Overheating 135 Mercury

[sag2]

I have a 1988 135 Mercury that turns on the overheat alarm when I am pulling a wakeboard just above planing speed. It works fine at idle and high speed. I have replaced the water pump housing and impeller each year, and it gets better after doing that, then slowly deteriorates. I have replaced the pressure relief valve once, thermostats once, replaced the temp sender once, and switched it to the other head. Nothing seems to make it any better. I use only OEM parts, and never run it dry. It runs 12 PSI water pressure WOT, and around 5 PSI at idle. You can see the pressure relief open and close on the gauge. Does anyone know what happens if you leave the relief valve out? Any other suggestions would be appreciated.

 

[glenns]

Re: Overheating 135 Mercury

I am having the same problem with the same motor. Rebuilt water pump, rebuilt poppet valve, replaced thermostats, checked that all lines are clear and I still have a weak telltale 50% of the time. Only difference is the weak telltale can be at any/all RPM's. I have run out of options. Any input would be great.

 

[glenns]

Re: Overheating 135 Mercury

The Chilton manual says if you leave the poppet valve out, your pressure reading will actually drop. They stress using the valve.

 

[dutch135]

Re: Overheating 135 Mercury

Same problem here,, on occasion. Never at high speed or idle. Can idle around the lake for an hour without a problem, but pull a tube or wakeboarder and the same thing will happen.. the alarm will sound for a minute or 2, then go off. If I open the throttle and go, it goes off sooner. I was going to replace the impellar this weekend, but from the sounds of it, this is an inherant problem with this engine! Think I will wait and see how everybody solves the problem, I am thinking it is in an electrical control box somewhere.<br />

 

[glenns]

Re: Overheating 135 Mercury

Hey sag2, Did you have any luck with this issue yet? Must be a tough one, no one responding with any input. My local dealers don't even have any suggestions besides "bring it in and we will try". Is it possible to contact Mercury directly? We cant be the only owners with this problem. I will let you know if I get it resolved. GOOD LUCK!

 

[clanton]

Re: Overheating 135 Mercury

Test the thermostat by place stat in hot water until open, place a piece of thread in the opening let stat close, hold stat in hotwater, stat should start to open at 143 degrees and drop off of thread. Stats check ok reinstall with correct gaskets and seals. The thermostat should have a vent hole, may be round hole or V notch in the valve that opens. Check poppet valve, seat in block , diaphram, and spring. Then verify engine temp with melt stick or lazer temp checker. I think there may be service bulletin that cover some overheating problems, but I m not sure. Alarm system needs to be checked ,there is problems with alarm module giving false signals, if you are using premix you can bypass the alarm module. I have heard of removing 1 coil or heating 1 coil and making the poppet valve spring a little shorter.

 

[sag2]

Re: Overheating 135 Mercury

I stopped by the dealer that I have purchased all my parts from. I thought I would give it one more try. Well, seems this time he remembers a bulletin. It covers 1988 Sea Ray 135 and 175 outboards. This is what I have. Because the Sea Ray used a one piece cowl, they have an aluminum plate under the powerhead. When I pulled the pressure relief valve last night, I noticed that there was a round hole on the outlet side going through the plate. The fix is to remove the powerhead and file the hole to the shape of the gasket. I would guess this would at least double the outlet size. The dealer only had one gasket, so I got one and ordered another. In the mean time when I got home tonight, I pulled the relief valve and broke a round file to fit in the opening. I am trying to file the hole without removing the powerhead. It is slow going, but is working so far. The bulletin is Mercury 90-16, dated Jan 91. Wish I would have known about it about $500.00 ago.<br />Thanks for all the input. If you would like, I could scan the Bulletin and send it. Send me a note to stangozzi@aol.com

 

[sag2]

Re: Overheating 135 Mercury

Looks like I got it fixed. I should note that this is a Sea Ray motor (135 Mercury V-6 with one piece cowl) and the problem was due to no hole being drilled in the plate that Sea Ray added between the powerhead and motor leg. Not enough cooling water was reaching the powerhead.