The only possible explanation for a rapid build-up in pressure is a leakage of combustion gases into the cooling system. This is typically caused by a head gasket leaking from a cylinder into a nearby water jacket.
I do not see any “leak stop in a bottle” products solving this problem. I also would caution against using any of these type of products on a good car, which someone may want to properly repair in the future. If you are unable/unwilling to pay for a proper repair at this time, then just sell the car “as-is” as a mechanic special, and buy a different car.
Glad to hear a new radiator cap fixed it. fyi, I have a 30 year old Corolla and have had only a couple of coolant system problems over the years
replaced the temperature switch that turns on the cooling fan at the 9 and 18 year mark.
radiator replaced (along w/cap) at the 20 year mark.
water pump replaced this year
Regarding the water pump, the symptom was there’d be maybe 2-3 tbsp coolant under the car in the morning, but it would never leak when I was watching for it. No leaks at the shaft seal no matter what I did, tilting the car this way and that way etc. As I had time during the pandemic for car experiments I decided to figure it out once and for all. About the only place, beside the shaft seal, it could leak was the gasket that held the two pieces of the water pump together, and the two o-rings where the water pump attaches to the engine. I removed the old water pump and replaced the gasket and the o-rings, then reinstalled it. The o-rings were definitely shot, so I figured that was the problem. I ran it for 45 minutes, no leaks. The next morning, a few tbsp of coolant on the ground. It still leaked! This time however I had a lot of stuff removed that in otherwise in the way, so I was able to spot a drip coming from the water pump weep hole, meaning the shaft seal was the culprit. I replaced that part of the water pump w/a new mechanism, reinstalled, no more leaks, good to go.
Maybe the OP means they noticed steam suddenly coming up from the engine and the coolant temp gauge went into the red zone. A faulty radiator cap could cause that symptom. I’d recommending asking the shop to check the thermostat condition, and that the radiator fan is turning on correctly as well.
Those comments were from the testing of the cooling system.
Sudden rise with engine running, Pressure failed instantly when pressure tested. It would seem to me the radiator cap would be off with the testing instruments in its place.
The sudden build up of pressure is a red herring. You’re driving down the road and the temp gauge suddenly starts going to the hot end of the scale. How would you know what the pressure is doing? How many people have a pressure gauge attached to their cooling system?
I have seen several cases where a sudden rise in ECT (engine coolant temperature) at higher speeds or going up a hill was caused by a bad radiator cap, that is why it is the first thing I check. BTW, there is no rise in pressure, in fact a lack of pressure is the culpret.
Toyota vehicles switch on the radiator fan at 204 degrees, with a broken radiator cap in city traffic the problem goes unnoticed.
On the highway during a hot day these cars will get hot enough to boil out the coolant if the cooling system is not under pressure. I have seen a number of vehicles with damaged engines due to a broken radiator cap.
Driving with a broken radiator cap will result in warped cylinder heads. With the pressure tested installed, if the pressure climbs significantly during the first 15 second of operation, there is a problem with the head gaskets.
$45 later that’s it folks !