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Merc 135 optimax overheating

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kairey

New member
I have a 2000 merc optimax 135hp that is experiencing dramatic overheating. It started at the end of last season, the boat would overheat at idle and low speeds but would cool down at high speeds or if I revved the engine in neutral. I replaced t-stats and water pump but this did not solve the problem. Before the beginning of this season I replaced the water pump (again) the poppet valve and the spark plugs and I am now experiencing worse overheating at all speeds. The tell tale is strong and the boat does not over heat when hooked up to the hose in front of my house. Most forums suggest that all overheating issues can be solved by one of those 3 things (t-stats, water pump, poppet) so any further suggestions on what it could be would be greatly appreciated.
 
Finally figured it out! I have a 2000 135hp optimax. Mercury has a quicksilver part #884236A called a "fitting kit". Cost me $72.00. Before the kit I did all the water pump, thermostat, and popit valve replacements with no luck at all. Here's the scenario. The original water route is from the top of each side of the engine down to a tee fitting where it is now just one water tube (the same diameter as the first two) returning into and cooling the engine. What I heard is that the restricted flow of just one tube is the problem. I just installed this kit on my engine. Now I have a water tube coming from each side as before, but now they stay separate and attach to their own fitting. The old fitting is removed and plugged, and with the kit (as per instructions) you drill and tap two new holes in the engine. I went from chronic overheat problems to; I can't get it to overheat! I needed to be at 1700 rpm's or better prior to the kit install or I would overheat. It was most noticeable when going from cruising speed and dropping to idle. In this case the engine is turning lots of rpm and the sudden drop to idle speed wouldn't get enough water in to cool the engine. Temp would rise fast and overheat horn came on. If you are handy and comfortable with drilling into your engine and tapping aluminum, you can do it yourself. If no tap and die set, shouldn't be much labor for your mechanic to install. I'm no mechanic but did it myself in about two hours. Good luck.
 
Hi there - I am having the same problem. Had the poppet valve cleaned out as it was stuck with marine sediment. When I run the engine at 1500+RPM, my temp drops and I dont get the alarm sounding. I was hoping you have some pictures or how this was done as I have already spent over $1000 with my mechanic trying to get this problem fixed.
 
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