Water temp not registering on gauge

[Nick36]

Hey everybody, I'm new to the boating world and posting in forums, so bear with me...

I have a 1998 mercruiser 3.0l and I'm having trouble getting the water temp to read on the gauge. I've checked the gauge by shorting the send terminal to ground and seeing the needle pin to full scale, and I've also put a 90 ohm resistor from the send terminal to ground to simulate approximately 180-185 degF from the water temp sender. Again, the gauge was reading what I expected. I did both these tests locally at the gauge and from the end of the tan water temp sender wire in the engine compartment. Ive also replaced the water temp sender. The problem is, whether I put the muffs on and run it for 10 or 15 mins or take it out on the lake (3 times for 2 hours per trip), the water temp never measures above 100 degF (bottom end of scale) and doesnt move from there. I've read that normal operating temp 175ish?

I guess I should check the thermostat, but not really sure how to determine if it's any good or not. Any suggestions?

Thanks!

 

[Maclin]

Welcome Aboard!

I think 175 would be high. More like 140-150, the thermostat is specced at 140 deg. I would use an infrared thermometer first if you can rent or borrow one, to measure temps at the thermostat housing and other spots around the engine. I have seen some for as little as $20.

Then if the temp gauge is correct and the temps are low, then go after the thermostat. The thermostat housing itself may be missing a check ball or similar inside, the Merc guys here can help as you progress through your troubleshooting.

Good luck!

There are Stickies at the top of the forums, posts that are stuck at the top, with lots of good information and links.

 

[alldodge]

I guess I should check the thermostat, but not really sure how to determine if it's any good or not. Any suggestions?

Put the thermostat in a pan of water o the stove with a cooking thermometer or use a IR temp gun. The thermostat should open real close to 140 degrees (that is if the stat is a 140). If the stat is slightly open when you remove it cold, then its either no good or its hung up by something

 

[tpenfield]

Maybe the thermostat is missing, thus the low temperature?

 

[thumpar]

Nevermind.

 

[Nick36]

Thanks for all the great advice, and based on the manuals I've read, my stat should be the 140degF type.

So I did take apart the thermostat housing and low and behold........nothing (tpenfield called it!). No thermostat. I guess that would explain why the temp was never getting above 100degF, especially here in NS, the water is still pretty bone chilling.

I would venture a guess that wide open is not the ideal running condition for the cooling system in the merc 3.0l? Do some people take them out on purpose? Cant understand why mine would be gone.....

 

[alldodge]

It should always be there to make sure the entire block and heads stay full of water. If stat is not there air pockets can form and cause hot spots. Many think the motor will be better, but every change is a risk

 

[thumpar]

Sometimes people will take them out because they are having overheat problems and think that will fix it.

 

[achris]

The thermostat is required to get the engine up to its operating temperature, the temperature the various components of the engine were designed to operate at. Things like pistons are actually machined at 'room temperature' in such a way that when they heat up and expand they are THEN the right size and shape. Running without a thermostat stops those parts expanding properly. This leads to excessive component wear. Also, the carburetor choke system will still operate as if the engine is not warmed up, and the engine will burn more fuel. This is even more of a concern with injected engines.

For a one off diagnostic test, running without a thermostat is ok, but for sustained operations, not a good idea..

Chris........

 

[Nick36]

Thanks Chris, I'm going to hunt down a new t-stat today!

Nick

 

[Nick36]

Well, I just installed the new t-stat and ran the engine for about 15mins with the muffs on. The temperature climbed to just under 100degF on the gauge after about 10 mins and then stopped. I would expect that's the moment the t-stat opened, but it doesnt make sense on the gauge. Also, the resistance from the sender was around 200 ohms, which for 100degF probably makes sense since my manual says the the resistance should be between 121 and 147 ohms at 140degF. Still, why would it stop at such a low temp? I dont have a temp gun, but the t-stat housing is definitely hot so it seems higher than 100degF but it's not registering on the gauge......kinda confused at this point. Makes me think the sender is bad but its brand new. Any suggestions??

Thanks!

 

[alldodge]

Electric gauges are approximations, and they can be real close or off some. Since you cannot keep your hand on it, then its more then 100 and probably 140 as the stat opened

May need a new gauge to get closer, but IR gun would verify

 

[tpenfield]

wonder if it is a European style sender and a US style gauge or vice versa?

 

[Nick36]

AllDodge, I picked up an IR temp gun and the temp around the t-state housing is in the 140degF range and holding. The gauge reads just under 100degF.

I think Ted might be onto something as i have a faria gauge and a quick silver sender. I think there may be a mismatch?? Going to do some research, but if anybody can confirm it would be much appreciated.

Thanks again!

 

[thumpar]

I doubt there is a mismatch. My boat has faria gauges and stock engine sensors. If the resistance is not correct for what you measure with the IR gun I would think bad sensor.

 

[Nick36]

The sensor is brand new. I replaced it as part of the troubleshooting for this problem. Actually, I put in the old sensor as well and they both read 100degF when the engine is up to temp.

 

[alldodge]

Probably the gauge is just off, could replace or just know that 100 is 140.
The European resistance works the opposite way, it pegs cold when shorted (goes high to low resistance)