I took our 2003 Ford Taurus SES (6 cyl, DOHC) to our mechanic yesterday, presenting the symptom described in the question above. After driving the car for about 45 minutes Saturday afternoon (mostly interstate) and noticing the behavior of the gauge, I parked the car in our driveway and opened the hood. No steaming or hissing at all under the hood. The oil dipstick showed absolutely normal oil (no coolant in the oil). Exhaust was not abnormal (no steam, no “white smoke”). I let the car sit overnight – no fluid leaks onto the ground. At that point, I had assumed a problem with the thermostat, or maybe air pockets in the coolant lines.
Long story short: the mechanic performed some kind of chemical test of the coolant (akin to this). He showed me the results, and told me the car had a blown head gasket**. The fluid test revealed that exhaust gasses (as opposed to ordinary air) was getting into the coolant.
Now, OK. The mechanic proceeded to top off the coolant and tell me that the temperature gauge will stop spiking. He then recommended that the head gasket be replaced at another shop (his shop didn’t work with or disassemble blocks) for an estimated $1,500-2,000.
Because (a) the engine showed no visible/audible signs of overheating, and (b) I couldn’t detect coolant in either the oil or the exhaust, and © there wasn’t fluid leaking onto the ground … it occured to me that if the head gasket was indeed blown (cracked, sealed poorly, whatever), the car might have the World’s Most Fortuitous Head Gasket Issue and might actually be driveable for some time. So I asked. The mechanic told me that short trips would likely be OK for a good while. I then asked if I could safely drive the car for a few hours on the interstate (we’ve got a vacation planned soon). The mechanic said that was chancey, and didn’t recommend it. The shop manager was listening, and walked over to opine that taking the car out of town would probably be fine so long as we brought extra coolant along and paid attention to the temperature gauge – if it stated spiking again, pull over at the nearest exit and top the coolant off again, just as the mechanic had done five minutes earlier.
As I research the issue on my own, there does seem to be a kind of just-so head gasket problem involving the water jacket that can present the symptoms described above. I also read through this page which described cases where exhaust gasses in the coolant cause true overheating (signs visible upon raising the hood).
What I can’t seem to figure out on my own (excepting the shop manager’s opinion above) is just how truly risky driving the car is in this state. If I saw some of the usual signs of a messed-up head gasket, I’d feel more definite about getting the car repaired ASAP. But being that the one and only sign was a single incident of the temperature gauging spiking/dropping repeatedly during the course of a drive … I dunno.
*** - I do not know if the mechanic performed a true pressure test of the engine block, or any other test besides the coolant analysis. Calling the shop now to find out.*