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Compression test questions

Old 08-19-2011, 04:24 AM
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Default Compression test questions

So I am goning to look at a boat I am interested in purchasing this weekend. It is a 99 25 outlaw with a 502 mpi 240hrs. I have been trying to get a mechanic out there all week but they are either to busy, out of town, or the one that said he would inspect it for me turned out to be a flake.

So I have been researching and I have been seeing that some people say to have the motor up to operating temp before doing a compression test, and then I read a merc service bulliten and it says to do it cold.
Which way is the best?
Is there a difference in pressure readings cold vs warm?
Where is the best location to ground the coil wire?

Thanks in advance, I hope all turns out well with this boat!

Last edited by T-RAV; 08-19-2011 at 04:26 AM.
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Old 08-19-2011, 05:45 AM
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Originally Posted by T-RAV
So I am goning to look at a boat I am interested in purchasing this weekend. It is a 99 25 outlaw with a 502 mpi 240hrs. I have been trying to get a mechanic out there all week but they are either to busy, out of town, or the one that said he would inspect it for me turned out to be a flake.

So I have been researching and I have been seeing that some people say to have the motor up to operating temp before doing a compression test, and then I read a merc service bulliten and it says to do it cold.
Which way is the best?
Is there a difference in pressure readings cold vs warm?
Where is the best location to ground the coil wire?

Thanks in advance, I hope all turns out well with this boat!
mr. t-rav...
you have started out trying to do the right thing. you recognize you need a professional to confirm what you are buying because you don't have the skills. SMART... very very smart. don't screw up now because you are impatient. take the time and spend the money to get a well qualified guy to inspect the boat and validate its condition for you. that exercise will be like buying 100 dollar bills for a dollar in the long run. you started out being smart... stick with it.
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Old 08-19-2011, 06:31 AM
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cold = a little less compression, warm = better. watch your finger on exhaust !

if you are aware and know a little the basic of mecanic, you can do it , if not... pay and have someone to look not only compression but also have a quick eye on the rest!

good luck!
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Old 08-19-2011, 08:04 AM
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If you hire someone, watch them do it. I bought a boat off a broker and they would only allow an insured licensed mechanic in the place. I payed a guy $150 to do a compression check and found out later he never did it. Now almost $10k later I'd like to sue the guy.
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Old 08-19-2011, 01:22 PM
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Originally Posted by jeff32
cold = a little less compression, warm = better. watch your finger on exhaust !

if you are aware and know a little the basic of mecanic, you can do it , if not... pay and have someone to look not only compression but also have a quick eye on the rest!

good luck!
Thanks for the info!
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Old 08-19-2011, 01:24 PM
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Originally Posted by ThisIsLivin
If you hire someone, watch them do it. I bought a boat off a broker and they would only allow an insured licensed mechanic in the place. I payed a guy $150 to do a compression check and found out later he never did it. Now almost $10k later I'd like to sue the guy.
Ouch, that hurts, Im sure he will get whats coming to him some day.
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Old 08-19-2011, 01:26 PM
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I suggest doing the compression test yourself. What could it hurt? If you have around 155 pounds each cylinder, you are OK (125-160 is cool). If you have some really low cylinders (90-120) ask the seller (he may be a little afraid of losing the sale by now) to pay for a mechanic to do a compression test.
Do it cold with a screw-in tester. Inspect each spark plug to see if all are in the same condition (all are burning the same).
Bring along new spark plugs just in case you buy the boat.
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Old 08-19-2011, 01:41 PM
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Originally Posted by Dennis Moore
I suggest doing the compression test yourself. What could it hurt? If you have around 155 pounds each cylinder, you are OK (125-160 is cool). If you have some really low cylinders (90-120) ask the seller (he may be a little afraid of losing the sale by now) to pay for a mechanic to do a compression test.
Do it cold with a screw-in tester. Inspect each spark plug to see if all are in the same condition (all are burning the same).
Bring along new spark plugs just in case you buy the boat.
Dennis Moore

"I suggest doing the compression test yourself. What could it hurt? "

let me answer that for you... not being a mechanic and not knowing what he is doing or looking at, he can make a mistake and either convince himself something is good that isn't or he can be staring at something thats obviously bad and not know what he's looking at. and then instead of making an informed decision on the purchase , he can end up screwing himself very very badly... for the sake of saving a $ 150 bucks.

i freely admit that sometimes i don't understand the advice given here... everytime someone posts about a bad boat or motor they bought, the first thing everyone does is beat the sht out of him for not having it surveyed. here is a guy looking to have it surveyed and the general consensus is that, even though he doesn't know how to do it, he should just have at it himself...

i just don't get it...
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Old 08-19-2011, 02:17 PM
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Steve,
I've got a feeling there are a lot of things "you just don't get"

I don't get why people would ever own a Ford, why anyone would ever vote Re-buba-lican and why anyone
would ride a Harley.
But I bet stevesxm does those things.

Ha Ha
Dennis
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Old 08-19-2011, 02:23 PM
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It's not like I am mechanically dumb, I just read conflicting information and wanted conformation. I did fiberglass and gel work for 5 years through college. Which wasn't that long ago! So I feel I am capable, thanks for the waste of bandwidth steve.
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