Merc 200 Overheating... Revisited

I have read through quite a few posts on Merc's overheating.... Not sure if any of them directly apply to my situation so let me lay it out... Hopefully someone can point me in the right direction.<br /><br />I recently aquired an overheating problem, only when I get it above 3000 rpm. After about 30-45 seconds above 3000 rpm the alarm sounds. I back off the throttle and the alarm goes off in 5 - 10 seconds. I have replaced the impeller, t-stats, poppet valve and diaphragm. Ofcourse, it is still overheating. <br /><br />I took manual readings of the temp via infrared thermometer and it shows that the starboard side is running 40 - 50 F hotter than the other side!?!?! <br /><br />I did see a post about a blown exhaust gasket, also another one about a mud dobbbers nest. What do you guys think?<br /><br />Thanks in advance!

Re: Merc 200 Overheating... Revisited

I would think it is time to pull the heads and change the gaskets after look at the cylinders. Also check for blockage in the water passages. I normally change mine every 3rd year just for insurance, especially if you run in saltwater.

Re: Merc 200 Overheating... Revisited

Time to pull the head covers and replace all the gaskets and clean the water passages while your in there...<br /><br />Not a tough job. Good luck...

Re: Merc 200 Overheating... Revisited

I have had the "EXACT" same problem with my '92 200 Offshore. In fact, I have burned about 4 pistons within the first 100 hours. I did everything you did also.... A tech at Fond du lac told of some problems with double firing, but I was unable to confirm. One thing that did help was backing the Max timing off about 3 to 5 degrees. It just knocks to RPM's down about 500, and on this Center Console, it only equates to a knot or two.... I have successfully ran the engine another 150 hours with no problems (So far). I also installed a water pressure guage, which I have come to believe is more important than a temperature guage. You must be sure that when you get up and go, your boat and motor are planing through the water correctly. Any way, you are not alone brother......... Clear back to 16....

Re: Merc 200 Overheating... Revisited

same prob here with my 225...someone else told me something bout the bottom gasket also possibly causing this...any ideas?

Re: Merc 200 Overheating... Revisited

Had the same problem on my mariner 40 Hp. I realize that it is a different animal than your 200, but my mechanic seems to have solved my problem. His explanation was that the spark plugs were gapped too tight causing them to foul and run hot. He cleaned and regapped them and did a decarb job on it. I have only had it out a couple times since, but so far so good. Hope this helps.

Re: Merc 200 Overheating... Revisited

You don't mention what year or model your engine is, but I can tell you what I have seen in some of the bigger Mercs that relate to temperature and overheating.<br /><br />A difference in temp of 10 - 30 degrees from one head to the other is not unusual. The way the coolant goes through the block is the reason for this. If you experience overheat above 3000 rpm, you may have a couple of things possible. <br /><br />Knowing that you have performed the normal maintenance on the water pump, thermostats and poppet valve, next you should check the cylinder head gaskets (o-rings). These wear in time and can cause a slight leak of combustion gasses into the cooling passages, leading to areation of the coolant and an overheat situation.<br /><br />If that doesn't correct the problem, then you may have to lift the powerhead and look for a leak at the adaptor plate/gaskets. <br /><br />Good luck!

Re: Merc 200 Overheating... Revisited

Wow, thanks for all the responses. <br /><br />The motor is a 91. I purchased the boat about 6 months ago. Apparently is sat for quite some time, so I had a mechanic give it a service and it was running great.<br /><br />I let is sit for a month, and made a rookie mistake. I went out and cranked it, just to turn the motor over, but I did this without the muffs on. This is my first boat.... and this is something you can do with jet ski's without a problem.<br /><br />When I replaced my impeller it was not that bad, but maybe it was enough to clog something up.<br /><br />I am going to start pulling the covers and taking a peak inside. Is there anything I should use to flush out the passages?<br /><br />Also, where do I get a water pressure gauge, and where do I install it?<br /><br />Thanks.

Re: Merc 200 Overheating... Revisited

If and when you find an impeller missing its blades, its a good idea to remove the thermostat housing and thermostat and stuff a garden hose into the housing and back flush the cooling passages. Put a bucket underneath the middle unit (where the lower unit was bolted) and look to see if you catch any pieces that may be flushed out. Worst case scenario, if after doing this you still have reason to believe that something is clogging your passages, remove the cylinder head for direct inspection of passages. This is relatively simple if no bolts break off and only costs you the cost of a head gasket.

Re: Merc 200 Overheating... Revisited

hello<br /> did you really inspect both housings for signs of melting? it only takes a little melt to alter the pump housings drasticly. if they are fine, both uppr and lower housings. then pull heads and clean and inspect. pay close attention to the cyl walls<br /> good luck and keep posting

Re: Merc 200 Overheating... Revisited

I found a problem with my lower pump housing. after a few times of tearing lower unit down I noticed the lower water pump housing was warped on the top where it seals. I bought a new one separatly (lower) merc.I also found my impeller jumped the grove some how and made a new one. I put in a new keyway also