Q: Water pump replacement after coming loose?

Water pumps do NOT get loose on an engine excepting an unusual circumstance where the pump was improperly installed. So, the mechanic might be referring, in your circumstance, to a seized water pump or a situation where the water pump pulley and/or pump impeller have come loose. Certainly, if the water pump pulley is loose or the pump impeller has failed, the pump is not re-usable and must be replaced. However, if the pump itself, that is the aluminum pump housing, was loose (i.e., the bolts holding the pump to the engine block were loose) unless it is a relatively new Toyota pump, it is wise to replace it simply because the pump shaft seal does not last forever and if the pump has any significant number of miles on it will be cheaper in the long run to just replace it now when the mechanic has to already go in there anyway to re-install it. If you are re-using a pump, you have to remove it to re-gasket it and re-install it so better to just install a new pump and save yourself the possibility of the used part failing "soon". As far as engine cooling system thermostats are concerned, if you install a new water pump, the best practice is to install a brand new thermostat (use Toyota OEM ONLY; save yourself a headache and do NOT use aftermarket) because the engine thermostat performs a critical function in terms of optimizing and regulating engine operating temperature. Thermostats do not last forever and when they fail they often cause engine overheating which is VERY bad. The reason the best practice is to replace the thermostat when the mechanic is already in there, and the cooling system is already drained, is the thermostat housing is often adjacent to the pump and so it is also efficient and economical to replace it. Thermostats are not expensive: the Toyota OEM thermostat might be around $25 with a mechanic’s discount. As far as this job being costly, as you noted in your question, yes if you use a shop, or worse a "dealer" where the mark ups are huge, you can be sure you won’t be eating lunch for the next month once you have the job done. However, in your circumstance you have a choice: a water pump and thermostat replacement, plus a new serpentine belt, is a relatively easy task on a vehicle such as yours and so is an ideal in the field, mobile job. You can not only save money but get personalized service if you replace the water pump through YourMechanic and the money you save can be put toward OEM parts which a mechanic can get for you typically at a 25% discount off the dealer list price. If you do the job and you want the fewest headaches, but the water pump, thermostat, and belt from a Toyota dealer. Finally, IF the pump was in fact loose, the REASON it was loose has to be investigated. In particular, a loose assembly raises the possibility of stripped internal threads in the block thus causing the bolts to eventually come loose. So, if in your case, there were loose bolts, the internal threads have to be carefully inspected and repaired if necessary, otherwise you will be completely wasting your money on the repair because the pump WILL come loose again. Any further concerns or questions, don’t hesitate to contact us and I hope you are able to get this all resolved to your satisfaction.