Steam from telltale and no compression

[Nitro13]

I have a used 150 mercury that was over heated 2yrs. ago always ran alittle
rough at idle but ran good.When cranking can hear lack of compression in at least one cyl.If it was a car engine I'd say head gasket,(steam from telltale,cleaned spark plugs and pistons etc).I'm prepared for cyl head removal and replacement of the gaskets.What i don't know is can it be something more than the head gasket.I've read on some of the posts about exhaust gaskets and crank gaskets.Also can the heads be milled if warped?
Can the block be straightened with a mill or long board? Is there a shim available to maintain normal compression if milled?Note engine still runs very good .:

 

[Silvertip]

Re: Steam from telltale and no compression

You based all of your information on "I think" methodology. You need to do some testing. Steam from the tell tale says the water pump isn't working. It also means the engine is likely overheating (again). The overheat 2 years ago probably started the issue on the engine that "runs very well". Sorry to be blunt but you need to start doing some real diagnosis before you totally destroy the engine. Compression test is a starting point. How did you clean the pistons if you didn't remove the head? Ports don't move so they don't wear. Piston rings loose their temper when overheated so you lose compression. Rough idle is probably due to that one cylinder that you can tell is down on compression or has zero compression which means the engine is NOT running very well at any speed. Water pump impellers need replacement on a regular basis to maintain water flow through the engine. Get a service manual to guide you through the diagnostic process.

 

[Nitro13]

Re: Steam from telltale and no compression

I have 20 psi on water pump at operating speed,just replaced it.The clean piston I could see through the spark plug hole,and the spark plug was spotless compared to the others.Thanks for your help,I'll be doing some testing this weekend,to try and narrow down the exact cause,before any parts removals.Again thanks.

 

[Silvertip]

Re: Steam from telltale and no compression

What you just mentioned about the clean piston and spark plug is the answer to your question. Water is passing through the combustion chamber due to a blown or leaking head gasket. A compression test would show that. Water in the combustion chamber immediately turns to steam. Water in the combustion chamber also means that cylinder is not being lubricated properly so I again suggest you stop running this motor to avoid any further damage. when you have it apart, figure out why the leak/blown gasket occured.

 

[Nitro13]

Re: Steam from telltale and no compression

Thanks again forthe help found copression ring on headgasket cracked and gasket damaged.I checked the block deck its warped less than .001,but the heads are warped .008-.010 my local machine shop straightened and milled the heads .The cylinders aren't scored,and all looks ok,my question is my motor had the thermostats removed(old trick that doesnt help on cars)how did the flow characteristics get affected.I have new ones ready to install,but am looking for the magic bullet that warped the heads if the previous bad water pump and over heat wasn't the problem.Thermostats probably removed after over heat,by the mechanic I bought the boat from.