1989 Merc. 200 HP Overheating Challenge

I'm stumped. I have a 1989 Mercury 200 HP engine that is overheating while boat is underway (not so much when at idle). <br /><br />I have replaced the water pump and rebuilt the poppet valve. The timing is correct and the carburator is adjusted correctly. <br /><br />I have a large amount of air bubbles that are coming from the lower unit. The air bubbles are coming from the exhaust slits (holes) that are on the lower submerged portion of the lower unit.<br /><br />I have a low water pick-up on this engine (if it matters)<br /><br />The compression is between 120 and 122 on all cylinders so the head gaskets should be OK.<br /><br />I don't know what else to check or replace - any suggetions?

Re: 1989 Merc. 200 HP Overheating Challenge

Dunno if it will help but.....I bought this Merc (different hp and year) and in reading my service manual noticed it had a thermostat and a pressure relief valve.<br /><br />Upon taking this newly purchased hot dog to the lake,(on a cold day) I launched it and pretty much immediately put it thru it's paces to see what it'd do.<br /><br />I ran around the lake for like 30 minutes at varying speeds, but was well over 30 mph which I found out later, provided sufficient ram water to the engines skeg water intakes to force the pressure relief valve open.....no overheating problems.<br /><br />Later I slowed down to just below plane and after just a few minutes the over temp horn sounded.<br /><br />Back home I tore into it and discovered that the thermostat was stuck closed.<br /><br />Now if my pressure relief valve had frozed and my thermostat had remained active, I would have had a problem similar to yours...<br /><br />Other thing I learned is that the tell tale stream just says water is available at the head; not that the engine is actually being cooled..........my tell tale was real stout.<br /><br />HTH,<br /><br />Mark

Re: 1989 Merc. 200 HP Overheating Challenge

What's the strength of your tell-tale at WOT, how about idle after the tstat have opened. When you replaced the water pump did you make sure that water tube was aligned with the water pump, if so make sure the water tube fits snug inside the grommet instead of just folding the grommet over. Did you use all new gaskets? Was the old impeller intact?<br /><br />When replacing the poppet valve did you replaced the grommet, carrier, diaphragm, spring...etc.<br /><br />Have you replaced the t-stats?<br /><br />You could have a clog somewhere, try dropping the L/U and flushing water down the tstat holes.<br /><br />This sounds like alot to chew on, but it's just that it could be mulitple things. Systematically go through each component of the cooling system until you nail it! If you have a water pressure gauge it would be easier to figure out if/when things are opening and closing, i.e., poppet and tstat.<br /><br />If you are running in saltwater and you don't know when the last time the head gaskets were replaced, I would do it anyways. It's good to replaced every three seasons.<br /><br /><br />Good luck

Re: 1989 Merc. 200 HP Overheating Challenge

TexasMark-<br />At least with the 80's thru early 90's model Merc's, if the tstat is closed your tell-tale will be weak. Merc's have a different design then say an OMC. With OMC's the tell-tale water you see hasn't gone thru the block yet, just up the water tube and out the tell-tale...so it will be strong even if tstat are closed (like at start up). With Merc's water circulates thru the block before exiting the tell-tale. At startup (closed tstat) you will see a weak tell-tale because the tstat has a vent that let's a little water pass when it's closed. But once the engine warms and the tstat opens you will get a full telltale.

Re: 1989 Merc. 200 HP Overheating Challenge

Andy,<br /><br />On this '02 the tell tale was strong from idle thru full blast.....obviously stronger at WOT.<br />My '02 90 hp 3 cyl 2cycle can't circulate prior to the tell tale cause the tstat was stuck closed (even in a pot of boiling water on the stove) and the tell tale was strong.<br /><br />Mark