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Overheating, flow problem is past the thermostat

C

cassenbl

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2003 mercury 40hp 4 stroke 3 cylinder serial #0T783233. Engine overheats at idle after a couple of minutes. Replaced water pump twice. Ran scope up water tube, everything OK. Engine is overheating, too hot to put hand on. Replaced thermostat. Will overheat with thermostat in or out. Have a lot of flow coming out of telltale. Yesterday, I wanted to check water flow so I removed thermostat cover, took out thermostat and left thermostat cover off. Started engine and water started come of thermostat hole and telltale. Engine was nice and cool. I ran it 20 minutes and no Adams and everything nice and cold to touch. My guess there is some king of obstruction so water can not get past thermostat. I have not taken not exhaust cover yet. What is past Thermostat? I do not think this model has a poppett. Service manual does not show one but does not that system well. Any ideas?
 
Sorry for typos. Hope I am explaining OK. With thermostat cover on, engine overheats. Leave cover off, water flows out of that hole and telltale. Engine stays nice and cool. I can not find any information of what the water flow is past the therm9stat. What could be obstructed past that Point? Any help what be great.
 
Try a compression test, What do the plugs look like? A good compression test would rule out a bad head gasket
 
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Thank you for the reply. I will check but I think the compression is OK. I originally had this in to a Mercury shop. They had check it and said it was good. I spent close to $1000 and they good not solve the problem. Their next suggest was to spend more money to pull the power head. I just brought it home. This was 3 years ago and I had medical issues and have not been able to work until now.
 
Thank you Johnny. You were correct. Top cylinder pressure 120, middle 117 and bottom 0! Took off head. All pistons are scored brown. Bottom cylinder wall has a Ridge around half of was. I think this Ridge is too bad to allow boring. Block is junk. Can't believe dealer thought compression was fine. Looking for new motor. Next question. Would my electric parts and trim and tilt Fit on a 2006 f25l? This 2006 is at a dealer not from me and is supposed to be brand new. Overstock.
 
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