200 HP Mercury overheating between 1400 to 2500 rpm's

[jdupree]

I have posted this on several boards including TMD.  Just wanted to see if there are any Mercury guys out there who can give me some suggestions.

I have had this problem for some time and still can't figure it out. This motor will overheat between 1400 to 2500 rpm's after several minutes if you run it in that range. At WOT she runs around 140deg and at idle she will be around 140 to 180deg.

The motor was re-built right before I bought the boat which was about 18 months ago. Within the last year it has had water pump, t-stats, poppit. I also have a water pressure gauge installed. At idle showing around 2 to 3 psi which I think is normal. Between the 1400 to 2500 rpm's the gauge is showing 5 to 10 psi.

The motor runs great but will overheat in that range. The motor is a 1992 200 HP Mercury Offshore. Any input would be greatly appreciated.

[GoneFission]

JD - makes no sense that it overheats with 5-10 PSI water pressure.  Are you sure the temp meter is correct?  You can check it by heating some water up to boiling in a cup using a microwave, then take it out and stick the temp sender in the water.  I should read 205-210 degrees + or - on the gauge.  If it reads more, the sender may be defective or even the gauge.  If it's a Teleflex gauge/sender, you can get the specs for Ohms at different temps and see if the sender is giving the right reading.  

Otherwise, you may have a blockage somewhere in the cooling system - is the right side hotter than the left?  Still strange, because usually those problems show up at high speed, not midrange...  

[jdupree]

GF - Thanks for the reply.  The gauge is a Teleflex but the sending unit is the factory one located in the port head below the top plug.  My buddy has a temp gun.  We tested that temp on the gun against the gauge and they were very close.  We did notice during that test that the port head was hotter than the starboard head by about 15deg.

The overheat alarm has not actually gone off yet, but I did notice last week when the temp gets close to 200 the motor will start missing.  When this happens I can either bring her on up to higher rpm's or back off and the temp will start coming down.  

I am going to drop the lower unit tonight and double check the water pump.  This really has me  

[GoneFission]

Open up the poppit valve and see if there's mud or something in there.  It could be the poppit is not opening until high speed - and it usually does not open at idle.  A stuck poppit is about all I can think of that would cause a midrange high temp.  

Once it's open, run a hose through there and wash it all out - even mud daubers can get up in a motor sometimes...   :x

Hope this helps - see ya on the water!

[jdupree]

Thanks GF :!:   Will let you know what I come up with.