Checks After Coolant Flush

For gen 2 prius, after the initial changes, a coolant change every 50k miles or 5 years (whichever comes first) is what Toyota USA recommends. There's no mention of numbers (presumably freeze protection in degrees Fahrenheit) such as your mechanics, it's just miles/months, change it.

Gen 2 Repair Manual Repair Manual info, and my spreadsheet summary of Toyota USA's maintenance schedule (which you can also find in the toyota usa publication Warranty and Maintenance Booklet*), is attached below, in excel and pdf formats. Note: I've extrapolated the official schedule to 240k miles or 24 years.

* The Warranty and Maintenance Booklet, Owner's Manual, etecetera, can be downloaded in pdf format, from Toyota Tech Info website, under the Manuals tab.

That said, if you look through the engine coolant change instruction from the Repair Manual, there's mention of esoteric device connected via the obd port, which I suspect your mechanic does not have access to. At this juncture, I'd have a dealership service department check it over.

 

First off, the coolant system doesn't need to be flushed. Them telling you that the glycol concentration is wrong; -15 vs -35; all you need to do is adjust the concentration. The proper test to see if coolant replacement is required is a litmus test; to make sure the coolant PH hasn't move toward the acidic side. That would cause deterioration of your plastic and rubber parts the coolant would come into contact with.
A simple drain and fill is all that's required, per the OEM service manual. Your not going to get all the coolant out doing this, but your also less likely to screw up the coolant concentrations. The concentrations don't move on their own, unless someone is adding distilled water to top off the cooling system. I'm talking about quarts of water, not a few ounces.

I would probably take the car to another shop with more careful and honest mechanics.

Just my two cents..........

 

Low pH will increase corrosion of the metal too. I wouldn't expect it to hurt the sorts of plastics used in cars much, but rubber, sure.

For the OP, it might be necessary to add more coolant later if there is air in the system. Park the car somewhere flat each night. Place a Sharpie mark exactly halfway up the side of the overflow tank, in a location you can see easily looking over the passenger side fender when the hood is up. Each morning, when the car has fully cooled off, open the hood and examine the fill level in the overflow tank. If there is air in the system it will push it into that tank and when it cools off will suck in coolant. This assumes that there is coolant up to the neck of the radiator. Check that once (only after the car is completely cool) just in case they woefully under filled the radiator. Anyway, while there is air the level will fall in the overflow tank each day. When it falls below the mark open the overflow tank, put in a funnel, and fill back to the Sharpie mark. In a couple of days the level should stay constant and you are done. Also, good idea to run the heat for 5 minutes or so each day in order to help get any air out of the heater core.

 

Just open after it cools down once you get in for the day there's not coolant standing in the filler neck you'll be pouring cooling into the tune of about close quart probably . Once you get it full everytime cap off water standing at top of neck ..