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View Full Version : HELP overheating merc 200hp



nirto shop
05-03-2001, 06:03 PM
I have a 85 200hp merc overheats at
high speeds i think i tried everything
The boat has a jack and next thing i was
going to try is a low water pick up.
I have about 10 to 18 psi at all times.
Need so help???
jerry

drewvir
05-05-2001, 08:27 AM
do you have thermostats in the motor?if so boil them in water with i thermometer to see when they open,if not put them back in,the water is going through the motor too fast to remove enough heat.you didn't say if the gage says your too hot or the buzzer is going off.10-18 psi should be enough water pressuer,also check the waterjackets for blockages.
drew

nirto shop
05-05-2001, 01:56 PM
Yes i changed the stats 2 times and then
cut the center out of the stats so it
would act like the stat was opened.
And i have a gauge and a factory buzzer
one sensor on each head and 2 seprate
circits.
The gauge goes into the RED (no #s)on
the gauge just cold and hot.
And the buzzer goes off.
I run the jack up or way down and still
overheats.
jerry

BillyD
05-09-2001, 09:03 AM
I've seen several posts on this and other boards about this problem. Is water coming out of the "tell-tail" hole like it should or is it obviously restricted? In any case, I will give my usual anwser to this problem. On my old Merc 175 V-6, my overheat problem was caused by clogs in the internal water flow holes in the block that are under the exhaust cover plate (the tall verticle plate between the two cylinder banks in the rear of the engine). My engine got so hot that the black paint came off of the block up near the top of the heads. If you remove that plate, you can access these holes which can then be cleaned/reamed-out with a small (1/8", maybe) drill bit turned by hand/fingers (NOT in a drill). I forget how many holes there are, but 3 or 4 sounds about right. As I recall, these are only about 3/16" in diameter, but don't hold me to that exact number. If these holes are clogged, engine will overheat despite good water pressure. Will become clogged simply from corrosion buildup; this is hastened if boat is used in water that is salty or brackish, but I believe it can eventually happen in any water. You will need new gasket to put the cover back on and proper specs for tightening the bolts back (due you have a sevice manual? If not, you really need one for stuff like this). Be careful not to shear off bolts (during both the "off" and "on" process). If things look real corroded in there, may want to also take water jacket covers off of your heads and clean that area out as well (again, will need new gaskets). Another possibility, I guess, is something going on with your poppet valve. Could try replacing that (or by-passing it). I assume that you see no leaks when running the engine with its cover off while on the garden hose on your trailer? Good luck.