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Overheating - Was It The Thermostat?

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minuteman62-64

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I've got a 1982 30 HP Mariner. Took a run in San Diego Bay this a.m. All seemed normal. Good flow from tell-tale. Didn't notice the exact head temperature, but didn't recall it being excessive (normally shows in the 145-150 degree range, again, on my home made head gauge). Shut the motor down to do some fishing, and, a few seconds later my home made overheat alarm went off. Since the motor was shut down, I figured the thermistor had crapped out so I pulled the fuse to shut it off.

Started the motor and began running at low speed to change location. Noticed the head temperature reading was about 180, but, still strong flow from tell-tale. When I speeded up, the temperature began to drop and got back to the 145-150 degree range.

We fished some more, then decided to cruise by the Spanish Landing area, about 1/2 mile down bay. Same thing with the head temperature, except after a few minutes it wouldn't drop even going at a higher speed. Still strong flow through tell-tale.

Limped back to the launch ramp with a run/shut down/run shut down sequence. Got to the dock with head temperature showing about 200. Running OK, no steam or smoke, still good flow from tell-tale, but definitely way too hot. Let it cool for awhile, then pulled the thermostat. Housing was bone dry, and thermostat was wide open.

Buttoned it up w/o thermostat. Head temperature was still showing 140 degrees, but dropped like a rock as soon as the water started pumping with out the thermostat.

Ran OK to put back on trailer and to flush back home.

Any ideas on problem? New thermostat, last I looked, was about $140. Cheap for peace of mind, but, I'd like to make sure I've zero'd in on the problem.
 
put it in a pot of water on the stove.heat it slowly and see if it opens correctly...use one of your wife's thermometers..it would be best to suspend it in the water and get it off the hot bottom of the pot..
 
papyson said:
put it in a pot of water on the stove.heat it slowly and see if it opens correctly...use one of your wife's thermometers..it would be best to suspend it in the water and get it off the hot bottom of the pot..
Click to expand...

It is old, it was put in already used, with unknown mileage, by the guy who renovated my motor. I found replacement on-line for $62, so I'll just replace it.
 
Apparently it was not the thermostat. Got a new thermostat (found one for $62). Installed it, and, running on muffs, still overheated - temperature ran up to about 200 F in 5 minutes - usually runs at about 145 F on the muffs.

Did find out one interesting thing. Ordering the new thermostat I looked at the parts diagram and noticed a metal washer that sits on top of the thermostat. Didn't previously have that. What it does is hold the thermostat down so the only water flow occurs when the thermostat opens. Without it, the thermostat gets pushed up and allows some water flow between it and the housing. Here I'd thought I had one of those motors that began flow through the tell-tale right upon startup. Nope. Oh well, live and learn.

Anyways, back to the drawing board on the overheating. Next step, drop the LU, check the impeller and blow water through all available orifices. If nothing shows up there I'll be back asking for more advice.
 
Assuming your home made temp gauge is accurate, don't just check the impeller, replace it. Also, inspect the impeller housing surfaces for any wear, replace as needed. If your cooling systems is working correctly, you can place your hand on the head and cylinder surfaces without it being too hot. I checked mine with an infra-red thermometer and it tops out at 120 F. Also, does your engine have a water pressure gauge. This is valuable as the pressure should be around 2-3 psi at idle and 6-8 psi at WOT. If you don't have access to a Mercury factory manual you need to get one.
 
unclebob said:
Assuming your home made temp gauge is accurate, don't just check the impeller, replace it. Also, inspect the impeller housing surfaces for any wear, replace as needed. If your cooling systems is working correctly, you can place your hand on the head and cylinder surfaces without it being too hot. I checked mine with an infra-red thermometer and it tops out at 120 F. Also, does your engine have a water pressure gauge. This is valuable as the pressure should be around 2-3 psi at idle and 6-8 psi at WOT. If you don't have access to a Mercury factory manual you need to get one.
Click to expand...

Temp gauge is a Faria(sp?) w/cylinder head sensor. Checked w/HF remote sensing "shoot and read". Thermostat is 143 F, so probably runs a little hotter than yours.

Impeller new in April 2015, only a little over 1 hour run time on it. It's a 3'd party (Sierra) and I've had issues with them before (also had issues with my last OEM impeller). Anyways, I'll be checking it and have a replacement on hand if necessary.

I'd really like a pressure gauge, but no port for sensor fitting or tube to tell-tale on my motor. If I have to pull exhaust jacket or head for further diagnosis I'm going to look at locations for drilling/tapping a port.

Squirted water at about 40 psi in all orifices yesterday. Looked like passages were clear. I'll be checking the water pump next. Then, test in barrel in case post-thermostat overheating was caused by poor supply from muffs.

Appreciate your input.
 
Here is a link to a factory manual that might include your engine. Check your serial number against the one listed in the manual front page. The link is; http://www.boatinfo.no/lib/mercury/manuals/30-40_2cyl.html#/0/. If not the right manual then the link to master list is; http://www.boatinfo.no/lib/mercury/manuals/. If your motor has been used in salt water, you could have some corrosion or restrictions that might require some disassembly.
I had similar problems on a '94 Mercury 60 HP. First it ran hot while on the water and the alarm went off. I replaced the impeller and also changed the thermostat, (which was O.K.). When your out on a big lake, you don't want to be stranded. I bought this boat second hand so I didn't know the history but it has been used on fresh water only. The next year I started getting false readings from the overheating alarm in a short period of time. It turned out to be a faulty solid state control module that also serves as an alarm for low two cycle oil. By the way, when trouble shooting my original problem, I would momentarily start the engine in a barrel without the thermostat and it's cover before and after replacing the impeller. There was a huge difference in the volume of water being pushed through after the impeller was changed. You can't just judge cooling efficiency by the engine just peeing water. The more experienced contributors may offer some suggestions as there are some real outboard gurus that frequent this site. Good Luck.
 
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