I have since gone back to the quick lube place in question and they assured me that they check and service these types of cooling systems all the time and have never had this problem develop. They said they couldn't see how air would have gotten into the system from merely opening that overflow reservoir.
The bolded statement above, is wrong. This is the standard response you will get from any "quick lube" station on this matter. Our cars are VERY
prone to sucking air into the coolant system if you open the tank when the engine is warm.
Rob is correct about the temperature readings in general.
However, if you now notice the coolant temperature will rise while sitting at a stoplight and it did not before: Then you probably now have some air in the system. If a large enough air bubble gets into the system, then the heater will start to malfunction as Rob stated. DDM has a video posted on the forum on how to remove the air yourself. I also have videos posted with another method to remove the air. And of course, you can also order and install the check valves to help prevent the problem from reoccurring.
My knock story:
One problem I recently found is that the plastic nipple in the turbo air inlet pipe that normally snaps off, had snapped off and the inline one way valve was stuck. I have no idea how long its been broke as I can count on one hand how many times Ive had the hood up since Ive owned her. When I saw that I thought, there's my problem, unmetered air getting into the system causing a lean condition. After fixing it the problem did seem to be less severe but, when I had the inlet pipe off I had noticed that their was a little puddle of oil that had pooled on the inlet of the turbo itself.
My guess is that the check valve for vent tube has been stuck for a long time. When this happens an excessive amount of oil will make it's way into the intake. (As Carbon Sky said: the check valve is supposed to let air into the head, but not let oil back through at this location) I had a failure of this check valve and it did result in oil pooling at the turbo inlet as well as coating the upper charge pipe from the turbo to the intercooler. And excessive oil in the intake system may contribute to your pinging problem. It would be a good idea to take the charge pipe from the turbo to the intercooler loose and inspect the inside of the pipe. If you see a trail of oil, then I would suggest you inspect the the charge pipe from the intercooler to the intake as well. If you see a mist of oil covering the inside of the pipe: Then you need to also take loose the catch hose on the bottom of the Intercooler and see if any oil drains out (if so, I would strongly suggest you pull the IC and have it flushed/cleaned). Clean the oil out of the charge pipes and put everything back together. Double check all clamps, especially the throttle body clamp; and make sure you don't have any air leaks. A small air leak could be causing A/F issues and aggravating ping. (if you have a leaky clamp, it will eventually get bad enough to throw a P0101 code)
If you have corrected the the faulty check valve, cleaned the charge tubes and upon re-inspection at a later date find oil pooled at the turbo connection again, then you will know that you are getting excessive blow by from the second line, as Carbon Sky suggested.