Community Forums

dawoody · 05-07-2025, 12:13 PM

Our 2012 phaeton / freight liner Cummins 8.9l doesn’t seem to run warm enough. The only time it gets up into the center of the temp gauge is if you’re doing a long hard pull on a hot day and then only until you level out. Mostly runs at the first line above the blue cold area. Is this normal or does it need a thermostat

m0v1 · 05-07-2025, 12:51 PM

The sender or gauge could be the problem. I would take actual measurements before and after the thermostat housing using an IR gun or thermal cam before doing much of anything else.

shepardmike · 05-07-2025, 12:53 PM

Maybe thermostat. Either way it this can lead to poor fuel efficiency, increased emissions, and reduced performance etc. I would get it looked at.

wolfe10 · 05-07-2025, 12:58 PM

Quote:

Originally Posted by m0v1

The sender or gauge could be the problem. I would take actual measurements before and after the thermostat housing using an IR gun or thermal cam before doing much of anything else.

Yup, you need to find out if this is an actual coolant temperature issue or issue with sender unit/gauge.

Then you can address the real issue.

bpu699 · 05-07-2025, 01:00 PM

My cat 3126 gauge is the same. Its in the bottom 1/4 the entire time, on a heavy slope hits the middle of the gauge.

Got the electronic reader, and temps were in the correct range... gauges just set to read low...

**Lt Dan** · 05-07-2025, 01:48 PM

Agree that you need to find out the actual temperature, not just the gauge. Your dash display should be able to show coolant temp, or you can add something simple like a ScanGaugeD to monitor the temps.

ernieh · 05-08-2025, 11:30 PM

Your LBCU has the ability to read out the actual temp detected as well as a test feature for the gauge.

shootist · 05-08-2025, 11:56 PM

Cool is good as long as it's up to operating temperature. Heat destroys engines. The newer stuff runs way too hot. It's all to supposedly lower emissions.

wolfe10 · 05-09-2025, 07:50 AM

Quote:

Originally Posted by shootist

Cool is good as long as it's up to operating temperature. Heat destroys engines. The newer stuff runs way too hot. It's all to supposedly lower emissions.

Yes, heat destroys engines.

But, so does running too cold (like if the thermostat is stuck wide open).

You want (and need to prioritize) having your engine run at the proper/manufacturer's recommended operating temperature.

Said another way, having an engine run too cold is NOT something to aspire to.

Model A Man · 05-13-2025, 11:05 AM

I have a 5.9 12 Valve in a Dodge truck and it never gets up to where I would like it except (as stated above) it is really called on to do some serious work. Ideal temperature for a diesel is in the 200 degree area (hence, 195 degree t’stats). Too cold is obviously better than too hot but it’s not real good either. I have replaced the t’stat, I have 2 temp gauges (which agree). Some guys complain that they can’t even get anything out of their heater when the weather is cold. I blocked off a big portion of the radiator which helped some but not much. Bottom line is that the motor should be running at least 180 and preferably 200-210 but it doesn’t and I am out of ideas about what to do about it. It is hard to explain why a motor with a 195 t’stat in it can run at 140-160 but it does

Elictrix · 05-14-2025, 05:02 PM

I would check the accuracy of your temperature gauge.

Remove the sensor from the engine.
Run a couple of wires to the sensor, one from the vehicle wire to the sensor terminal, and the other from a good engine earth to the body of the sensor.
Hang the sensor in a jug with the terminal above the water level.
Connect the jug to power and bring up to boiling point (212F/100C).
Check what the gauge is reading.

If you have an accurate thermometer, you could also check the gauge calibration and various temperatures.

shootist · 05-14-2025, 05:23 PM

Quote:

Originally Posted by Model A Man

I have a 5.9 12 Valve in a Dodge truck and it never gets up to where I would like it except (as stated above) it is really called on to do some serious work. Ideal temperature for a diesel is in the 200 degree area (hence, 195 degree t’stats). Too cold is obviously better than too hot but it’s not real good either. I have replaced the t’stat, I have 2 temp gauges (which agree). Some guys complain that they can’t even get anything out of their heater when the weather is cold. I blocked off a big portion of the radiator which helped some but not much. Bottom line is that the motor should be running at least 180 and preferably 200-210 but it doesn’t and I am out of ideas about what to do about it. It is hard to explain why a motor with a 195 t’stat in it can run at 140-160 but it does

Those 200 degree temperatures are for emissions. My 2004 isl has a 180 thermostat. Going way back we ran 165's. Maybe sounds too cool but a half a million mile motor was the low end and a million miles the high end. They don't last like that anymore.

Jzack · 05-14-2025, 05:50 PM

Quote:

Originally Posted by Model A Man

I have a 5.9 12 Valve in a Dodge truck and it never gets up to where I would like it except (as stated above) it is really called on to do some serious work. Ideal temperature for a diesel is in the 200 degree area (hence, 195 degree t’stats). Too cold is obviously better than too hot but it’s not real good either. I have replaced the t’stat, I have 2 temp gauges (which agree). Some guys complain that they can’t even get anything out of their heater when the weather is cold. I blocked off a big portion of the radiator which helped some but not much. Bottom line is that the motor should be running at least 180 and preferably 200-210 but it doesn’t and I am out of ideas about what to do about it. It is hard to explain why a motor with a 195 t’stat in it can run at 140-160 but it does

Did you happen to put the thermostat in upside down? Don't laugh, I actually did this on my 5.9 and it does exactly what yours is doing. Maybe worth having a look.