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AQ145a pressure build up in heat exchanger

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Prscator

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I have a problem with excess pressure forcing raw water out of the heat exchanger cap at about 3500 to 4000 rpm. I am advised that this may be caused by a faulty exhaust manifold although there is no evidence of water in the oil. Also whilst the engine revs off the clock when in neutral it will not exceed 3800rpm when in gear. The boat is a Fairline Carrera 24' which when new should have pulled 5500rpm. i have checked compression and all cylinders are at original specification. Could the exhaust manifold be causing the lack of power? Any suggestions will be gratefully received.
 
Your exhaust manifold cooling water and your internal Engine Coolant are separated.
Unless there is a breach in this system....., one should not affect the other regarding pressure.... other than heat related pressure.
Heated engine coolant does and will increase in pressure within the Closed Cooling system side.... but does so proportionate to the engine/coolant temperature.
IOW, the pressure should not continue increasing once engine temp is reached and levels off........ if it does, you likely have cylinder pressure leaking into this side of the system.... Head Gasket, perhaps!

The RPM not reaching WOT may be a result of several things:
  • Dirty hull or dirty prop.
  • Wrong prop pitch.
  • Wrong over-all gear ratio.
  • Poor fuel delivery and/or engine tune.
  • Poor spark or inadequate spark lead..... aka Ignition Advance, of which is quite common with an older Bosch distributor.
 
you might want to try another compression test to make sure. Sounds like a head gasket to me. You could try a pressure test to by pumping air pressure into the spark plug hole and see if it increases pressure in the cooling system. That will tell you which cylinder is affected. Not that it matters on an inline motor. Just went through this with my Ford truck blowing coolant out of the reservoir (similar closed cooling system) the pressure test told me it was the number 1 cyl on the passenger bank on my V8. If your 4 cylinder is not quite firing on all fours that might be your sluggish performance problem as well.
 
Prscator said:
I have a problem with excess pressure forcing raw water out of the heat exchanger cap at about 3500 to 4000 rpm. I am advised that this may be caused by a faulty exhaust manifold although there is no evidence of water in the oil. Also whilst the engine revs off the clock when in neutral it will not exceed 3800rpm when in gear. The boat is a Fairline Carrera 24' which when new should have pulled 5500rpm. i have checked compression and all cylinders are at original specification. Could the exhaust manifold be causing the lack of power? Any suggestions will be gratefully received.
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Did you ever figure out what was going on with your engine?I had the same problem with my 145a and it was a clogged manifold right at the end of where the exhaust comes out but the outer part where the raw water ecapes.I had also verified that my oil and antifreeze heat exchangers were flowing by just draining them out and seeing how fast they drained out.
 
Generally a good idea to start your own thread rather than hijacking a 4 ++ yr old one. FWIW... a fair number of first time posters never bother to come back to the forum and report their results, so its unlikely the OP is still around.
 
sandkicker said:
Generally a good idea to start your own thread rather than hijacking a 4 ++ yr old one. FWIW... a fair number of first time posters never bother to come back to the forum and report their results, so its unlikely the OP is still around.
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Whatever floats your boat if it floats.
 
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