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Old 02-09-2012, 02:46 PM
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Low Compression

02 7.3 Okay I had my truck at a very good mechanic shop and I know he wouldn't bs me. My compression is 420 across the board except cylinder 5 she is at 390. What do you think I could do? The truck keeps smoking on start up due to this problem fuel smoke not oil smoke. The cylinder can't burn off the excessive fuel. Is the truck still healthy or is she on her way out? I don't have the money or the expertise to rebuild it myself. Thanks.

Chris
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Old 02-09-2012, 03:20 PM
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Wait a min, 420 - 390 = 30lbs difference? Like 7%....

Look, I too would like to see more consistent "across the board" numbers but 7% is NOT worth panicking over!

Relax If you've got a mechanic who's telling you you need to tear it down because of this, Go Find Another One who can find out what's going on.......!
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Old 02-09-2012, 03:31 PM
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Oh no he is not telling me to tear her down but he is telling me the truck will not stop smoking on start up unless I address the problem you know what I mean? Just want to be sure I am not hurting the truck by running her at 390 psi. What is the tolerance anyway?

Chris
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Old 02-09-2012, 04:54 PM
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I don't mean to be dumb but how did you figure out that I have lost 7% what is the equation?


Chris
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Old 02-09-2012, 05:52 PM
GAS GAS is offline
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390 is what percent of 420?
Divide 390 by 420 should come out to .92857,,, or 93 percent
7%loss

Another way to check loss % is to use a leak down tester, it helps you pinpoint the leakage.
With a regulated input to the cylinder of 100 psi the gauge connected to the cyl would in your case read 93 psi
This is tested with the piston is at TDC on the combustion stroke so both valves are closed.
Now you can take off the oil filler cap and listen for the hiss of escaping air past the rings
OR at the tail pipe for leakage at the exhaust valve
or at the air filter or intake for intake valve leakage

Last edited by GAS; 02-09-2012 at 05:58 PM.
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Old 02-09-2012, 06:33 PM
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Much appreciated. Like NoRalph said I am not going to worry about it what else am I going to do all the other ones are at 420 just can't see why this one is at 390 but oh well just run her right?

Chris
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Old 02-10-2012, 06:59 AM
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But you still feel there's actually a problem with smoking at start-up?

Are you sure it's not just normal cold weather "smoke"? I'm not 100% sure, but my "gut" tells me your mechanic is wrong. It doesn't make sense to me that, at 390 lbs of compression, that cylinder isn't capable of complete fuel burn. Just doesn't sound right....

I'd get another professional opinion because this guy's not making sense to me.

You either may not have a problem at all, or there's something else that needs fixin'.
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