Adding temp gauge...cylinder head temp vs water temp?

[shooter5636]

I've started looking into adding a temp gauge to my '78 Johnson 35hp tiller. Does anyone have any experience with the type of gauges where the sending unit is in direct contact with the cylinder head? Thoughts or reviews on reliability?

I couldn't find mention of any bolts that opened up to a water passage in the service manual, so my other option that I came up with was adding a "T" to the telltale tube before it exited through the cowl, and then install the temp sensor into that "T".


Thoughts or opinions?

 

[Vic.S]

The temperature of the tell tale water is of no interest. It has only passed through a small part of the engine and will be only slightly warmed.

 

[interalian]

Head temperature sensed on the outside of the cover vs the sender exposed to the water would show little difference in steady-state operation. Response will be much faster if the sender is exposed to the water, but I doubt you'd see a problem in normal use. You would need to drill into the water jacket and tap 1/8" NPT for a typical sender. Fitting a sender into a fabricated "Z" bracket will let you put the sensor in contact with the head. Best spot is above or next to the higher sparking plug.

As Vic said, the telltale water won't show you anything as it's tapped off the cooling supply before it goes anywhere warm. OMC probably did it that way to prevent burns when putting a finger under the stream.

 

[shooter5636]

There's something about drilling into an engine block that I'm not comfortable with yet. Looks like the head temp is the route I'm going to go.

 

[gm280]

I have to ask, have you had any temp issues with your engine? If so then go ahead and mount a head temperature sensor. If not, why try to install one? :noidea:

 

[bob johnson]

on motors with no port for a threaded sensor, they usually have a style that goes under a head bolt.... you want the temperature of the head.....not the water......

I have never seen a temp sender on anything but a head........its a great thing to have....a temp gauge.... I have one on EACH head on my motors...

 

[interalian]

^ Sender tip in the water is the fastest way for heat to get to the sensor - as long as there's water in the water jacket, the sensor tip is getting the head temperature correctly, since water is an excellent conductor of heat. And the sensor body would still be threaded into the head, providing the head temperature even if there's no water in the water jacket. I too have one on each bank of my V4, because temperature:

 

[interalian]

There's something about drilling into an engine block that I'm not comfortable with yet. Looks like the head temp is the route I'm going to go.

I can appreciate that. I wouldn't be comfortable drilling into a block blind - the head would have to be off if I'd never seen inside.

 

[shooter5636]

No temp issues. Simply for peace of mind.

Thanks for the input. Are there any brands that are typicall reccomended over others for the under-head-bolt style?

 

[bob johnson]

you need to make sure the ohms of the sender match the guage.... at least that's what I read when I was doing my initial installs.....and there is no better reason than piece of mind.... you watch the temps rise and you have the OPTION, to just turn the motor off, before it destroys itself....because if you wait and it destroys itself....not only are you looking to get towed in, youll need a new motor next week!!!! at least if you turn it off....youll have a chance to repair the problem

 

[shooter5636]

After doing some searching, I found the Trail Tech Digital engine temp sensor.... it's a 14mm ring that slips around the spark plug threads and reads head temp. Question though.... if you use this style gauge would you have to cut off the crush washer that comes preinstalled on the plug?

 

[interalian]

I think a sender that reads the spark plug threads is going to show a higher temperature than the rest of the head, maybe more suited to air-cooled motors? I've pointed my IR thermometer at the base of the spark plugs and it shows about 30 degrees hotter than the water jacket, which would be normal as there's no water jacket around the plug threads. Still, it would show overheat if you monitored baseline temperature in normal conditions and watched for rising above that.

 

[Sirrobin53]

Ultralight aircraft use the under spark plug sensors. I believe they are reliable. As stated above, baseline temperature is key.

 

[mlaajanen]

No temp issues. Simply for peace of mind.

Thanks for the input. Are there any brands that are typicall reccomended over others for the under-head-bolt style?

I would get the original one(part number 174682), comes with all that is need to fit it!

 

[bob johnson]

I would get the original one(part number 174682), comes with all that is need to fit it!

actually looking at the package it has several mounting options!!!!!!!!!!!

just in case your outboards head isn't tapped for that 1/8th-27 pipe thread

 

[Sea Rider]

Will any of those mentioned systems shut engine down when an overheat is detected, if not it's useless counting with one.

The only possible way for any 2 strokes engine to develop an overheat is when :

(1) Out of correct carb/advance timing synch.
(2) Bad impeller.
(3) Excessive salt formation or obstuctions on powerhead as to cool less than factory should.
(4) Debris obstucting water intake ports and melting impeller running dry inside liner "at speed"

On points 1,2,3 will see temp gradually rising to be aware of. 4 being the worst case scenario, temp will rise so quick that won't protect engine. The damage will already have been done as from experience.

What did moons back as I'm a impeller champ screwing them due to running OB's on kelpy seas at speed was installing a thermo fuse secured onto cylinder head bridged to CD ground that blew inmediately soon after temp started to rise from normal operating temp. Saved changing a lot of impellers and recurrent powerhead damage.

Happy Boating

 

[Fed]

I've never seen an outboard that will shut off because of an over temperature situation, I think there's a reason for that.

 

[Sea Rider]

Yep, OB's don't shut down, just switches into slow rpm mode when an overheat is detected. The issue with portable 2 strokes carbed OB's is that doesn't have a electronic system to put OB into low rpm operating range when an overheat is detected. The temp kit shown is only a temp gauge as in a car, right ?

Installing a temp gauge on a tiller OB will be a bummer if being bulky, unless going for those electronic compact ones designed to fit on tight spaces.

Happy Boating