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Old    adam (03suprassv)      Join Date: Jan 2009       10-19-2010, 6:35 AM Reply   
I want to do a compression test on my engine and need some input. I think I have it for the most part but want to be sure there are not anymore steps I need to do.

1. get the boat up to operating temp
2. remove the spark plugs
3. disconnect the main wire to the distributor
4. disconnect the power to the injectors

Do i need to disconnect the power to the fuel pump to or is just disconnecting the injectors enough? I want to make sure I am not injecting fuel during this process.

Old    Kyle (RidingTheNW)      Join Date: Apr 2010       10-19-2010, 12:04 PM Reply   
This is all you need to do for a compression check...

1. Take the throttle and put it on the full throttle position. This allows more air into the engine.

2. Take all the spark plugs out.

3. You just need to jump the yellow wire to the red wire on the starter. that will engage the starter but not the ignition. The ignition wire is purple. If you put power to the purple wire you will give power to all things like the computer and fuel pump. The Red wire is constant power straight from the battery, the yellow is not. It might be Yellow with a red tracer on it too. You can do this at the starter or at the ignition switch.

4. screw in the compression gauge and crank the engine over until the gauge stops moving. Its a GM 5.7 so the PSI per cylinder should be around 175. Hopefully even if its lower then that, all cylinders are within 10% of each other.

Thats pretty much it.
Old    Kyle (RidingTheNW)      Join Date: Apr 2010       10-19-2010, 12:05 PM Reply   
Oh when you put the throttle in the full throttle position, make sure you are on the idle only (not in gear) otherwise the engine will not crank over.
Old    SamIngram            10-19-2010, 12:40 PM Reply   
While you are going to all the trouble of doing a test that won't tell you anything you might as well do a leak down test too, it will actually tell you what is going on.

Get yourself a leak down tester and actually figure out exactly what is wrong with the engine.

A leak down test requires the removal of all the spark plugs. The crankshaft is then turned so that each piston is at top dead center (both valves closed) when each cylinder is tested. Most people start with cylinder number one and follow the engine's firing order.

A threaded coupling attached to a leakage gauge is screwed into a spark plug hole. Compressed air (80 to 90 psi) is then fed into the cylinder.

An engine in great condition should generally show only 5 to 10% leakage. An engine that's still in pretty good condition may show up to 20% leakage. But more than 30% leakage indicates trouble.

The neat thing about a leakage test (as opposed to a compression test) is that it's faster and easier to figure out where the pressure is going. If you hear air coming out of the tailpipe, it indicates a leaky exhaust valve. Air coming out of the throttle body or carburetor would point to a leaky intake valve. Air coming out of the breather vent or PCV valve fitting would tell you the rings and/or cylinders are worn.

A leakage test can also be used in conjunction with a compression test to diagnose other kinds of problems.

A cylinder that has poor compression, but minimal leakage, usually has a valvetrain problem such as a worn cam lobe, broken valve spring, collapsed lifter, bent push rod, etc.

If all the cylinders have low compression, but show minimal leakage, the most likely cause is incorrect valve timing. The timing belt or chain may be off a notch or two.

If compression is good and leakage is minimal, but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery (bad injector) or ignition problem (fouled spark plug or bad plug wire).
Old    Sparky Jay (wake_upppp)      Join Date: Nov 2003       10-19-2010, 1:34 PM Reply   
Compression check will tell you alot, but not as much as a leak down test. It depends what problem you are actually trying to diagnose. For general health of an engine, the first thing I hook up is a vacuum guage. This gives a great overall picture of the condition of an engine in a very short amount of work time. It's always the first guage I hook up.
Old    SamIngram            10-19-2010, 1:56 PM Reply   
I have had this discussion many, many times....

What will a compression test tell you? What will a leak down test tell you?
Old    Murphy Smith (murphy_smith)      Join Date: Dec 2005       10-19-2010, 2:08 PM Reply   
Wow Sam - great information
Old    Johnny Zero (dirtrider)      Join Date: Sep 2008       10-19-2010, 2:47 PM Reply   
Sam is correct a leak down test will tell you more about a problem, but in my opinion it is a more dificult test to perform for someone who is not
a mechanic. I would do the compression test first and if you find that there is a problem (low or uneven numbers) then go after the "trouble cylnders" with
the leak down.
Old    adam (03suprassv)      Join Date: Jan 2009       05-13-2011, 5:03 AM Reply   
I never got to test it last year so I am back at it now. My starter is not wired up like Kyle (RidingTheNW) Stated. It has the large red wire from the batt and the a smaller red and an orange wire on one post. On another post there is the yellow wire. It looks like a ground wire on the bottom. When looking at it yesterday I noticed that he large power wire was touching the prong next to it that has nothing on it and was arching to the point that the bolt is burnt. How can I do the compression test with it like this? Is this wired correct? The engine is a 2003 indmar 5.7 tbi.

Also on the other side of the engine I found a wire that is not connected. If you were looking at the motor frmo the front it would be on the rear right hand side. There is a blue wire that looks just like it connect to something but the brown wire with the blue stripe is not hooked to anything. Also attached picture of it.
Attached Images
Old    Jeff D (Jeff)      Join Date: May 2010       05-13-2011, 9:42 AM Reply   
I think you can just pull your kill switch lanyard and turn the key instead of messing with all of that.
Old    adam (03suprassv)      Join Date: Jan 2009       05-13-2011, 11:10 AM Reply   
My boat is a 2003 before the kill lanyard came around. So no kill switch.
Old    Jeff D (Jeff)      Join Date: May 2010       05-13-2011, 1:29 PM Reply   
Originally Posted by 03suprassv View Post
My boat is a 2003 before the kill lanyard came around. So no kill switch.
I thought they were mandatory long before that. My boat's an '00 and it has one. All of the boats I every went on when I was in jr. high and high school (Circa 1995) had them.
Old    Sparky Jay (wake_upppp)      Join Date: Nov 2003       05-14-2011, 2:23 PM Reply   
My 06 Sanger has no kill switch. None of my Sangers had one.
Old    adam (03suprassv)      Join Date: Jan 2009       05-16-2011, 5:25 AM Reply   
unfortunately I was not able to do the compression test because the wiring was different on my starter. I was not sure what to do so I didn't even try. I did notice if I pull the throttle back all the way right before it goes into neutral where it is still in gear but idling about 600rpm it does not have the problem with the rpms dropping and occasionally stalling. Could this have something to do with the throttle cable adjustment or some kind of neutral switch?


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