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. :bsflag:

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!:drink:

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.

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!

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.

Thanks for the info!

Ouch, that hurts, Im sure he will get whats coming to him some day.

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...

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

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.

i say do the test yourself.if you pluck it up,you got nobody to blame but yourself,at least you will learn.i have to agree w/dennis ,why would anyone drive a ford or a harley?lol jus kidding guys.

yes. there are a lot of things i don't get. and for sure, stupid for the sake of being stupid is one of them. and dennis, for the record, like it or not, your politics, car of choice or motorcycle preferance not withstanding, when a guy tries to hire someone to do the work for him because he wants a professional opinion ( smart) and asks a most basic question that might suggest he is unsure of the procedure ( his words )
then your advice that he do it himself because " what can it hurt" is stupid. and i don't care how many books you wrote 30 years ago.

"then your advice that he do it himself because " what can it hurt" is stupid. and I DON'T CARE HOW MANY BOOKS YOU WROTE 30 YEARS AGO!!"
Steve, will you adopt me??? YOU ARE MY HERO!!!!!!:evilb:

T-rav-find someone qualified and do a full survey, transom replacements are exspensive and should really be checked for moisture on a 14 year old boat and they could check compression and hopefully do a leakdown test too as the 502 mpi's used metric low tension rings and are prone to leakdown problems stock. If you do it yourself pull the fuel pump fuse or unplug distributer signal wire and un-plug the coil so you don't burn up the coil (I have done this) and you don't wash motor with a bunch of fuel every time you crank it over, Smitty

There is a certain speed that is expected with every make and model of boat depending on the HP. If the boat does not hit that mark then somthing should be looked at.
Listen for a mis under load and listen for anything unusual. The transom and stringers should be looked at, take a rubber mallet and tap around, listen for hallow or different tones. If you can get a moisture meter that would help.

I don't think the OP was saying him doing a compression test himself will replace a full blown survey. I think he was simply asking if he chooses to do a comp test himself should the engine be warm or cold.

IMO warm is better as problems show up as the metal expands. But the problem is too warm and it's a PITA on a boat.

I always check my sleds warm as they are obviously easy to get to the plugs. I've seen wounded sleds have decent compression cold then warm them up and it drops significantly. But ask 5 people the proper method and you'll get 5 answers. Some say warm, some say cold, some say throttle wide-open, some say it doesn't matter, some say 3 pulls, some say pull until the gauge quits going up, etc, etc.

Smitty gave you good advice. And don't get hung up on the actual number as gauges can vary(unless they are all very low then try a different gauge). The important part of a compression test is the difference between the cylinders.

And what's wrong with a guy doing his own comp test? If a boat I was thinking about buying was near me I would check it out first myself. If it checked out then I may have someone more experienced survey it. If it doesn't they you simply walk away and save yourself the money for a survey. Seems logical.

Because they are smarter than you.

:lolhit: +1 that's awful rude of him