you'd get some compression.

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Anonymous Poster
#6

Re: Cylinder compression

11/20/2009 9:32 AM

Not enough information.

If the vehicle uses hydraulic lifters, the possibility of a valve getting stuck exists. This is not possible with an overhead cam style engine.

You need to give us this information.

I question whether the compression test is valid. Were all plugs removed first? Was the throttle open? Was the compression guage completely sealed? Was the lube oil at operating temperature prior to testing? Did the crank turn four complete revolutions before taking the reading?

Compare cylinder readings. The reading should not vary by more than 10 percent from one to the next. If so, the engine failed the test, even if compression is reported in each cylinder (head may be warped or gasket problem).

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Member

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Posts: 5
#7
In reply to #6

Re: Cylinder compression

11/20/2009 1:54 PM

The engine is a Jaguar 4.0 liter, 6 cylinder, overhead cam. Test was conducted by California Smog Inspection Station. Test vacuum cold 14" vacuum hot 18", smoke test for leaks and no vacuum leaks detected. cylinder 5 KV 10 all other cylinders 16KV. Cylinders 1-120, 2-125, 3-120, 4-130, 5-0 and 6-125 cold. 1--150, 2-145, 3-135, 4-145, 5-125 and cylinder 6-150 hot. Valve clearance is good 013 intake and .014 exhaust.

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Anonymous Poster
#8
In reply to #7

Re: Cylinder compression

11/20/2009 3:29 PM

If the cam lobes loose lubrication (usually when RPM drops below 1,500), the hardening (it is only 0.001) can get scuffed. Then the cam lobe can wear off (without the hardness). I had a six cylinder BMW that this happened to. You will have to replace the cam shaft. It my case it was #6.

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Member

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#9
In reply to #8

Re: Cylinder compression

11/20/2009 6:15 PM

Thank you very much.

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Commentator

Join Date: Jul 2009
Location: Northern CA
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#10

Re: Cylinder compression

11/20/2009 11:04 PM

Is it possible the valve was stuck in the open position until the engine warmed up and the valve finally broke free and started to work normally again?

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#11
In reply to #10

Re: Cylinder compression

11/21/2009 12:44 AM

Yes, I suppose so. I would I verify that?

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Commentator

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Location: Northern CA
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#12
In reply to #11

Re: Cylinder compression

11/21/2009 1:06 AM

I'm not sure what year your car is, but I think you could remove the valve cover (cold) and look at the valves for the offending cylinder while turning the crank by hand through two full rotations. This should allow you to see if anybody is sticking. I guess my next question would be if the engine consistently produced these compression results cold and hot. If so than A valve problem like I'm describing could be caught in the act reliably.

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