Why engine stays so cool?

[CountryFit]

my isc 350 engine temp had been at around 165 degrees on flat roads, occasionally touched 180 deg on climbing hills. to correct it, i replaced the t-stat two weeks ago. well, on the first few hundred miles i did see stable 180 degrees from eugene to seattle (almost flat isn't it?). but the following 600 hundred miles were at 165 deg again. the chance of both t-stats being wrong is slim i think.

i observed that the side radiator fan was in low speed when engine just started. since the t-stat would be closed before 180 deg, the speed of the fan won't matter to the coolant temp. it seems the fan speed control module isn't the gremlin.

so what could it be? any comments/suggestions are appreciated.

[jleamo1]

Countryfit, how are you monitoring your temps? My ISL runs cooler than expected also. It has a 180 degree thermostat and on a cool day will hold at 175 degrees. 99 degrees out in the sun pulling a hill I cant hit 200. No complaints I also found it odd.

[96 Wideglide]

Were you running dash air? Believe that can put your side rad fan on hi-speed.

[CountryFit]

Originally Posted by jleamo1

Countryfit, how are you monitoring your temps? My ISL runs cooler than expected also. It has a 180 degree thermostat and on a cool day will hold at 175 degrees. 99 degrees out in the sun pulling a hill I cant hit 200. No complaints I also found it odd.

i only have temp gauge on cluster panel, no additional tools. by looking at the level of coolant overflow reservoir, it appeared the coolant level and the temp did show correlation - the level was higher when temp was higher, vise versa.

[CountryFit]

Originally Posted by 96 Wideglide

Were you running dash air? Believe that can put your side rad fan on hi-speed.

no, i weren't running dash a/c.

[dons2346]

The temp system has the gauge and the temp sensor. Both of them have a 10% plus or minus tolerance so if both are on the low side, that could give you the 160.

When my controller went TU and ran all the time on high, the highest temp I would see was 180. I don't think that with the fan running high, you could ever get the temp down to 160.

My first suspect would be the temp sensor.

[Skilletface]

Had the same issue with my isc 300 side radiator. While at Cummins for an air compressor repair, I had few things reprogrammed and while hooked up it was discovered the fan was programmed on all the time.
After the fan issue was changed I did begin to see a steady 180 and 195 while in a hard pull.
Note: my temp gauge ran at 170 to 180 all the time and that is what cought my attention.

[TR4]

This is one of the best upgrades I installed. I don't use my dash gauges anymore (except for air pressure). They seam to only give general readings.
SilverLeaf Electronics, Inc. | Knowledge is Horsepower

My fan controller kept my temperatures under 180. I replaced it and now runs between 190 to 200. Fan comes on at 201 and off at 189.

When dash A/C is on, the fan controller will go to medium fan speed to draw air across the condenser.

I agree with dons2346 that the dash gauge might not be accurate. The dash gauge and fan controller use two separate sending units.

Bill

[CountryFit]

thanks everyone!!
don's explanation makes perfect sense! the poor accuracy of the sensor (and the gauge) will show the number off, but the actual temp is regulated by t-stat so i might be ok leaving it as-is (btw, i have not noticed the loss on mileage yet).

skillet - before you had them reprogrammed it, had you noticed the fan spinning fast though engine was cold? in my assumption, in that case, obviously you'd lose some horse power to the fan, but since the t-stat cut off the flow to the radiator when cold, the fan wouldn't have cooled the coolant inside the engine or in a small circulation. am i making this right?

tr4 - silverleaf is looking very cool . i read a lot of positive comments about it here before, apparently it's worth a lot (i remember don has it too)! i'll keep it on my radar maybe one day the dw will loose her lips .

[jleamo1]

Here is another option, not nearly as nice as a Sliver Leaf, but will work for pulling codes and monitoring gauges digitally. My gauges are not so good either, I mounted this above my camera monitor.
Scangauge D - Diesel Engine Multi-Function Monitor-pplmotorhomes.com

[Skilletface]

Originally Posted by CountryFit

thanks everyone!!
don's explanation makes perfect sense! the poor accuracy of the sensor (and the gauge) will show the number off, but the actual temp is regulated by t-stat so i might be ok leaving it as-is (btw, i have not noticed the loss on mileage yet).

skillet - before you had them reprogrammed it, had you noticed the fan spinning fast though engine was cold? in my assumption, in that case, obviously you'd lose some horse power to the fan, but since the t-stat cut off the flow to the radiator when cold, the fan wouldn't have cooled the coolant inside the engine or in a small circulation. am i making this right?

tr4 - silverleaf is looking very cool . i read a lot of positive comments about it here before, apparently it's worth a lot (i remember don has it too)! i'll keep it on my radar maybe one day the dw will loose her lips .

Countryfit, the fan was fully engaged with or without the dash A/C on as soon as the engine started. On my dash A/C it is a stand alone system located in front of the steer axel using an electronic fan for cooling.
The chassis cooling fan is hydraulically driven and circulation of coolant would begin around 180 degrees prior to the reprogramming. And yes it did have some impact on horse power.

[CountryFit]

Originally Posted by jleamo1

Here is another option, not nearly as nice as a Sliver Leaf, but will work for pulling codes and monitoring gauges digitally. My gauges are not so good either, I mounted this above my camera monitor.
Scangauge D - Diesel Engine Multi-Function Monitor-pplmotorhomes.com

thanks. the pricing looks good. i remember reading people asking about it not long ago...

[CountryFit]

just finished the tank full after 912 miles. filled to neck with 94 gallons of d2, which yields to 9.7 mpg, exceeding my expectations . now i shouldn't need to worry about the coolant temp; the cause has got to be the sensor accuracy.

[dons2346]

Your 9.7 beats my 8.5, I wouldn't gripe