I bought a cheap little $40 gun. Tried it out on my scion after a drive. The upper rad hose was reading at 160 F and the lower was reading at about 110 F. So it seems fairly accurate. I'm going to put some water to boil and test take a reading just to get an idea of how accurate the tool is. After that I will take the MR2 for a drive and use the gun. By any chance do you think that my overflow cap coming off while driving introduced air into the system? Is that even possible?Point the gun at any part you want to look for abnormal high/low readings. Saves a heap of guesswork and allows real time diagnois of the engine cooling system. The inlet/ outlet radiator pipes are probably the most important as this will tell the differential between engine temp and how well the radiator (plus pump, tstat, etc) are operating.