I'll have to put some miles on with the heater going to see what happens but although I agree with what you are saying regarding the affect the heater should have on engine temps, don't forget that a DII has a way more messed up cooling system than a DI. My engine runs at 190 to 195 with a 180 degree thermostat, new radiator and fresh coolant. The thermostat hanging in open air behind the radiator is not the same as in the block where it belongs. It works if the system is in good shape, but not like one would expect.

 

Hey guys I saw my first temperature "spike" today. Since I've had the ultraguage I haven't seen the temp go above 194. However today when I was approaching stop lights it went up to 197-199. I still didn't break 200 but I found this unusual.

This was one of the first times I drove without the heater on entirely. When I notice the temperature I decided to experiment by putting the heater on and my temperature went all the way back down to 188.6. This was in city driving, 20-25 mph with stops.

When I got home I idled the truck trying to get it go up again. After sitting for a few minutes it went to 195.8 and at that point it wouldn't go higher. Revving it lowered it to 190.

Thoughts? Am I being paranoid? Premature fan clutch failure? Radiator?

Expansion tank was full, no waterfall noises.

 

For over a month after installing my temp and oil pressure gauges I kept myself stressed out watching the gauges for bad numbers. I wouldn't stress over those numbers at all. It is way too easy to get paranoid the more info you have.

 

Radiator not 100%. Might be 80%, etc. If turning on heat (making fan blow air thru heater core) drops temps that much AND keeps them there, under steady state operation, then maybe rad is not quite clean enough inside. Revving engine increases air flow over radiator, and the viscous clutch may need to be checked as well. Also, if you have one reading at 50 mph, and it increases when you stop, and does not come back down, the air flow at 50 is a whole lot different than the fan alone at idle. Wimpy fan clutch and slightly clogged radiator come to mind. But you would expect the hot running motor to need some time to cool down with the lower airflow at idle.

If you have an IR thermometer, with truck warmed up, switch off engine. Measure fins top and bottom (not tanks). If clogged up, fins usually read more than 10F colder on the bottom.

You doing the right thing to question performance. However, it is also true that you are using an electric temp sensor converted to digital by the ECU, and read by the ECU when it is not busy with something else, like fuel trim; and spit out to the OBDII port and read by the UG. And in effect you are reading the changes brought about by a blob of wax inside the mechanical thermostat cooling or melting. It won't be exactly repeating, and you will notice some variations. Plus the ambient air fan blast is changing how the thermsostat operates.