1997 Eagle - Cummins c8.3 Thermostat Question

[jd956jd956]

Greetings,

I am the second owner of this coach and when I bought it 10 months ago I did
many preventative maintenance tasks, including replacing the thermostats with
Cummins brand. Over the past 10 months, the engine temperature has ALWAYS been
a rock solid 180 degrees except during a trip to Yellowstone where it would go
up to 185 while climbing in the mountains - never any higher. Very recently
however my engine temperature began fluctuating constantly on flat ground. It
would initially rise to about 195, then go down to about 183, then rise back up
to near 195, then go back down to 183, continuously moving up and down.
Assuming one or both thermostats were failing, I replaced the cummins brand with
two new NAPA brand. They do the exact same thing - opening and closing
continuously. Any suggestions to get back to that rock solid 180 degree reading
that I once enjoyed? P. S. - my radiator is clean, my hydraulic cooling fan is
working properly, etc, etc. THANK YOU, Jeff...

[jimkate]

I would check the electric temperature sensor, and the connection at that sensor. If that's not the problem, I would check the ground connection to the electric temperature gauge in the dash.

[dieselbeaver]

Did you verify the gauge using an infrared thermometer at the location the sensor is mounted? Is the sending unit wiring secured from the sender to the gauge in the dash? And are all the connection good (low to no resistance)? Did you use a heating element, a pot of water and a thermometer to check the opening temp of the new Cummins brand thermostats? And the NAPA's? And the originals? Have you checked the hydraulic fan with a photo tach against the manufacturers specs at normal operating temp for both the hydraulic system and cooling system? What is the temp drop across the radiator core? What is the block water pressure? Is the water pump impellar secure on the shaft. Is the serp belt tight?

Don't assume old parts are bad and new parts are any better. Proper troubleshooting begins with knowledge of the system at hand followed by utilizing sound practices to narrow down the choices till the least likely are excluded leaving only the most likely. Best of luck

[jd956jd956]

Thank you for the suggestions! I did use a laser thermometer and the guage is working properly. I did not have the special tach to verify the "purely" hydraulic fan speed, but its RPM's do increase more than substantially as the engine RPM's are increased. Today, I removed both thermostats and put it back together with none installed, then test drove it to see if it would still fluctuate. It never exceeded 160 degrees and did not fluctuate wildly as it did. I am going to buy two new cummins thermostats Monday, install them, and hope they BOTH are good.

[sc3283]

I know with some of the vehicles I have owned with fast acting gauges....in certain weather conditions (outside temps) I can see the gauge fluctuate as the T stat opens and closes.......just as your describing. This is more noticeable in fall and early spring...You may be more "in tune" with the gauges now and noticing something that has always been there????

Just a thought

[deandec]

Those temperatures are easily within the temp range of the recommend thermostat(s) in the C8.3L engine.

I would think a single temperature readout would be hard to attain.

Our C8.3L has ranged from 180 to 225 (engine auto shut down) depending on the air temps and vertical grade and length of that grade.

[Perry White]

Try turning dash air on when having trouble. With AC on fan is constant. Thermostate will keep steady temp. With fan cycling on tempm control, thermostate opens sooner than the fan coming on. I don't think you have a problem.

[jimkate]

Thermostats have a high failure rate right out of the box. Next to windshield wiper blades, they are the cheapest manufactured product in your vehicle. OEM thermostats are no better than aftermarket, in fact, they are often worse, because you don't know from one purchase to the next who actually manufactured the thermostat. The OEM people don't care because their name is not riding on the product.

Your situation puzzles me. Part of the reason for two thermostats is to get away from the "open-close" temperature fluctuation that you often see with one thermostat - the difference between the thermostat's opening temperature and its closing temperature, which often can be perceived on the gauge as a big temperature swing.

[jd956jd956]

Greetings... Well, I changed the thermostats again (Cummins brand) and the fluctuation is still there but does not seem so drastic / fast... Perhaps because summer is now behind us. We did take a near 400 mile round trip this weekend, and all seems well as it can be for now. If I learn anything in the future, I will be sure to share. THANK YOU ALL

[mharner]

I read this thread with interest because I've had problems with erratic coolant temp gauge readings. The gauge always seemed high after a few miles, starting at 180, then rising to 220 (approximation on analog gauge). I had the motor rebuilt. I expected that afterward those readings would be more normal at 180. However, the readings were the same. When questioned, the Cummins shop told me that when they were testing the rebuild, they had a scare when the gauge crept up, but it returned to normal, and there was no indication that the temp was actually high. I had the Cumins shop install a second gauge in the back, so I could pull over and verify that the temp was not high, and sure enough, when the dash gauge crept up to 220, the back gauge would reliably show 180. I've been guessing the sensor is bad, but after reading some posts, maybe the dash gauge ground is degrading. I saw some good input here on the subject. Thanks.

1997 Country Coach Allure
Cummins C8.3 Turbo RV 325
90K miles

[dons2346]

A bad fan controller will cause a low temp and can also cause temp fluctuations