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Author Topic:   Temp and RPM
70_Katama posted 05-04-2001 12:53 PM ET (US)   Profile for 70_Katama   Send Email to 70_Katama  
I have a 1988 90hp evinrude. I dont have a temp or rmp gauge to monitor the engine. Any idea how difficult it would be to install these gauges? Car motors I can get around ok, but this is my first experience with boat motors.

Thanks Tim

lhg posted 05-04-2001 05:20 PM ET (US)     Profile for lhg    
Not difficult at all, especially the tach. Take a serious look at the new Teleflex glass domed "Pro Series". Really sharp, and easy to keep clean. Available at Cabelas or Bass Pro. (also at West, but badly overpriced) Just put some of these in mine and they are super. Best instruments I've ever used.

The cylinder head temp gauge (called and Outboard Temp Gauge) is more difficult to install, since you will have to mount the sensor (supplied with the gauge) on the engine. Instructions are provided how to do this. Do not confuse this with the Water Temp Gauges sold for inboards. Not the same thing.

The OB gauges read Cold-Hot.
The inboard gauges read in water temp degrees.

Many of the newer engines come with the temp sensor already a part of the engine, with a wire in the harness, and often an overheat warning horn. Check to see if yours has that, otherise a wire to the engine is needed.

70_Katama posted 05-04-2001 10:19 PM ET (US)     Profile for 70_Katama  Send Email to 70_Katama     
As usual..Thanks for the advice/help!
Hank posted 05-04-2001 11:44 PM ET (US)     Profile for Hank  Send Email to Hank     
My 1984 90HP Evinrude has an aftermarket temperature sensor installed. It's a contact sensor bolted to the head. It sends to a gage on the dash which reads in degrees F from 120 to well over 240. My trailer is being fitted to my boat at the trailer dealer's. As soon as I can it back I'll post the manufacturer of the gage and describe how the sensor is attached to the engine.
Tom W Clark posted 05-05-2001 06:48 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

I consider both of the gauges to be absolutely mandatory equipment. You'll be glad you installed them.

As Larry indicates, both are very easy to install. All the wiring is at the console for the Tach. There should be a capped purple wire that is the tach lead. This wire is part of the motors wiring harness. The outboard temp gauge will need one, tan I believe, wire to be run from the console to the power head of the outboard. As Hank says, the sensor bolts to one of the cylinder heads. There is a z- shaped metal bracket that you atatch to the head by simply removing one cylinder head bolt and mounting the bracket with that same bolt. Tighten to the specified torque and you are ready to screw the sensor into the bracket. The tan wire simply plugs onto the back of the sensor.

Of course you will need + and - power which is already at the console and you have the option of wiring the internal lights to come on with your nav lights for night running. The internal light may, or may not, be an extra cost option with either gauge.

70_Katama posted 05-05-2001 07:30 PM ET (US)     Profile for 70_Katama  Send Email to 70_Katama     
What I need is all the people from this site to come over to my house and help me fix these little items I have left to complete! Steaks and beers for everyone!
lhg posted 05-06-2001 12:54 AM ET (US)     Profile for lhg    
Hank - It sounds like your cylinder head temp sensor is hooked to the wrong gage, as all outboard temp gauges only read Cold-Hot. With Teleflex, at least, the inboard engine water temp gauge also has a different sender, so somebody may have given you a wrong installation.
Tom W Clark posted 05-06-2001 11:41 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
If Hank has an outboard temperature gauge like the standard OMC (Teleflex) that I had on my last four Whalers, then it probably is just as he describes it. The sensor is a cylindrical piece of metal that measures the water temperature by means of the heat conducted through the head to the sensor. The installation instructions tell you to scrape the paint of the head where the sensor makes contact. I presume the paint may act as an insulator.

The early gauges that I had were indexed with degrees F, but I suspect that it became clear that the ability of the sensor to measure the water temperature in the head was compromised by the vagarities of each installation. Thus the degree marking were omitted and the gauge is used as a means of measuring the engine temperature relatively. By this I mean that after using your boat a while you learn what the normal range of temperature is and can note a departure from this norm. You can also tell when the engine is warming up by seeing when the needle comes off the peg.

Kim posted 05-06-2001 02:14 PM ET (US)     Profile for Kim  Send Email to Kim     
how about a water presure gage ? tee in the water line for the tell tail, this what some have done. telflex has them . no water presure / you got problems / ...
I have a set of gages 4/sale from my motor swap. tac,temp. (head sencer) trim &hrs. telflex white . used 124 hrs.
can send pic. of them ...
lhg posted 05-06-2001 06:37 PM ET (US)     Profile for lhg    
Tom - according to my Teleflex manual, the temp sensors for both the outboard cylinder head temp and the inboard water temp look the same, but are different units, with different part #'s. Don't ask me why, but that's why I don't think an automotive/marine inboard style WATER temp gauge should be used as cylinder head temp.

Kim, I have water pressure gauges on both of my boats, but the engine take off must come off a cylinder head fitting, preferably a top cylinder. Putting a tee in the teletale hose does not work! Most engines have provided such a place for this fitting, plugged at the factory. For anybody running an engine at elevated transom heights, this gauge is a must to make sure you're not getting reduced water pickup.

Tom W Clark posted 05-06-2001 09:51 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
lhg, I understand your differentiation between those two gauges, and you are correct. An inboard water temperature gauge will not work in this application. My point is that this is not what Hank has. Call it what you will, but the idea is to monitor the temperature of the water in the motor. An inboard temperature gauge has a sensor that screws into the block and makes contact with the cooling water. The outboard temperature gauge does not, it merely senses the water temperature via conducted heat throught the head. The sensor varies (somehow) the electrical resistance that is sent via the tan wire to the gauge.

I have had the outboard water pressure gauges on two of my boats and the instructions expressly dictated that the hose be connected to the supplied tee which is installed in the tell tale hose. I'm curious, why is that a poor choice? I never really thought much of these gauges precisely because of the fact that your measuring at the tell tale, but does that necessarily mean it will not work? Or is it merely unreliable? At any rate I put more stock in a temperature gauge.

Dan posted 05-07-2001 11:49 AM ET (US)     Profile for Dan  Send Email to Dan     
On my just purchased Whaler with a 115 Merc, the temp guage never moves from the left, cold position. Is my guage bad? I called one Merc place and they said the 115 has "such good cooling abilities that the engine rarely gets hot enough to register a reading and at speed the guage sensor is bypassed by the thermostat." What gives?
lhg posted 05-07-2001 01:24 PM ET (US)     Profile for lhg    
Tom - regarding water pressure gauges, my first experience was installing them on Merc V-6's, using Mercury branded gauges and instructions specific to Mercury engines. As I have said, there is a removable nut on the top of a top cylinder bank for the take-off fitting and hose. First of all, water pressure needs to read where it's most critical, at the top of the engine. If a problem exists, this is where a decreased reading will first show up, with inadequate pressure to push the water all the way up and cool the top cylinders.

I liked these so well, that I decided to retro-fit my older in-line six Mercs. But this time I bought the Teleflex gauges, and their instructions were not as good, nor as "engine specific". So, like the instructions said, I used the "tees" and spliced them into the teletale hose. No readings! So I took the boat over to the Mercury Dealer, and right away they knew that wouldn't work. No pressure build up at that point because the water is being relieved a few inches down the line. He knew where the "plugs" were on the engine block (top cylinder, where the pressure is "confined") and re-installed the take-offs. Worked perfectly.

For those interested, the water being taken off does not actually travel all the way in the tube to the gauge. It simply compresses the air in the tube, causing the gauge to register.

lhg posted 05-07-2001 01:29 PM ET (US)     Profile for lhg    
Dan - one can get an outboard to run the hotest by idling along in gear at about 1200-1400 rpm. Unless the water is really cold, the temp gauge should register. On a new 1997 Merc 200 EFI, I also had a temp gauge that wouldn't register, as you are reporting. Took it in for a checkup, and the sensor on the engine was a dud. So look into that also.
Tom W Clark posted 05-07-2001 01:53 PM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     
Dan, I'd be concerned about a "no reading" on the temp gauge. It is possible to have a thermostat stuck open which can let the motor run too cold, and this is bad.

lhg, I like the system you describe much better. Set up that way it would definitley be a useful gauge. I wonder if this can be achieved on other brands of outboards.

lhg posted 05-07-2001 02:20 PM ET (US)     Profile for lhg    
I would imagine that all engine brands have a "plug fitting" on the block or head area for the water pressure gauge.

On a related subject - Do the new four strokes use the same cylinder head temp and water pressure gauges. Or do they use the water temp gauge of the inboard four strokes?

Kim posted 05-07-2001 07:29 PM ET (US)     Profile for Kim  Send Email to Kim     
I used a sencer that goes aginst the head on the 70hp omc. it worket ok. ran about 120 to 140. never had a water preasure gage.
now on my new Suzuki DF70 there is not a temp. gage that you can buy , so I'm told. but there is one built in the tac, that comes with the motor. if the thmp. gets to high a buzzer (opchenal) goes off . if no buzzer (alarm) which I do not have.when the motoe gets hot it will shut down... untill it cools down.
Hank posted 05-08-2001 01:26 AM ET (US)     Profile for Hank  Send Email to Hank     
It's interesting that my OMC Service manual for the Evinrude engine indicates that engine temperature should be checked by Thermomelt sticks. These in effect check the water temperature via the cylinder water jacket.
They suggest using two sticks, one melting at 125 deg F and one melting at 163 deg F. To test they recommend running in a boat at a max speed of 3000 rpm for 5 minutes. The mark of the 125 degree stick should melt; that of the 163 degree stick should not .
The contact sensor I mentioned above is used in the same way. It measures the cylinder water jacket temperature. If the contact is good and clean then the gage can give a good indication of the coolant condition. If the gage doesn't get to 125 deg F then I'd suspect a faulty gage or a stuck thermostat. If it reads over 160 deg F I'd shut down and try to find out why. The engine also has an over temperature sensor inside each cylinder cooling jacket which is set to close when overheated and sound a horn. These are set to close at 211 deg F and open at 175 deg F.
Note that the horn signal isn't set to sound till its temperature is well above the 160 degree mark.
dgp posted 05-08-2001 07:30 AM ET (US)     Profile for dgp  Send Email to dgp     
Larry, the first Mercury supplied water temperature gage that came with my 50 four stroke was a 120-240 degree range gage. The dial scale was marked C to H. With the engine at operating temperature the gage pointer barely moved off the C mark.
My dealer supplied me with a different gage with a 60-200 degree range. The scale is marked C-H with 5 increments:
#1..60 degrees
Now, at normal operating temperature it's reading about 110-120 degrees.
The temperature sender will read about 600 ohms with cold engine at ambient temperature.
The guy at Teleflex said " most O/Bs operate on the cool side. My own Evinrude 88 SPL runs about 135. If you can hold your hand on the cyl. head its running cool."
simonmeridew posted 05-14-2001 08:20 PM ET (US)     Profile for simonmeridew  Send Email to simonmeridew     
A couple of questions:

Internal combustion engines (read auto engines) are most efficient at a high operating temperature. All new cars have a 192 or higher thermostat, to assure they run at the optimum temperature. Try running a car at -20 without a thermostat and see what kind of gas mileage you get. So: Why do OB's operate at 125 or so degrees rather than 192or even 180? It isn't the 2 cycle thing: My 2 cycle lawn boy mower is air cooled and is made to run hot.

2nd question: Would a coolant(water)pressure guage be able to predict when the water pump is starting to go bad in my OB? If so it seems that it would be a good reason to get one. Comments??

lhg posted 05-14-2001 10:24 PM ET (US)     Profile for lhg    
Yes, one of the main reasons for a water pressure gauge is to reveal water pump impeller condition. Since the take-off place is on a top cylinder, it will be the first to show reduced cooling pressure. They are also used in high speed applications to show the same thing, or if the engine is mounted too high, or raised to high by a hydraulic jack plate.

Regarding engine cylinder head operating temp, you've got to remember that a car is a closed radiator system, and simple for the thermostat to regulate and keep from overheating. Outboards don't seem to operate this way. I have noticed, that when operating in cold water, you can never get the temp up very high, even though the water pressure gauge shows everything is OK. The faster you go, the colder the engine runs. Sometimes in the Great Lakes, with 50 degree water, the C-H temp gauge barely registers at all at planing speed. Whereas idling along in S Florida in 85 degree water, the gauge will show 7/8 to the "H" side.

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