Mercury outboard overheating

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BikerBen

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Re: Mercury outboard overheating

I wonder if a loose connection at one of the three temperature sensors could trigger an alarm, especially since the alarm is not triggered unless there is actual physical movement of the boat. Also, it doesn't seem as though the motor is actually overheating since the water "peeing" out is only lukewarm.
 

hkeiner

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Re: Mercury outboard overheating

I leave the dock and have to cruise along for awhile at under 6mph in a no-wake zone. The water pressure is about 3.5 psi. If I throttle up to about 7 or 8 mph the overheat alarm sounds.
A big part of troubleshooting with a water pressure gauge is to monitor the changes in PSI at various RPMs and then consider the troubleshooting chart to either eliminate or narrow down the possible causes. If the PSI is acting normally at all RPMs and you detemine that the motor temp is OK (using alternative means such as a hand held IR temp meter or wax temp stick), you can pretty much narrow down the problem to your sensor or wiring. A multimeter will help in troubleshooting that stuff.

I suggest that you report back on the PSI readings at various RPMs and the motor temp when using alternative means to measure. This information will help those giving advice on what repairs to try next.
 

fpdiver

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Jul 13, 2008
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Re: Mercury outboard overheating

Fighting same problem (with Merc dealer working on it) with a 2003 125 Carb. (have posted). Replaced alarm switch (t-stat switch with single wire) today and new one did the same thing...I guess it is definitely getting hot.

Have also lowered prop pitch and put in another new poppitt. All other parts are new.
 

CharlieB

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Re: Mercury outboard overheating

Many of you may have the same gasket failure under the powerhead.

The water passage is next to the exhaust passage, as RPM raises, exhaust bleeds into the cooling system, water pressure remains about the same bby cooling is radically reduced until engine speed (and exhaust pressure) falls low enough for full water flow again, and the engine cools.

Lift the powerhead and change the gasket pictured in mosi's second pic

The exhaust adaptor plate that fits here also has a gasket on the motor side of the plate, change both gaskets.
 
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BikerBen

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Re: Mercury outboard overheating

I haven't yet solved my overheating problem, but I now have a readable water pressure gauge and temp gauge. They're in an LCD display which quickly becomes unreadable when exposed to sunlight, but it's good enough for troubleshooting if I cover it up until I want to read it. I find that my water pressure never goes beyond 4-5 psi, and the temperature slowly creeps up as I'm moving along at channel speeds at about 1000 rpm. As soon as I throttle up, the overheat alarm sounds, not surprisingly because at that point, the temperature is already near 180 degrees F, and the alarm sound at near 190. However, if I put my hand in the pee stream, I find that it's only lukewarm, and if I take the motor out of gear, and run it at higher rpm (about 2000 rpm) the temperature quickly drops down to the 140's. I've removed the thermostats and run water up the water pipe successfully, put in new thermostats (even though the old ones still worked properly), replaced the poppet and spring, rebuilt the water pump, even checked out the oil injection system for leaks, and there's new oil in the lower unit. Now where do I look?
 

BikerBen

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Re: Mercury outboard overheating

(continued) A couple things I forgot to mention...I cleaned the air compressor strainer screen, and the pee comes out nice and strong.
 

SURFERPUNK12

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Re: Mercury outboard overheating

I'm having the same problem. (overheat alarm comes on above idle and shuts off at idle). Merc 2003 150 saltwater carburated V6. I put a new water pump and new stats and it runs great on the hose but overheats in the water. I haven't checked the popet valve yet but i will as soon as I get off duty wendnesday and i'll post results.
 

gerryw39564

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Re: Mercury outboard overheating

I have been working on a similar problem 1998 Optimax. I first developed the problem running on plane at approx 80% power for about 15 minutes. I suspected thermostats. They were dirty but appeared functional. I replaced them anyway. The problem still existed so the next step was to replace the popit. The problem was still there so I checked the screen on the back of the power head at the top (cooling for the pump). It was clean. Something I noticed today was that the telltale stream was normal at first but then quits. I?ll let it cool down and restart and it does the same thing. Is doesn?t matter the rpm it will start fine and then quit. It will idle fine but will alarm and heat up when the throttle is advanced.
 

gerryw39564

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Re: Mercury outboard overheating

Ran the engine again after a cool down. The telltail is flowing water fine and it appeared to have a clog that passed. pressure is 5 psi to 8 depending on rpm. I have a warning light on the gauge that looks like a crank and piston. It is on when I turn the key in the on position prior to cranking and stays on. It came on after the temp light last run it the water but went out after the start during the run with the water hose after checking screen but came back after the cool down indicating a problem (possibly a sensor). Engine appears to be running fine other than the lights and the temp.
 

Faztbullet

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15,590
Re: Mercury outboard overheating

The cooling system on the newer engines uses volume not pressure due to floppy vane water pump thus the lower water pressures on guages. The poppit valve is a pressure relief valve and does not come into operation untill block pressure overcomes the spring. Most of the overheating problems I see are due to a obstruction, air leak,weak impeller or poppit valve seat not flush in block or damaged and most are in the exhuast cover area. On a V-6 water fills the center of block and then into cylinder area thru exhaust cover. The holes to the cylinders are only 1/4-5/16 in dia and any restriction reduces flow around cylinder and to the heads. Water exits heads 2 ways ,thru the t-stat(top) and the poppit(bottom) I have a factory water flow chart if anyone needs a copy for new and older V-6 engines.
 

BikerBen

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Re: Mercury outboard overheating

I tried lifting the powerhead but I couldn't get it, even with both hands. :) Seriously, I don't have the facilities for that, so I was forced to turn the boat over to the pro's. So far the problem remains unresolved, but it appears that only the starboard head overheats. I'm wondering if it could be a lubrication issue in one or more of the starboard cylinders ... lack of sufficient 2-cycle oil = increased friction = heat. I'll post again once the issue is resolved and I've paid the bill. :)
 

j_martin

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Re: Mercury outboard overheating

I tried lifting the powerhead but I couldn't get it, even with both hands. :) Seriously, I don't have the facilities for that, so I was forced to turn the boat over to the pro's. So far the problem remains unresolved, but it appears that only the starboard head overheats. I'm wondering if it could be a lubrication issue in one or more of the starboard cylinders ... lack of sufficient 2-cycle oil = increased friction = heat. I'll post again once the issue is resolved and I've paid the bill. :)

Naw, it's coolant flow, likely between the head and the block. Plugged ports.

By the time you get enough friction to generate noticable heat, you have enough melt to stick a piston.

hope it helps
John
 

gerryw39564

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Re: Mercury outboard overheating

This really sucks. I found a manual online from iboats. It looked like a deal all you to do is put in your type of engine and you would have 1 year of online manuals for your engine for 19.95. What a crock of crap. The diagrams are not even of my engine. The #1 Site For everything boating my butt. Should have stuck to the horses mouth Mercury Marine.
 

BikerBen

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Re: Mercury outboard overheating

The so-called pro's were no help, but they did take lots of my money, mostly doing things I had already done, but never finding the real problem. All they did was make the temperature rise a little slower. However, the problem has now gotten worse, which is a good thing. Now, at idle speed, I can see water trickling from one of the bottom bolts of the exhaust manifold cover. I'm kind of kicking myself for not noticing the rust and corrosion down there before, but nevertheless, I've now started to work my way in there for the seal/gasket replacement. I'll probably do a nicer repair job than the pro's would have done anyway. After all, it's my boat and I care!
 

CharlieB

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Re: Mercury outboard overheating

Each of you guys need to start a NEW thread of your own, list the problems, what you have tried, and we will give each individual suggestions on how to proceed.

It is poor form to high-jack another thread, even tho you problem is very close to his, it may have different cause(s), each need to be troubleshot and dealt with individually.

All of you need to at least test compression on each cylinder and WRITE IT DOWN.

Verify your link and sync, and that WOT timing is dead-on.

Odds are at least one of you has a small head gasket leak that only blows a very small amount into the cooling system until the throttle/load increases, then the leakage is great enough too displace a measureable amount of cooling water and temps seriously rise.

HOWEVER, without even a compression test I wouldn't recommend that you all arbitrarily start yanking cylinder heads off without some sort of verification, i.e., lower compression on a cylinder on the hotter side of the motor.
 

BikerBen

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Re: Mercury outboard overheating

I replaced the exhaust manifold seal and gasket, the boat is now running great and I learned a lot in the process.
 

john h miller

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Re: Mercury outboard overheating

I have 1996 mercury carb outboard 150 overheats or runs about 200 or higher against tide otherwise stays in middle of gage when plained off water pump and themostats fairly new seems like enough water comming from motor HELP.
 

Fishslick

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May 12, 2010
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Re: Mercury outboard overheating

John,
I was having the same problem with mine.

Did you change the poppet valve?
When you installed the lower unit did you use silicone on the divider plate that seperates the water pump area and the exhaust area?

I worked on mine for about two weeks due to the same problem you are having. I finally got it fixed this morning.

If you do not have the poppet valve kit, you can always start with removing the lower unit and using a marine grade silicone on the divider plate.
Below is a link to the original thread I posted and what my solution was.
Also there is another link that shows the divider plate I am talking about. It is part number 8 and is called a filler block.

I wish you the best of luck. Don't give up, keep at it and you will get it.

http://forums.iboats.com/showthread.php?t=398059&highlight=150+mariner+running+hot

http://www.crowleymarine.com/mercury-outboard/parts/1679_300.cfm
 
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