cracked head

[dtr352]

I just did a compresion test on my motor and got some scary. The number three cylender makes no compresion, doesn't move the gauge at all. Every other cylender is between 125 and 150 psi. Is this a cracked head problem. How can I be sure. I just bought the boat and if the head is cracked Im going to be pissed. Would this cause my oil to get milky. Please help.

[CHEETER-JOE]

Rings??

[b's sanger]

Do a leak-down test and it will show you exactly what the problem is. Ideally it should be at running teperature, but you can get a rough idea doing it cold.

[roostwear]

I would venture to guess that if you have ZERO, you have a severely jacked up valve, bent pushrod, or a ventilated piston. Pull the valve cover... if the pushrods are ok, time to yank the head!

[dtr352]

I replaced the head gasket and still got water in the oil so I did the compresion test. Pushrods looked ok at reasseambly. The motor fires right up and idles. I always thought that no compression meant it wouldn't run and if it did it would be very rough. If the problem was rings wouldn't I get at least some compresion.

[b's sanger]

Again, do the leak-down, it will tell you if it's the rings, intale valve, exhaust valve or head gasket.

[dtr352]

I have never done a leak down test. How do you do it. How does it tell you what the problem is. I know it souns stupid but I am new to this. Thanks for the help

[b's sanger]

You can buy a tester from Jegs for about $100. Basically it is a valve with two gauges. You need an air source (conpressor). Attach the hose adapter to your spark plug hole, connect to the tester and fill the valve until the right hand gauge reads 100psi. The left gauge will tell you the percenatge of the air you are losing from the cylinder following the chart in the instructions. Listen from where your air is escaping. Air coming from the exhaust means exhaust valve, through the carbs means intake valve, into the crankcase is likely your rings. You can listen through your breather element or oil fill for this. Headgasket may be harder to identify without water in the block.
Yours' sounds somehat previlant so if you cant hear anything through the first choices, I would lean toward the head gasket.
Hope this helps B

[Fiat48]

If the head gasket is blown between cylinders, this can be caught with the leak tester by closing the valves on the cylinder next door to the suspect leaking cylinder and see if you get air coming out the spark plug hole of that cylinder next door.
If no time to go get a leak tester...using a regulated air supply of about 80 lbs...take the shrader valve out of the end of your compression gauge..then take the gauge off...then adapt that to your regulated air supply. Put the suspect cylinder at tdc and give it some air. See where it goes.

[dtr352]

Does the piston need to be at tdc to do the leak down test. If so how do you know when it is.

[roostwear]

And then pull the head..............

[Fiat48]

Originally posted by dtr352
Does the piston need to be at tdc to do the leak down test. If so how do you know when it is.
If in doubt of finding tdc for that cylinder...simply remove the rocker arms so the valves on that cylinder can't open and stick the air to her. The reason for tdc is to make sure both valves are closed.
And like he said...then pull the head unless you find a broken valve spring or something.

[Rexone]

Leak down test thread (http://www.***boat.com/forums/showthread.php?s=&threadid=41809&highlight=leak+down)
:)

[victorfb]

just a thought, but are you sure you didnt over tighten the valve on that cylinder? try pulling the coil wire and turning it over, then listen if your getting air back through the carb. im assuming you looked at the valves before you installed the heads, did you make sure they were seating properly? maybe lap them? when running are you getting any popping? if not id look closer at the exhaust valve and port. see if raw fuel is escaping.