I need some help. Overheat alarm only @ WOT.

[KCLOST]

Re: I need some help. Overheat alarm only @ WOT.

mc, that's a good test, as it tells you what the water temp is at the time (or shortly after) the alarm goes off....<br /><br />However, your overheat sensor is measuring metal not water... So, either the metal is getting up to 260F and the water isn't getting anywhere near that (unlikely), or you're sensor is installed improperly or malfunctioning. Did you install the sensor? If so, did it include a new wire harness?

 

[mcgurk34]

Re: I need some help. Overheat alarm only @ WOT.

KC, I replaced the original sensor which was probably OK due to this problem. Both the new sensor and original sensor came with a 4" tan wire lead terminated with a round lug which attaches to the engine's wiring harness at a small terminal block. The sensor is treaded into a blind hole tapped into the water jacket cover and is about two inches away from the water jacket outlet hole for the telltale hose. The telltale outlet is at the very top of the engine's water jacket above #1 cyl. How could the water jacket metal plate get 150 degrees hotter than the coolant water passing over it? This plate is not thermally connected to the cylinder top which should be the hottest part of the engine. What am I not understanding?

 

[KCLOST]

Re: I need some help. Overheat alarm only @ WOT.

OK, maybe we are getting somewhere. First when you installed the overheat sensor, does it completely go through the water jacket through a factory recess (actually never touching water) and set itself against the cylinder block or the metal plate? Meaning, if you pulled the water jacket cover off, is there gasket material around the sensor to prevent water from comming in contact with it? Or does it actually enter the cover and measure water only?<br /><br />I'm trying to understand your sensor, since my old 50hp never had a problem and so I never took it apart. But on my 175 the sensor never touches water and it's rated for 240F. <br /><br />If yours is in any contact with water, and being a 260F sensor for metal, I think this could cause it to ground itself and cause the alarm... Maybe the gasket around the sensor is leaking.... Even a little water may cause this, and then go away when you throttle down. <br /><br />OR MAYBE I was wrong all along and it's suppose to measure water, but 260F!!! that's crazy for water only... Has the water jacket gasket ever been replaced (they are notorious for leaking by the way)<br /><br />Let us know.... and don't give up...<br />Anyone else have any thoughts???????

 

[andrewkafp]

Re: I need some help. Overheat alarm only @ WOT.

On a 50, the pee'er comes out of the top of the motor and down near the coils to the bottom cowling. The water coming out has been right through the block. When I stop my 50 from near WOT, the water is pretty hot and on a cold day has a bit of steam coming from it, but not hot enough to cause hand discomfort. This continues for about 30 seconds at idle and as the vanes return to normal position then drops to warm. My impeller is 3 seasons old and has a pressure of 5-6lbs at WOT and is in need of replacement soon. If yours is 110f 15 seconds later, you don't have a cooling problem.

 

[trumanlake]

Re: I need some help. Overheat alarm only @ WOT.

hey kclost, we did have quite the ordeal last year with overheat sensors didn't we. you explained metal and water differences quite well. if that 50 is anything like the 115 i just worked on, that sensor screws right into the metal. Mc could confirm this by hooking up the muffs or starting it in the water with the sensor removed and look for a stream of water out of the sensor hole. my guess is that he won't have any. ya suppose? a couple of other thoughts that just came to me, have the plugs been change (hotter or cooler) from oem, second, just for grins he might try running with the sensor disconnected for a short period of time to make sure that is what is actually setting the alarm. if it still goes off inspect insulation on the wires, ect... just a thought.

 

[KCLOST]

Re: I need some help. Overheat alarm only @ WOT.

I'm just convinced something is wrong with that sensor, or the lead to it, or the way it is connected....

 

[mcgurk34]

Re: I need some help. Overheat alarm only @ WOT.

The sensor is measuring metal temp and is not in contact with water. It's threaded into a blind hole. I am becoming convinced that it's a false alarm, not an overheating condition, but the time delay from WOT to the alarm sounding is throwing me off. The delay both on sounding the buzzer and on clearing the alarm sure feels like a temp issue not a wiring short circuit. I'm going to disconnect the sensor and see if the alarm still sounds. BTW Plugs are new and correct type also all water jacket covers have new gaskets, no leaks.

 

[jim dozier]

Re: I need some help. Overheat alarm only @ WOT.

I don't know any specifics on your engine sensor by my Suzuki has a temp sensor that measures the heat of the metal on the head not the coolant. Mine is set to alarm at 237 degrees F. and the alarm won't shut off until the head drops below 194 degrees F. So you should expect a delay in the alarm shufoff after it has been activated. These things can go bad and I had to replace mine once.

 

[UpstNYer]

Re: I need some help. Overheat alarm only @ WOT.

Don't know this engine, nor any outboard well; but is it possible that corrosion, sludge, old impellor vane, old gasket piece or whatever might be impeding jacket water flow in the neighborhood of the sensor? How does the metal around the sensor feel right after a WOT run? Can he run it with the lid off to check? The on/off delays mc is seeing does seem to indicate temp. problems.

 

[JimOHoffmann]

Re: I need some help. Overheat alarm only @ WOT.

mcgurk34, I also have an 88 Mariner 50/60 3 cyl I bought last year on an 88 Tracker. My first trip out it overheated, with only @ 5 lbs pressure. I took it back and the dealer replaced the water pump. Next time out, the alarm sounded again after 10 to 15 min at wot, 5700 rpm. It wouldn't shut off until below 2000 rpm. I took it back again and the dealer water tested it and reported no problem. They said they checked it with the thermal sensor gun and it wasn't overheating. When I got it back, it still did the same thing. But I found that if I stayed below 5000 rpm, the alarm wouldn't sound.<br /> Two weeks ago I spent a week at Reelfoot and the only time the alarm sounded was when we were in a hurry to get out of a bad rain storm and ran over 5000 rpm. The water pressure above 4000 rpm is 12-15 lbs. New plugs didn't make any difference. Water stream is good and the motor runs good. If you find a cure please let me know. I did notice that the dealer replaced a damaged prop that was on the boat,a 15 pitch with a 14 pitch. Maybe the higher rpm's make it run warmer?

 

[mcgurk34]

Re: I need some help. Overheat alarm only @ WOT.

Boat4fun, Looks like we have the same problem, and I no closer to a solution. I don't have a pressure gauge but based on my observation of my Telltale stream, I have at least 12 PSI above crusing RPM and the T/T water temp is only warm. At this point I am just running the boat just below the alarm sounding RPM. I'm hoping that the actual problem will become more evident with continued use. I'm also hoping that the motor doesn't blow up while I guess at what the problem is. Let me know if you figure it out.

 

[jim dozier]

Re: I need some help. Overheat alarm only @ WOT.

Since you have been through most of the obvious, have you checked the maximum ignition timing at WOT. Despite what KCLOST says, too much ignition advance can overheat an engine as well as cause pre-ignition, detonation, and worst case a fried cylinder. Generally speaking retarding maximum ignition advance a couple of degrees, all other things being equal, will lower combustion temps in the cylinder.<br /><br />Check the WOT timing. If the maximum ignition timing is advanced more than spec put it right. If it is on spec and you can't find anything else wrong, try retarding it 2 degrees. It is possible that you may loose 100 rpms but it will run cooler.

 

[KCLOST]

Re: I need some help. Overheat alarm only @ WOT.

Jimd and I may get into an arguement over this, and that may be good as one or both of us may learn something by it... <br />But I have never heard of an overheat condition or especially an overheat failure due to a timing issue... In my opinion we would see thousands of those type failures if that were true due to the fact that maximum ignition timing adjustments have to be done at WOT after repair/rebuild work is completed. The good thing is we know you're water temps are normal, so I'd suspect a sensor issue or possible hot spot...<br /><br />But anyway, I guess it's worth checking the timing anyway...<br /><br />I wonder if there is a hot spot on that #1 cylinder... Pull the water jacket and check it again... Also take a look into the cylinder with a dentist mirror, if you can find one that will work... Maybe you'll see something out of the ordinary... Also, Call Mercury and see if you have the correct sensor (maybe yours requires the 285F, maybe not).

 

[jim dozier]

Re: I need some help. Overheat alarm only @ WOT.

No need to argue. It seemed mcgurk34 had covered most of the obvious bases for overheating.<br />1 impeller<br />2 thermostat<br />3 poppit valves<br />4 coolant passages<br />5 sensor<br />6 leaking head gasket (there is none)<br />7 lubrication<br />8 fuel mixture<br />so I thought I would mention one of the less common potential causes of overheating. Different outboards use different methods of linkages for maximum timing. I don't even know if with this particular engine its possible to over advance the maximum timing. But if its possible to adjust the timing it should be easy to check and rule out. It won't cost anything either unless he has to buy a timing light. Depending on the engine linkage you may be able to test it in the driveway by disconnecting the carb from the ignition advance linkage and running the engine at full ignition advance but carbs left at idle (relatively low rpm).<br /><br />The head temperature on my Suzuki dropped 15 degrees with 2 degrees retard and I never noticed any differenc in performance although I don't have a tach so its possible I lost a 100 rpm and dont't realize it.<br /><br />KCLOST old VW bug engines (aircooled) had the number 3 cylinder retarded relative to the other 3 because number 3 cooling airflow was affected by the oil cooler and would tend to run hotter. You could get quite a performance gain by advancing the ignition on a VW except that they would then routinely overheat and blow up because the aircooling did not have as much extra cooling capacity as a water cooled engine.

 

[jim dozier]

Re: I need some help. Overheat alarm only @ WOT.

One other thing, with as much work as you've put into this it might be worth buying an infrared point and shoot thermometer from Radio Shack to accurately check whether or not this engine or part of it is actually overheating. If it isn't then you know you've got electrical/sensor problems only. If it is overheating, then you have more mechanical work to do.

 

[jim dozier]

Re: I need some help. Overheat alarm only @ WOT.

Run this engine at WOT until the alarm sounds again and stop the engine and pull the sparkplugs. Look at the plugs for any evidence of a lean condition at WOT. It doesn't sound like its lean but you should be checking everything at this point. If the plugs look lean, maybe your carb rebuild was inadequate. Maybe one cylinder has a float setting off and fuel can't keep up at WOT.

 

[jim dozier]

Re: I need some help. Overheat alarm only @ WOT.

Did you ever confirm that the seal between the water pump and the exhaust housing was adequate? Some engines require a bead of high temp RTV silicone there. If the seal is bad exhaust can be sucked up by the pump and you would get an overheat at WOT.<br /><br />Is there any evidence that the seal between the powerhead and the middle unit is leaking (such as soot under the hood or leaking oil at the gasket)?<br /><br />If this engine has an exhaust cover plate on the powerhead have you checked it for leaking?

 

[andrewkafp]

Re: I need some help. Overheat alarm only @ WOT.

If you are getting 12-15 psi from a 50hp motor, it is way higher than the manual specifies at 6-8psi. 12-15 is more in the 100+ HP range.

 

[Trent]

Re: I need some help. Overheat alarm only @ WOT.

Like I said before and jimd...also said.... Did you do this? When you did the waterpump impeller you did replace the wear plate? Gaskets? You also sealed the exhaust support tube and tube seal with sealant as required??? <br /><br />Did you ever confirm that the seal between the water pump and the exhaust housing was adequate? Some engines require a bead of high temp RTV silicone there. If the seal is bad exhaust can be sucked up by the pump and you would get an overheat at WOT.

 

[mcgurk34]

Re: I need some help. Overheat alarm only @ WOT.

Unicorn, My Clymer's manual specs the water pressure for this engine as "3 or more psi @ 2000 RPM; 11 psi @ 5000 rpm". I'm guessing the actual pressure but it's very strong and steady. <br />Jimd, Trent, as in my previous posts the water pump was rebuilt completely replacing all gaskets, seals, O-rings, wear plate and impeller. Where specified in my manual, Perfect seal, loctite A and Multi-Lub was applied. One more thing, the gasket on the exhaust cover plate is new, no leaks. There is no evidence that the seal between the powerhead and the middle unit is leaking. No soot under the hood and no oil at the gasket. You both mentioned checking the seal between the water pump and the exhaust housing. Do you mean the Lower Unit housing? How do you check that? My manual does not mention using RTV between the pump and the L/U housing. It does say to use Perfect seal. Is that the same material? <br />I'm thinking about taking up golf.