Originally Posted By: ericthepig
This has been discussed a lot by the BITOG sages, including some technical write-ups I think -
I can't remember the exact numbers, but it goes something like this -
The coolant reaches temp in about 10-15 minutes.
Oil lags the coolant by 10-15 minutes - so the oil is at OP temp in 20 to 30 minutes.
The engine does not reach thermal equilibrium for 30 to 40 minutes. I think that means a minimization of thermal differentials and full expansion of metals (ie, full seal by the rings, etc).
This may help in guestimating how much time is necessary to burn water and fuel from the oil. I would think if you reached OP temp on the oil, and then ran another 20-30 minutes - you've got some nice evaporization going.
These figures may be more applicable for a large, iron V-8 or V-6, but if you have a smaller, lighter Aluminum I-4, the heat-up times will be MUCH quicker.
On the ScanGauge-II I've added to my car, as one of the four parameters I keep as a "default" display, is coolant temp. In the Camry Hybrid, which has a 2.4L Aluminum I-4 on the gas side, I see 190F (average operating temp) consistently in 1.5 to 2.0 miles, even when it's cold out (well, Florida cold...).* I tried an experiment a few weeks ago after I got my little hand-held laser-IR temp gun. I stopped as soon as I could after reaching 190F coolant temp, and took an IR off the bottom of the oil pan. Several shots averaged just over 150F, not that far behind the coolant.
I think maybe, fundamentally, you're asking the wrong question. If you're worried about whether or not you're actually burning off water (or letting it build up), I would simply default to changing oil earlier. A UOA or two would also help you tell whether or not you were getting some water buildup. Of course, if you open your oil filler, and see "chocolate milkshake", you KNOW you have massive water buildup (and probably a coolant leak). If you see "vanilla milkshake," turn from the car, walk away, and never touch it again. . .
* EDIT: Oh yeah, this car does NOT have the hot coolant "thermos" like the Prius does. In that car, when you shut down, the car pumps 3L of the hottest coolant into the "thermos". Upon the next start, it injects that hot coolant right back into the water jackets. The op temp in that car positively
rockets up to normal, largely eliminating the cold start "penalty" most cars suffer.