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57 Running A little Warmer Than Normal

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eriefeelin

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"I have a 1990 MerCruiser 260

"I have a 1990 MerCruiser 260 that seems to be running a little warmer than normal. The impeller, water pickup hose, and thermostat have all been replaced. The temp gauge reads from Cold to Hot with just 5 markers between them.It's reading a bout in the middle. The gauge is the original I wonder if it could be going bad. Any Ideas?"
 
Get a Laser Thermal Temp Gun t

Get a Laser Thermal Temp Gun to check the temps all over the engine so you know what the dash guage means and the rest of the engine is doing temp-wise.
 
I removed the hose that comes

I removed the hose that comes from the transom and it appeared to me that I had good water flow coming from the impeller. I removed both hoses from the thermostat housing and the poppet ball assembly looked alright to me. I removed the circulating pump and it looked ok to me and reinstalled it. I have no obstructions as of yet and the hoses look good.
I used a laser thermometer on various parts of the cooling system and exhaust parts.
The temp. sender showed a reading of 144* at 1100 rpm and about 154* at 700 rpm.
The inlet for the circulating pump was around 156*
The hoses that go to the exhaust manifolds were around 162*
Now the strange part-the top of the starboard side riser was around 100*-102* and the port side was 130*-132* and was noticeably warmer to the touch.
The exhaust manifolds were 180*-192* with the port side running about 10 degrees warmer
I held the laser on the sending unit and when I increased the rpm's from 700 to 1400 it dropped from 155* to 145* in a matter of about 15 seconds. Even though the impeller replaced in the spring I think it may be going bad already. The flow of water at idle was a little weak and as i throttled it up a bit the flow was much stronger. Any ideas?
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I would not be concerned about

I would not be concerned about your temps. They look very good to me.

As far as why your guage is "not" reading correctly there may only be 2 reasons.

The sending unit is out of spec or your guage.

The sending unit is a varaible resister.

Its range is between ~ 60 - 80 ohms to ~ 240 ohms.
If you have a digital meter try taking an ohm reading while it is cold and warming up. Remember that if is has a range of 100 degrees to 300 on a guage then 1/2 the range would be 200.
So at ~ 240 ohms = 100 degrees and 70 ohms = 300 then 200 would equal ~ 155 ohms
So a motor running at ~ 150 degrees the ohm reading should be ~ 110 ohms. This is approximate.

Also make sure the threads of the sending unit and the corresponding threads it screws into are clean because this is the sending units ground!!

If you are getting a reading of ~155 ohms (200') then your sending unit is defective because your laser gun said it is 150ish degrees (110ohms) wich is perfect.

Do not sweat the other numbers. Nothing wrong with them.

Also try to get a guage with numbers on it for future use if possible.
 
What thermostat did you put in

What thermostat did you put in?

it should have been a 143 degree.
 
Double check the threaded part

Double check the threaded part of the sender.
Also check the back of the guage for good clean and tight connections.

If your ground is weak it will give a bad or wrong reading.

Only other thing to try is a different guage.......
 
I tightened the wires on the b

I tightened the wires on the back of the gauge. It still read about the same. What's the chance of the gauge going bad although I rather doubt it. When I pull the boat for the winter I'll probably put another new impeller in it. Thanks
 
"If I put the laser thermomete

"If I put the laser thermometer right on the temp. sending unit and say it reads 155 degrees there, should my gauge on the dash be reading 155 degrees. The gauge on the dash just reads from hot to cold so I'm trying to get an idea."
 
Update. I finally had a chance

Update. I finally had a chance to get the boat out on the lake today. Before doing so I cleaned up the poppet ball valve assembly.After coming down off of plane with the gauge on the dash reading close to 170 degrees I put the IR gun right on the brand new sending unit and it read from 136-145 degrees while running about 1300 rpm. The 170 reading on the dash gauge hardly moved if at all.
The elbows had a reading of about 125-135 degrees with the post side a tad higher. The exhaust manifolds had a reading of 185-200 degrees.
When I installed the new thermostat I cleaned both mating surfaces good and used a new gasket with the 4 copper rivets in it that serve as a ground. Before I replace the gauge on the dash as it's 19 yrs. old I might try adding a ground wire right to the sending unit and see if that makes a difference.
 
"I put the IR gun right on

"I put the IR gun right on the brand new sending unit and it read from 136-145 degrees while running about 1300 rpm."

That is correct w/the 143 T'stat. Your gauge has to be off.
 
I replaced the gauge and all i

I replaced the gauge and all is well. Now it reads around 150ish while idling at the dock. Need to run it out on the lake to make sure.
 
Its good that you posted a fol

Its good that you posted a follow up. It helps when guys are searching a similair problem and can see what the outcome for a particulair post is.
Chris
 
I tested the old gauge and it

I tested the old gauge and it tested out just fine. I decided to try a new one anyways and I'm glad I did. This thing was driving me nuts. I'm glad I didn't keep putting money in it chasing a problem that wasn't there. My parts counter guy said gauges seldom go bad. You never know.
 
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