posted 09-29-2007 11:42 AM ET (US)        

I purchased an OEM engine temperature gauge from my local Evinrude dealer, Lockeman's Boat and Hardware in Detroit. I wanted an OEM gauge so that it would harmonize with the appearance of the other OEM gauges on the panel. As it happened, Lockeman's had an appropriate gauge in stock that was about ten years old (c.1997) and from the OMC era. Thus its appearance was just about a perfect match for my 1992 OMC gauges.

I had previously mentioned to OMC Master Mechanic Ben that I was planning to install the engine temperature gauge and sought his comments. Ben's wry reply was that after I had installed the gauge I would be able 'to watch the temperature go up and down." At the time he made that comment I did not appreciate the accuracy of it.

Installation of the gauge was very simple. A brass sending unit was threaded into an existing boss on the starboard top cylinder. This location is monitored because it is generally the hottest region of the engine. A supplied light brown or tan wire was run from there to the dash board. The supplied wire turned out to be about five feet short, but since I had just purchased my own stock of marine grade wire in various common colors, it was a simple matter to splice on the necessary extension. I also installed a new ring terminal on the extension wire.

Adding the new gauge to the panel was just a matter of rearranging the existing gauges. I removed the voltmeter gauge from the dash board panel and moved it to the electrical panel, replacing the hour meter that was mounted there. The hour meter was then mounted in a new location, not on the dash board or console. Since the hour meter did not require constant observation while operating the motor, it was moved to an interior location inside the console but visible when the console door is open.

Changing all the locations and wiring gave me an excuse to do some more clean-up of the installation, to re-route some more wires, and to dress up the wiring on the back of the dash board panel (and use some more of that newly purchased wire I had in stock). After a couple of hours I was finished with the overhaul of the dash board instrument panel, and put it back in place. Now to see if the new gauge worked.

I turned the ignition key on and closely observed the gauge indicator for any sign of movement. If it moved at all, it was imperceptible. With the engine block at ambient temperature (about 80-degrees) the temperature gauge pointer rested on the left-hand peg of the dial. Curious to know that the gauge was actually going to work, I cajoled Chris into accompanying me to the local boat launch ramp, where we could back the boat into the lake and start the engine. (You might ask why I didn't just start the engine on a hose adaptor, and that is a good question. But the answer is also good. The noise of a V6 outboard running on a hose adaptor is rather loud, and for political reasons I do not wish to give the neighbors the sonic exposure to my motor that it would produce.)

At the lake we just backed the boat down the ramp and left her on the trailer. I hopped aboard and started the engine. After about a minute at a fast idle of 1,500-RPM, the engine temperature gauge began to climb. This verified the new gauge was working and the installation was proven. We hauled the boat back out of the ramp and went home for dinner.

I did not get a chance to actually run the motor under normal conditions for a couple of weeks, and on Labor Day weekend we found ourselves out in the rather cold water of Lake Michigan (50-degree-F). Idling out to the big lake from the marina brought the engine operating temperature up to about 155-degree-F. The weather that day was picture-perfect, and the lake surface abnormally calm. We got up on plane and headed north for a ten mile run along the shore. The fun and excitement of the beautiful day, the beautiful shoreline sand dunes, and the crystal clear water of Lake Michigan distracted my attention from the new temperature gauge. We had been underway and on plane for about 20 minutes when I finally thought to look at the new gauge and check the engine temperature. I was quite surprise to find that the cylinder head temperature had dropped to a very cool 90-degrees.

For the next three days I watched the engine temperature more closely. I began to understand what Ben had meant by his comment about watching the temperature go up and down. While idling, the engine temperature would rise and stabilize around 150 to 155 degrees. But when the throttle was opened and the boat run at planing speeds, the engine temperature would drop rather quickly to less than 100-degrees.

In a later consultation, Master Mechanic David Zammitt confirmed that this temperature swing is typical. At full throttle my V6 engine has "Power Cooling", a configuration where a water valve linked to the throttle adds greater water flow to the cylinder block water jacket. When at idle speeds this valve closes, and the engine temperature becomes hotter and is regulated by the action of the thermostat. The higher operating temperature at idle speeds helps improve the slow speed running characteristics of the engine. The extra cooling at high speed insures there won't be problems with overheating. I was surprised to find that the engine actually runs at a cooler temperature when operating at 4,000-RPM than it does at 800-RPM. Dave also mentioned that keeping the engine cool helps it to produce more horsepower output.

Now I have three indicators of the engine cooling system: the water being exhausted from the cooling system aspirator (or tell-tale) nozzle, the pressure indicated on the water pressure gauge, and the engine cylinder head temperature shown on the temperature gauge.

There is one final tweak that can be added. A second temperature head sensor can be installed on the port cylinder head, and the gauge can be switched to read from either port or starboard sensor. I'll leave that project for next season, although I already have the switch on hand.

posted 09-30-2007 09:32 AM ET (US)            

Damage to your engine occurs from excessive heat, which the temperature sensor will indicate. A drop in water pressure is one possible cause of excessive temperature, but low water pressure is not the ultimate cause of damage to your engine - excessive heat is.

posted 09-30-2007 09:58 AM ET (US)            

The brass sender located on the cylinder block cover failed within months of the new installation and was replaced. At the time of the failure my temperature gauge read over 200 degrees. I shut off the motor and called mercury from the water on my cell phone. They believed it was a sender failure as the over heat horn did not sound and the power head could be touched by hand without getting a burn. Mercury stated that if the power head had over heated to temperatures near 200 I would not have been able to touch it by hand which makes sense. Even at 120 degrees a person should be able to touch the power head without incurring a burn.

posted 10-01-2007 11:58 PM ET (US)            

When a motor is started the three indicators reveal themselves in this order:

--water is seen flowing from the tell tale or aspirator of the cooling system almost as soon as the engine begins to run; it is the first sign of operation of the cooling system;

--when the cooling system purges itself of air and begins its normal operation, the water pressure gauge stabilizes and shows the pressure. This indicates that the water pump is operating at normal level, and thus is the second sign of operation of the cooling system;

--when the temperature gauge comes off the dial rest and moves upward, it joins the others in providing an indication of operating of the cooling system. It is the third monitoring device.

I fully agree that having an aural alarm which sounds when cylinder block temperature reaches a high threshold is also important. Most engines have a separate thermostatic switch which operates the alarm. This is another element of the cooling system monitoring, possibly the most critical.

posted 10-02-2007 12:53 AM ET (US)         

With a Merc, Jim's description does not apply. It takes several minutes after startup for a telltale to even show. The water first has to go all the way through the engine, to the top cylinders, to be evident. When it finally does show, it's HOT. With OMC, there seems to be a short circuit directly from the impeller to the telltale, where it expells cold water. There is no way to know the upper cylinders are getting cooled, and the OMC design only shows the impeller is not blocked. Merc's design tells you the impeller is working and all cylinders are getting water. Also with my Mercs, there is very little water pressure indication at low RPM and startup, since the poppet valve prevents full flow of water pressure to the top cylinders, where the pressure take-off is done. There has to be enough RPM and subsequent impeller RPM to create the water pressure to open the poppet valve, which will then show operating water pressure up at the top cylinder. So with Mercs, once the telltale starts to show up, the temp gauge is your next indicator of proper cooling.

posted 10-02-2007 01:16 PM ET (US)            

Larry - Your statement "... THEORETICALLY, water pressure is more important, since the cylinder head temp can't rise out of control until water pressure is decreased. ..." is not correct. That is - for example, if the discharge were blocked, or et.al. .... the pressure does not decrease - but there is no flow and the engine temperature increases.

And yes - the engineers must have thought the temp gauge more important than pressure.

Newt - Your "... nagging unanswered question. ..." is very appropriate - and yes - it possible for the engine or parts of the engine to overheat even if the water pressure gauge shows adequate pressure. A stuck thermostat will do the job - as will any other blockage.

Now, in the engineering and control world - in this case, pressure is a feed-forward control signal - telling you of a potential problem - whereas temperature is a feed-back control signal - telling you what is REALLY happening. Proper/stable/el.al. control REQUIRES some form of feed-back control. Many will say that we do not have a control system - but remember, our mind and knowledge is a wonderful control system - and is the control system we are talking about here.

A potential danger of using a pressure indication is that one may become complacent with just looking at the pressure and ignore temperature.

posted 10-02-2007 09:12 PM ET (US)            

As mentioned by LHG, with the type of configuration used apparently by OMC and Yamaha, there is some sign of water output from the tell-tale or aspirator very soon after engine start, and on the Mercury motors there is often a long delay, upwards of a minute or so, before any water appears.

I think that it is a fair characterization that most operators check for water flow shortly after engine start as a sign that the motor is operating properly. On the motors where the flow begins right away, you get immediate confirmation that you have water flow in the cooling system. On the Mercury you might be running the motor for two minutes before you decide that something is wrong. With the Yamaha and OMC style configuration, you get much faster indication of a problem than in the Mercury configuration. If there is no water after 20-seconds, I would shut down to investigate the problem.

There are actually three different sources of water which feed the tell-tale on my OMC motor, and I have not made a study of precisely where the are all located in the cooling system in terms of their distance from the water pump. One hose appears to be a bypass from the Power Cooling valve which dumps excess flow when the valve is closed; a second hose is from some point in the cooling system water jacket which I assume is located fairly high in the engine so that trapped air is easily expelled--that is what the term aspirator refers to I believe--and a third is really a water suction pickup which is intended to remove any trapped water from the mid-cowling which might have entered if the engine were swamped in high waves.

Jerry--I very much like your explanation of feed-forward and feed-back data. Thank you for joining this discussion to add that.

The pressure shown on my water pressure gauge is initially a bit low, around 5 to 10-PSI, until all the air is purged from the system. At that point the pressure increases to about 15-PSI or more. When the throttle is opened and the Power Cooling valve opens, the pressure tends to drop slightly. In all cases the water pressure is correlated with the volume of flow from the tell-tale or aspirator. So in a sense you can say the pressure gauge is a sort of calibrated measurement of flow volume.

Again, I have to say that the pressure gauge gives you an index into the state of the water pump and will warn of problems in pump output which may not show up immediately in engine over heating.

posted 10-03-2007 12:56 AM ET (US)            

A poppet valve is operated by pressure.

posted 10-03-2007 09:06 AM ET (US)            

All this justification is pointless, lets just install them and enjoy the beauty of gauges. It makes us feel good.

posted 10-17-2007 08:56 PM ET (US)            

Last Saturday evening I ended up watching the prime time NASCAR race on television--yes, I know, this is somewhat out of character for me but I was really quite tired from hand-buffing the gel coat on the boat all day and I spent the evening lounging in an easy chair in front of the television, but I digress.

In the course of the race one of the cars had a very small piece of debris, a sheet of paper perhaps, which was plastered to the grill of the car and held in place by the wind pressure. The announcers pointed this out and explained that even this slight blockage of the air intake to the radiator was going to be a big problem, and they predicted that the car would soon have its engine over heat. I could not believe that such a small area of blockage in the grill could have such an effect, but I was wrong and the announcers right. A few minutes later the car was in the pits and water was spewing out of a relief valve in the hood; the cooling system was over heating. Of course during the pit stop the debris was removed, and the car got back in the race without damage to the engine.