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Old 04-06-2017, 10:39 AM   #1
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Engine thermostat

I have a question regarding the engine thermostat in my Cummins 300 ISB in our 2006 Holiday Rambler 34' unit. We bought it used with just over 11,000 miles and it now has just over 19,000 miles. It has always registered about 3/4 of the way over to hot on the engine temp gauge. It has never run hot even on long pulls up hills maybe ticking over just off of the 3/4 peg. Due to the age in years not miles I think I will replace the thermostat. I don't know what temperature rated thermostat is in it and I know some will say it needs to run on the hot side but I want to put in one that opens at a lower temp rating and keeps a lower temp. I would like more margin on the gauge than all the way over to 3/4. I would appreciate hearing from you folks on this issue.
Thanks,
Bill
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Old 04-06-2017, 11:29 AM   #2
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A lower thermostat won't give you what you want...as far as margin.
The radiator size and fan determine how hot the engine will get under load...if you are looking for margin.

A thermostat that opens at a lower temp will allow the engine to run at a lower temperature....but at this time, the stat is basically open (or almost open).

When the engine is under load, the stat is fully open and the size of the radiator determines how hot the engine can get. that is why they recommend a higher engine speed when climbing...so the fan runs faster and puts more air through the radiator. All of this is with a fully open stat.

I don't think you should go by gauge position...how accurate is the gauge?
Check it with a thermometer or temp gauge that reads the temperature at the water jacket...and then see what the mfg recommends to operating temperature.

Maybe you are fine now.

Dan
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Old 04-06-2017, 11:45 AM   #3
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My 2007 ISB 6.7 gauge reads a little less than 3/4 of the way since I purchased it in 2009 (4,000 miles).

Last fall I flushed the radiator and replaced the thermostat with one that when the temperature rises above 195 it supposedly opens quicker.

Last week was my first trip since the change, didn't see any difference.

I clean the CAC/Rad every year, so I guess I have what I have and need to slow down and keep engine speed up when going up a grade.

I have attached the invoice for the replacement thermostat.
Attached Files
File Type: pdf NW023886.pdf (66.7 KB, 20 views)
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Old 04-06-2017, 01:20 PM   #4
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My 07 Neptune with ISB 5.9 Cummins runs about half way up on dash gauge. My dash gauge is about useless. I have a Banks tablet display. It shows the engine runs about 195 most all the time. Maybe on a long climb in dead of summer I may hit 203. That's if I run it hard.

I don't think you want the engine running any cooler that factory setting. You'll get less fuel mileage and not good for engine I'm told. Maybe call Cummins with your engine serial # and see what they say.

Chad
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Old 04-06-2017, 06:53 PM   #5
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I agree with Dan. Those gauges don't tell you very much and are not very accurate.

Get this, it's one of the best purchases I made for my RV.
VMSpc | SilverLeaf Electronics, Inc.

And this to take a reading off the thermostat housing.
Non-Contact Infrared Thermometer With Laser Targeting

Bill
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Old 04-06-2017, 07:04 PM   #6
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Quote:
Originally Posted by TR4 View Post
I agree with Dan. Those gauges don't tell you very much and are not very accurate.

Get this, it's one of the best purchases I made for my RV.
VMSpc | SilverLeaf Electronics, Inc.

And this to take a reading off the thermostat housing.
Non-Contact Infrared Thermometer With Laser Targeting

Bill
X2 on the VMSpc, a great system with the ability to provide diagnostics and audible alarms.

Also suggest you join the Cummins Quickserve site, it is a wealth of information on the cummins engine specific to your SN.

https://quickserve.cummins.com/info/index.html
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Old 04-06-2017, 08:44 PM   #7
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Quote:
Originally Posted by beboboy9 View Post
I have a question regarding the engine thermostat in my Cummins 300 ISB in our 2006 Holiday Rambler 34' unit. We bought it used with just over 11,000 miles and it now has just over 19,000 miles. It has always registered about 3/4 of the way over to hot on the engine temp gauge. It has never run hot even on long pulls up hills maybe ticking over just off of the 3/4 peg. Due to the age in years not miles I think I will replace the thermostat. I don't know what temperature rated thermostat is in it and I know some will say it needs to run on the hot side but I want to put in one that opens at a lower temp rating and keeps a lower temp. I would like more margin on the gauge than all the way over to 3/4. I would appreciate hearing from you folks on this issue.
Thanks,
Bill
Bill replace it with one the same temperature as what is in there now. Have you cleaned the CAC and radiater? Do you down shift on hills to sped up the fan and get more air flow? Cheack and see (as outhers have said) what temp it is running at now.
Let us know what you find.
Bill
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