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Old 06-04-2023, 08:20 PM   #1
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Engine running cooler than normal

Our 2008 Tiffin Phaeton 36 QSH with Cummins 8.3 ISC engine temp normally runs from upper 190s to 212 when the fan speed increases. I've seen 216 before it drops quickly to the 190s. As soon as we started into Colorado (possible elevation to be the difference) I noticed the engine running about 187 even on long uphill grades. Every now and then it will act like normal and get to the 200s but several times just hanging around 187 +- 4. I'm concerned that the temp sensor is faulty and giving bad info to the engine and not operating the fan appropriately. I find it hard to believe that it would run under 190 even on grades. We are in Cody, WY now so we have been above 4k for a couple of weeks. During a thunderstorm with hail in Colorado, the engine was running in the 150s and the tranny in the 120s. Thats hard to believe. Question: is there any redundant temp sensors to keep the engine from melting down or is there only the one engine temp sensor? Has anyone ever heard of this issue? Usually its always an overheating issue on grades. I did a complete coolant flush, most new hoses, and new thermostat over a year ago. We have driven several thousand miles since then. This just started less than 2 weeks ago.
Thanks in advance!
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Old 06-04-2023, 09:10 PM   #2
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Have you replaced the engine thermostat? Sounds like it is stuck open.
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Old 06-04-2023, 09:41 PM   #3
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Engine running cooler than normal

Check the fan clutch. Sounds like it is stuck in high. When the fan clutch goes bad they default to fully engaged for safety reasons - ensuring the coach does not over heat. Same thing my 2017 Tiffin Phaeton 44OH just had happen. Your local mechanic can test to to make sure it is the fan clutch vs a wiring / fuse issue. Hope that helps.
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Old 06-04-2023, 09:58 PM   #4
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Engine cooler running?

Have you had your gauges checked for accuracy? Do you have a system like Silver Leaf to monitor everything almost? What about the accuracy of you temp sensors? The garage or mfg might have a complaint file on this subject. I wouldnt throw money at this problem, gets very expensive. Im sure you checked with the engine builder yes? You might have an entirely loose temp sensor wire or one that might have grease non conductor on the wire/s not letting it make full contact. Id be very concerned about a gauge reading low. Anyone put anything extra in the cooling system which would read a lower cooling coolant temp?
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Old 06-04-2023, 10:06 PM   #5
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I’d be having every wire and sensor checked for contact and maybe corrosion on those wires as well. Any engine manuals you can look at and follow each wire in the cooling system and trans wire from end to end. The Ohms values should be the same from one end to a connection. Your gauges are lying to you! That’s an expensive engine you have so please don’t ignore this important issue. The engine maker may have technical bulletins that haven’t amounted to a recall … YET. It’s going to be all your fault but you know that if the engine blows in one area or everywhere!
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Old 06-05-2023, 08:27 AM   #6
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Originally Posted by Ray,IN View Post
Have you replaced the engine thermostat? Sounds like it is stuck open.
Yes, I replaced the thermostat about a year ago when I did a complete coolant flush and replaced several hoses. We have driven it a few thousand miles since then.
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Old 06-05-2023, 08:29 AM   #7
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Originally Posted by rvrrat View Post
Check the fan clutch. Sounds like it is stuck in high. When the fan clutch goes bad they default to fully engaged for safety reasons - ensuring the coach does not over heat. Same thing my 2017 Tiffin Phaeton 44OH just had happen. Your local mechanic can test to to make sure it is the fan clutch vs a wiring / fuse issue. Hope that helps.
Ii never did hear the "roar" of the fan when it was sitting on 187 degrees. I have always heard it start "roaring" around 212. I am thinking that the fan is not speeding up because it is getting information that it is only at around 187 degrees.
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Old 06-05-2023, 08:31 AM   #8
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Check the fan clutch. Sounds like it is stuck in high. When the fan clutch goes bad they default to fully engaged for safety reasons - ensuring the coach does not over heat.

Disagree. The thermostat is still the "determiner" of water flow to the radiator. Excessive air flow through the radiator will not change that.


BUT, are you sure it is actually running that cool VS temp sensor/gauge issue?
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Old 06-05-2023, 08:39 AM   #9
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Have you had your gauges checked for accuracy? Do you have a system like Silver Leaf to monitor everything almost? What about the accuracy of you temp sensors? The garage or mfg might have a complaint file on this subject. I wouldnt throw money at this problem, gets very expensive. Im sure you checked with the engine builder yes? You might have an entirely loose temp sensor wire or one that might have grease non conductor on the wire/s not letting it make full contact. Id be very concerned about a gauge reading low. Anyone put anything extra in the cooling system which would read a lower cooling coolant temp?
I do not have the silver leaf. I do have the round "idiot" temp gauge and the LCD display that allows me to choose 3 things to monitor at a time. I monitor oil pressure, engine temp and tranny temp. Both the LCD monitor and the round gauge seem to give the same information. Therefore, I am assuming that there is only one engine temp sensor that both the LCD and the round gauge get their information from.

I have not contacted the engine builder yet. We are in Cody, Wyoming (2k miles from home) and the nearest Cummins service center is a little over 100 miles away. I really didnt know if this would be a Cummins issue, Freightliner, or Tiffin seeing how it could be in the controls not directly related to the engine.

I would think that if a connection was loose, it would jump around all over the place and even sometimes show "0" degrees. The temp has always been steady, just cooler than I think it should be.
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Old 06-05-2023, 08:39 AM   #10
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Disagree. The thermostat is still the "determiner" of water flow to the radiator. Excessive air flow through the radiator will not change that.
?

Not necessarily so. There is always a thermostat bypass built into the cooling system and larger diesel engines have a significantly larger bypass than your pickup for example. Thus you see radiator covers on some big trucks in winter months to help bring them up to a higher operating temperature where they operate more efficiently.

I agree one needs to verify the temp gauge is accurate.
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Old 06-05-2023, 08:44 AM   #11
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Originally Posted by wolfe10 View Post
Disagree. The thermostat is still the "determiner" of water flow to the radiator. Excessive air flow through the radiator will not change that.


BUT, are you sure it is actually running that cool VS temp sensor/gauge issue?
I do not know if it is running that cool or not, but we have pulled some grades that would always in the past, run the temp up to 212 or so then the "roar" of the fan would come on and quickly drop the temp back down to the 190s. I never heard the fan when it was hanging around 187 even so we were climbing moderate grades.

Both the round gauge and the LCD monitor coincide with each other making me believe they get their information from a single sensor. I would hope that their is a redundant sensor somewhere but maybe not.
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Old 06-05-2023, 08:53 AM   #12
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A little more info on our last travel day:

We ran into hail during a thunderstorm to the point the road was milky white and the engine temp dropped to the 150s and the tranny temp in the 120s. Our tire temp sensors dropped from the 70s to the 40s. After the roads cleared up, everything went back up to engine: 187, tranny: 160s, tires: 70s. That got me wondering if the air temp dropped so much that the engine actually was running that cool. But, we have driven in close to freezing temp before and it still would climb to the 200s on grades (which I would expect).

Part of the travel day, the temps were normal: engine around 197, climbs to 210-214 on grades, the fan comes on and it quickly drops to the 190s and stays there until the next hill.

One thing to note: BOTH the engine and tranny were running "cooler" together or normal together. Obviously they are different sensors. That is the puzzling part for me. But ambient temps have been in the 60s and 70s on our last couple of travel days with elevations from 4500 to 6300.
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Old 06-05-2023, 01:09 PM   #13
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I contacted Cummins Tech Support and he said that the engine has its own temp sensor that is controlled by the computer and that the chassis manufacturer would be the one to install the additional temp sensor that goes to the gauges. I still dont have a warm fuzzy because I never heard the fan go to high speed while climbing hills, but I could have missed it. We will be leaving Saturday moving towards Glacier NP, and we will be closely listening to what is going on.

Question, where would be the best place to use an IR temp gun to compare the temp with what the dash says? I would like to check it at rest stops just to have that info.
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Old 06-06-2023, 06:58 AM   #14
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I would get the temperature with the IR gun by checking at the metal stand pipe coming off the top of the engine thermostat. It measured very close to my dash gauge temp. when I replaced the engine thermostat and coolant.
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