2003 Lexus ES300 +200,000 miles, automatic
My guess is the new cap, while oem specified psi, is too high for what this cars current cooling system is used to having.
1 week ago I noticed a consistent leak of coolant from under the radiator cap as well as from the skinny hose that connects to the fluid reservoir. This had been going on for maybe 3 years, and it was slow. I decided to change out the radiator cap with a new 16PSI aftermarket cap and add coolant to the radiator.
The cap that was on the car for 3 years was an aftermarket 13PSI cap. When I bought the new cap I found out that Lexus specifies to use 16PSI for this car. The wrong cap had been on this car for 3 years. Also the radiator turned out to be very low on coolant. My coolant reservoir has always been at FULL, though. And my hoses have never swollen nor needed replacing these 3 years.
I topped off radiator, installed new 16psi cap, turned on heater full blast for a long time and things have been all good for a week until yesterday. The upper radiator hose has become swollen. No leaks anywhere. The other cooling system hoses are still in good normal shape, but the upper rad hose is bulged towards both ends of the hose. As the car cooled down overnight, the swelling went down to normal hose size. I also took the cap off and a small bit of coolant and light air gushed out.
So what's my problem here?
Radiator hoses deteriorate with age - and I would replace your 17 year old top hose right now, and then closely watch your coolant level, your oil for coolant contamination, etc, to find out if there are other problems .
@glen_stet
At that age and mileage it is probably best to just replace all the hoses and be set for another 17 years. (Well, at least for a few years anyway. Today's aftermarket parts are frequently not as good as the originals.)