What you want to do is test the system, from cold you need an infrared temp gauge, and a cover for the radiator, start it up and run it as fast as you feel comfortable with, monitor the top rad pipe it should stay cold test the oil cooler housing for temperatures when the top rad pipe gets warm that is the thermostat opening temperature. If it leaks coolant then either the thermostat is open or the seals are worn out, and need to be changed, next you test fan in temp and fan out temp. From memory an "E" is 220 to 225, fan in, and around 210 fan out. These engines are made to run above 200 degrees and do not like running cold, it will cost you a lot of fuel, and wear.
You can run it how you want , but an engine that is made to run at above 190, with a minimum 7 lbs rad cap, that is the thermostat opening temperature has low oil temperature and things like the piston does not grow to the proper shape until certain temperatures, are reached and a 190 thermostat is fully open at 208. Cummins the fan must be off at 194 degrees.
Does the temp change have anything to do with turning lights or any other acc. on? in which case I would check the ground(s) on the cab and guage panel. I would also say that 180 is a little on the cool side for an ATTAC engine.
I would recomend a good mechanical gauge. Get rid of the electric one, i really think they are not really accurate.
There is no reason for your guage to read 10* higher at night, other then a ground issue.
The mechanical is a little harder to install, but well worth it for accuracy.:thumbsup
When I was at Freightliner we got a notice that we were to check the voltage drop from the right front corner of all engines for needing an extra ground wire. We found it especially helpful on the Cats, to the point I use to make up extra wires, when I was not busy. I used #6 wire with a 3/8 eye on one end and 1/2 on the other soldered and shrink tubed. Stray voltage on every type of engine is a problem. Not only with the electrical system but with corrosion of internal engine cooling system pieces, the cover plate for the water pump is a common one.
Having it back feed threw sensors causing multiple codes is also quite common, that is when you have to turn on every light, and go looking for bad grounds on the cab, and engine. Freightliner use to put ground studs on the rear of the frame and battery cable joining the two. Electricity can do amazing things.
I believe that the guage change is due to a voltage drop at the power side of the "Modern" electric guage, I agrre with the mechanical being better, but the 23 year old electric guage in my 379 doesn't missread with the lights on even though the voltmeter shows a drop when the lights are on especially when hooked to an incandesent lighted trailer. if my electric guage fails it will be replaced with a mechanical cause I like mechanical things.:toytruck
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