Engine Coolant Leak

I have a 2002 Toyota Sienna with 131,000 miles. The engine coolant overflow tank had been drying up even when refilled several times so I assumed there was a slow leak somewhere. The dealer ran pressure tests and could not find any leaks but then determined the radiator cap was defective and replaced it, but that has not solved the problem. Another pressure test showed nothing and the dealer can’t see any leaks. What could be going on?

You can pressure test a cap to determine if it’s within spec. And if it isn’t, you replace it. However, failing to pressure test the new cap before it’s installed can come around and bite ya! I learned years you get two new caps. Because I’ve found new caps to have 25-30% failure rate in holding the proper pressure. So ask if they tested the new cap before it was installed.

Otherwise, you may have to leave it with them overnight with the cooling system under pressure. I’ve had some vehicles where the leak didn’t show up until twelve hours passed since the cooling system was put under pressure.


Are you saying the pressure test after the new cap was put on would not show the cap was defective? It could take 12 hours? You may have something there b/cthe first pressure test did not show any leaks. It was when they were running the engine to find out why the check engine light had come on they noticed coolant coming out of the old cap and replaced it. How do you test the cap alone?