Low operating temperature in a Mercury 90

[J*o*h*n]

To Whom It may Concern: I am new at forums so please correct me if I do something wrong.

I have a 2002 Mercury 90 two-stroke with three carburetors. Last season I installed an automotive-type electric temperature gauge. I mounted the sending unit in the 1/4 inch pipe plug at the top of the cylinder head. After the engine was running for a few minutes the gauge would not read and the minimum on the gauge is 100F. As near as I can tell, the powerhead is running well below 100 degrees F. I can touch the block and it is luke-warm and the telltale stream is just barely warm to the touch.

I replaced the old thermostat with a new one rated at 120F and the gauge still will not read and the telltale stream is still slightly above the ambient lake temperature. Factory spec. on normal operating temperature is 5 degrees over stamping. I assume that means the temperature stamped on the thermostat. The warning horn is supposed to sound at 180F

Also, I tested the gauge and it works. I removed the sending unit and jumped it to ground and then dropped it in a cup of hot water. It indicated approx. 140F on the instrument panel.

Theoretically, I should be showing about 125F on the gauge. Whats going on here?

I realize most people complain about running too hot, but I am hoping I can get better fuel mileage if I can raise the normal operating temperature to at least 120F. Any advice or information would be appreciated.

John 2-18-2010

 

[Silvertip]

Re: Low operating temperature in a Mercury 90

Are you using a sender matched to the gauge. Outboard temp senders are typically in the range 33-240 ohms. Those senders will not likely match the auto style gauge. Auto gauges are typically calibrated 120 - 260 degrees or so which is why they don't work well in an outboard application. A good marine outboard gauge (hard to find any more) is calibrated with 60 degrees on the bottom end. 100 degrees would be a reasonable minimum. The thermostat on an outboard controls idle temperature. As the engine speeds up and load increases, the bypass valve opens to dump more water through the engine. There is a possibility that the bypass valve has stuck open thus allowing water to bypass the thermostat.

 

[Texasmark]

Re: Low operating temperature in a Mercury 90

I have that exact engine and year. My OEM stat was (as was the replacement) stamped 143F for opening temp. Normal setting on a US domestic hot water heater in the home is 140F for an example of what the outlet temp should feel like after opening (and remaining open).

The pee comes off the exhaust manifold directly so you don't have to wait for the stat to open to get water like on some engines (as you already surely know). Therefore the water may seem cool before it has had time to warm up.

With it being winter time and all and depending upon usage, and with a 120 stat, your problems could be normal. However, like Silvertip said, your instruments need to be matched to provide a reliable reading.

My usual 2c.

Mark

 

[Faztbullet]

Re: Low operating temperature in a Mercury 90

Install a water pressure guage as readings are instant and you can moniter the wear on pump. Also it will tell you instantly if you run over a ice bag, a temp guage and alarm reacts to slow...

 

[J*o*h*n]

Re: Low operating temperature in a Mercury 90

Are you using a sender matched to the gauge. Outboard temp senders are typically in the range 33-240 ohms. Those senders will not likely match the auto style gauge. Auto gauges are typically calibrated 120 - 260 degrees or so which is why they don't work well in an outboard application. A good marine outboard gauge (hard to find any more) is calibrated with 60 degrees on the bottom end. 100 degrees would be a reasonable minimum. The thermostat on an outboard controls idle temperature. As the engine speeds up and load increases, the bypass valve opens to dump more water through the engine. There is a possibility that the bypass valve has stuck open thus allowing water to bypass the thermostat.

I got this name brand temp gauge in an auto parts store for around $60 and it was a matched set. My boat uses a 12 volt system so I could not think of any reason why I could not install it in a boat. This is an old fiberglass boat, so I had to run a floating ground from the instrument panel back to the block. I think I'm just running to cool for the gauge to register as the minimum on the gauge is 100F. I am wondering about the bypass valve you mentioned. Is that bypass valve the same as what some people call the poppet valve? Also I am wondering if somebody has an exploded view of a poppet valve for this 2002 carbureted Merc two cycle. John 2-21-2010

 

[Silvertip]

Re: Low operating temperature in a Mercury 90

Its right under the thermostat as shown. And just to clarify, a water pressure gauge has absolutely no bearing on water temperature. The gauge merely tells you the pump is working. An engine can still overheat and run too cool with perfectly normal pressure.

 

[J*o*h*n]

Re: Low operating temperature in a Mercury 90

Thank a lot. Now I won't get any sleep at all worrying about my engine. Seriously, to answer "Faztbullet", I had never even considered the prospect of having a ice bag, or any plastic garbage bag wrapped around my lower unit to block my raw water intakes. Now I can understand the importance of a water pressure gauge. Problem is that I'm not a gauge-watcher, so I would need a red flashing light to alert me to pressure loss.
John 3-8-10

 

[4JawChuck]

Re: Low operating temperature in a Mercury 90

I have a 2003 90Hp Merc and also have an automotive gauge installed in the head, it rarely exceeds 100F unless I let it idle for extended periods. It has a new thermostat also which didn't change a thing.

The water temp here rarely exceeds 70F even in the depths of summer and was told this is totally normal.

Don't worry about it.

 

[achris]

Re: Low operating temperature in a Mercury 90

You can download a service manual for your engine HERE. The password for the file is ribforum. You'll find a diagram of the poppet on page 4-56.

The temperature indicated on your gauge will vary widely depending on where you mount the sender. Some parts of the cylinder head are very cool and even at high rpm don't get very warm... Sounds very much like your engine's running normally...

Chris........

 

[n2wly]

Re: Low operating temperature in a Mercury 90

I have a 2003 90Hp Merc and also have an automotive gauge installed in the head, it rarely exceeds 100F unless I let it idle for extended periods. It has a new thermostat also which didn't change a thing.

The water temp here rarely exceeds 70F even in the depths of summer and was told this is totally normal.

Don't worry about it.

Hello I do realize this is a very old post I’m writing it in the hopes you still around to read it. Would you have a picture of the gauge and how did you wire it when you screw it in that would be the ground the other wire runs back to one side of the gauge while the other side of the gauge goes to ground. I have the same engine and would like to do the same.

 

[Texasmark]

Re: Low operating temperature in a Mercury 90

I have a 2003 90Hp Merc and also have an automotive gauge installed in the head, it rarely exceeds 100F unless I let it idle for extended periods. It has a new thermostat also which didn't change a thing.

The water temp here rarely exceeds 70F even in the depths of summer and was told this is totally normal.

Don't worry about it.

Idling for extended periods , or running below 2500 RPM (Serv. Man. #) allows the T stat to control block temperature. Above that RPM, the larger diameter Poppet Valve opens and the block temp runs just a few degrees above ambient surface water temp.

 

[Dave1027]

This is what I have connected to my 100 hp 4 cylinder. Works perfect with the built-in sending unit.
Faria Euro Black 2" Cylinder Head Temp Gauge (60 - 220 F) 12819

 

[Prof T]

This is what I have connected to my 100 hp 4 cylinder. Works perfect with the built-in sending unit.
Faria Euro Black 2" Cylinder Head Temp Gauge (60 - 220 F) 12819

I'd like to add the same gauge to my 2016 Mercury 90 Fourstroke. There is presently no dash temp gauge. Is the wiring already included in the harness and not connected or do I have to run additional wiring to existing sending unit?

Thanks....new to forum

 

[alldodge]

Welcome, and note not ok to post same question in 2 areas

Gauge is pretty simple, Connect sender to sender connection on gauge.
Connect 12V and ground to gauge connections and its done.
Do use a 12V connection that is switched ON when key is turned to ON

 

[jbcurt00]

Closed
@Prof T started a new topic