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Old 10-23-2020, 11:43 PM   #1
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Engine running cool

We just took a trip from the Syracuse Utah area to northeast Oregon and back and my cat 300 horse power C7 engine ran so cool that it never read above halfway between cold and the middle of the gauge.

The heat coming out in the cab area was not very warm and it was hard to keep it warm for my wife and I. We ended up running the main furnace quite a bit. On the way back we got 10.58 mpg which was probably pretty good but I think if the engine was running a little hotter it would also do better on mileage.

Any suggestions on what I should do to bring my engine temperature's up in cooler weather??

Also should I be looking for the temperature to be at the halfway mark?
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Old 10-23-2020, 11:51 PM   #2
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Probibly the thermostat is stuck open
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Old 10-24-2020, 12:12 AM   #3
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To answer any questions on gauge accuracy , we'd need info on you coaches year and chassis manufacturer .Chassis manufacturer usually supplies the gauge package to the coach builder.

I've been in a similar situation 20 f coming down from Jackpot NV to Vegas . On an icy road , only able to do 40 mph due to conditions , the engine just would not warm up , and using the defrost just made things worse .
Defrost in my side rad coach, runs the dash A/C and puts the hydraulic engine cooling fan on high speed .
We had the furnace running and , found the only way to get any heat out of the engine to the dash , was to run in 4th gear , keeping the RPM up.
BTW: I know the engines temperature " regulators " ( Cat's name for thermostats ) are good because the were only a year old at the time and when ambient temps are 75+ the engine runs at a steady 185 .

JMHO: If the engine operating temps are normal ( 185 ) when your driving at highway speeds in 80 degree weather .I don't think replacing your regulators would improve the cold weather heater performance.
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Old 10-24-2020, 12:08 PM   #4
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We have the 2000 Tiffin Allegro Bus 37M with the 300 hp Cat engine and it runs beautiful with just over 60,000 miles on it, but it really seems to run too cool... Is the thermostat sticking wide open common??
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Old 10-24-2020, 12:13 PM   #5
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Thermostats ARE a wearing item (metal on metal).


It is not uncommon to need to replace them.


BTW, Caterpillar calls them REGULATORS. Some Caterpillar engines have one, some two. Be sure to get the gasket as well.


If you have access to the top of your engine, easy to replace after draining out a couple of gallons of coolant. Strongly suggest replacing when you change coolant as preventive maintenance.
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Old 10-24-2020, 12:16 PM   #6
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10 MPG? Wow, I would do cartwheels across the truck stop if I had those numbers! Usually down in 7s.

Should see the temp needle move around a bit.

In my 2005 Tiffin with rear radiator & Cat C6 I see the needle move from bottom of "Normal" as marked on the gauge to middle of Normal.

Engine speeds above 2000 RPM will actually cause the needle to drop down to lower range of Normal.

Little to no dash heat is ever felt. Would think the heater core would get the coolant prior to radiator to compensate for the long distance it flows.
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Old 10-24-2020, 12:37 PM   #7
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Quote:
Originally Posted by PastorB View Post
We have the 2000 Tiffin Allegro Bus 37M with the 300 hp Cat engine and it runs beautiful with just over 60,000 miles on it, but it really seems to run too cool... Is the thermostat sticking wide open common??
Our 3126Bs ( C-7 wasn't the series of the 7.2l Cat engine till 2004? after many modifications) have a pair of temp regulators and Cat says to replace both every two years ( I believe that would apply to truck use of the motor ) . There have been several cases where one regulator stuck closed and the engine would , overheat under load , but maintain normal operating temps when cruising at lower RPM. So nothing to say that one of the two couldn't stick open and have the engine not reach full operating temp. Particularly in cool/cold ambient temps.
Only way , I can think of to check for correct opening of the regulators and the gauge accuracy at the same time , would be to get access to the regulator housing with a laser temp gun , when the gauge is at it's highest reading . That could prove problematic , because by the time you've slowed and stopped the coach and accessed the engine , it's already cooler than it would be on the highway.
Cat P/N for the items on the 3126B
Regulators ( 2 ) 115-4223
Housing gasket 136-0812
It's been a number of years since I purchased these parts , so the numbers may or may not be any good. When ordering Cat parts always have your engine serial number handy.
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Old 10-24-2020, 01:04 PM   #8
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Quote:
Probibly the thermostat is stuck open

In a modern diesel engine cooling system, the thermostat is supposed to be "stuck open". Typically it opens at around 165 and stays that way, with the temperature controlled by fan speed. The simplest systems might use a viscous fan clutch or similar, but electronic control systems more likely switch the fan on/off or have a speed controller to select different speeds. The a/c compressor clutch may also be linked to fan control so that the fan runs whenever the compressor is engaged.


Don't know how the cooling system in PastorB's coach is designed - cooling and fan control is as much a chassis function as engine. Merely knowing it is a Cat C7 doesn't give all the answers.
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Old 10-24-2020, 01:08 PM   #9
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To get your correct engine series and it's serial number , look for a decal on the valve cover .As per photo 1.
If your coach is on a Freightliner chassis, and your gauges look like the second photo , this pdf on gauge testing , may provide a number for the reading your seeing on the gauge.
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Old 10-24-2020, 01:24 PM   #10
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Quote:
Originally Posted by Gary RVRoamer View Post
In a modern diesel engine cooling system, the thermostat is supposed to be "stuck open". Typically it opens at around 165 and stays that way, with the temperature controlled by fan speed. The simplest systems might use a viscous fan clutch or similar, but electronic control systems more likely switch the fan on/off or have a speed controller to select different speeds. The a/c compressor clutch may also be linked to fan control so that the fan runs whenever the compressor is engaged.


Don't know how the cooling system in PastorB's coach is designed - cooling and fan control is as much a chassis function as engine. Merely knowing it is a Cat C7 doesn't give all the answers.

Let's look at this:


Thermostats begin opening at a pre-set temperature. As engine temperature rises they go to full open. They work between full closed and full open to maintain the manufacturer's desired operating temperature.


A properly set up fan (viscous hub, hydraulic controller, etc) in a DP should be on low speed (need air flow over the Charge Air Cooler even when coolant is not over-temp).



It should not go to high (or gradually ramp up on some hydraulic systems) until the thermostat(s) is/are fully open. If they did go to high fan speed while the thermostat(s) are able to control engine coolant temperature, you would be WASTING HP and fuel!
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Old 10-24-2020, 02:04 PM   #11
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Before I jumped into messing with thermostats, I would get a mechanical gauge on the engine and see what temp you're actually running. The transmission temp gauge on my 2005 Monaco Diplomat NEVER went past a quarter in 60 degree weather or 100 degree weather. When I eventually installed the SilverLeaf VMSpc program, I could see that all of my temps and pressures were exactly where they were suppose to be.
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Old 10-24-2020, 04:35 PM   #12
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  • The temperature gauge for my Caterpillar C7 engine normally has the needle pointing toward the "N" in Normal. According to the file Skip426 posted ( ) that indicated 190+- degrees.
  • This agrees with the Medallion information display which shows my engine temperature cycles from 185 to 195 degrees. The highest I have ever seen it go was 205 degrees.
  • If you have the Medallion information center, you can set it to constantly display the engine temperature.
  • We often have to run the propane furnaces in conjunction with the dashboard heating system.
  • My Cat C7 engine has a direct drive fan. There is no viscous fan clutch or similar or any electronic control systems to switch the fan on/off. Just simply a fan belt and a BIG fan.
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Old 10-24-2020, 04:39 PM   #13
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Quote:
Originally Posted by Gary RVRoamer View Post
In a modern diesel engine cooling system, the thermostat is supposed to be "stuck open". Typically it opens at around 165 and stays that way, with the temperature controlled by fan speed. The simplest systems might use a viscous fan clutch or similar, but electronic control systems more likely switch the fan on/off or have a speed controller to select different speeds. The a/c compressor clutch may also be linked to fan control so that the fan runs whenever the compressor is engaged.


Don't know how the cooling system in PastorB's coach is designed - cooling and fan control is as much a chassis function as engine. Merely knowing it is a Cat C7 doesn't give all the answers.
I'm not familiar with it either, but I would try to discover what it is supposed to be and then accurately measure it somehow prior to doing any repairs (not even sure what type of OBD-like devices work with that engine).

As to dash gauges many GM vehicles basically have dummy gauges. They'll move to showing normal temperature when it's still 50 degrees cooler, and still not move when it's 25 degrees warmer. I'm not even sure what it takes to move the gauge on my truck towards the hotter ranges.
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Old 10-24-2020, 08:13 PM   #14
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Quote:
Originally Posted by Bigd9 View Post
[LIST][*]The temperature gauge for my Caterpillar C7 engine normally has the needle pointing toward the "N" in Normal. According to the file Skip426 posted ( ) that indicated 190+- degrees.
Pete; the file I posted , was for the gauge package, prior to the setup in your coach . Check the dates on page one.
The VDC ( VDU in some documents ) was only used till mid 03 production then switched the gauges and info center you have; MMDC used till 2007 .
After they switched to the LBCU . and I don't know any dates on that , or if it's still in use.
RV Tech Library - Freightliner MMDC Information Display
RV Tech Library - Freightliner LBCU Information Display
The display you've pictured ,and the rest of the gauges, was huge improvement, over the VDC package. .
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