Hey Dudes,

Today was the 3rd OEM radiator cap I've found that has broken. The spring and another plastic piece that holds the spring in place breaks free of the cap and ends up in your radiator. The parts can't really go anywhere because they're too big to get stuck in the cooling channels but your cap is not going to be working properly.

I not well-versed in radiator cap engineering so I borrowed the following knowledge I found via an internet search:

When the pressure inside the cooling system increases above its psi rating, the pressure cap releases some of the coolant into the overflow tank. It does this by use of a spring loaded valve. Once enough coolant has exited the system to reduce the pressure, the valve closes again.

So Dudes, I suggest you check your radiator cap and see if it's still intact. If you have a decent amount of miles on your rig and your radiator cap is original, I suggest you replace it as preventative maintenance. The obvious time to renew the radiator cap is when you get yourself a new radiator, but if you haven't replaced your radiator yet, replacing the cap soon wouldn't be a bad idea.

In the picture below, the spring and other small plastic piece were inside @infamousRNR old radiator.

This is normal when they get old. My oem made it 250k miles before it failed.

They might not show any signs of trouble in normal usage. But work it a little harder than normal, or go up in the mountains, and it can start boiling over.

I'll add I was losing coolant in the over flow. Thought it had a crack. Replaced cap when it failed like above and now my overflow stays full.

My experience was the same as @JayRolla - lasted somewhere around 230-240k miles.

I had this exact thing happen to me while on a trip in Colorado this last summer. I had zero issues all trip until we went up Pikes Peak. Towards the top I was boiling over really bad. Once we got back into town I took a look at my radiator cap and realized it was useless, and appears to have been that way for a long time. A quick trip to O'Reilly's had everything going operating as it should again.

It makes sense. When the system is under pressure it can heat up to much higher temps before boiling, but even slightly elevated temps will build enough pressure to start boiling over and bubbling out of the overflow reservoir. The rest of the trip was spent offloading over 6 different Continental Divide passes without a single boiling over incident. I'm actually pretty amazed that I hadn't had that problem sooner considering the condition of my radiator cap.

Huh, how ironic. My cap broke on me going through Colorado in January on my move to Arizona. I started seeing white smoke come out of the hood, pulled over and found out the cap had split in two.

Makes sense now why 2 months later my radiator needs replacing.

Mine was broke just like the pictures above around 178k when I did my waterpump, timing belt, radiator, etc. Cap broke once I took it off to drain the coolant before swapping radiators. I'm gonna pick up a spare to keep in the truck just incase. Not sure how much OEM vs stant or so is, but I might just get a cheap one that will get me home where I can replace it with an OEM.

Some Toyota dealerships offered a "premium" mileage service that included replacing the radiator cap. Seems like they just have a normal lifespan.

Good idea to keep one as a spare. The first time I noticed this cap failure on a guy's truck I was working on, I went out and bought myself a new one and kept the old one as a backup.

Some have said this is a normal thing for a radiator cap to fail like this but I've never experienced this before. I've taken two other vehicles to high mileage, 240,000 on a Honda Accord and 325,000 on a Subaru Outback and neither had a radiator cap failure. @infamousRNR rig only has 160,000 miles on it and his failed so it's definitely not just an issue for people with 200k or more miles.

New radiator, cap, LBJs, and TB/WP every 100k miles? Seems perfectly reasonable to me, if you love your 4Runner Maybe throw VC gaskets and PCV/grommet on the 100k mile maintenance interval list too.

When I purchased the 97 puppy hauler I noticed the top hose sucked flat when the engine cooled off and found pieces missing. It was a low mileage rig 120,000 miles) but cap was still old.

And I also never had this happen on any other vehicle.


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My rule of thumb is OEM, 10 years or 100,000 miles, whichever comes first.

Like the PCV valve the radiator cap is vital to the health of an engine yet rarely is ever replaced for optimum engine performance.

This is my plan!

Mine broke same way. Drove to parts store for cheap one till I could buy an OEM. That same day I go test drive it, come back and pop hood to find fluid everywhere. I forgot to put that cap back on. Could not find it anywhere, bought a second cheap one.

Few weeks back was doing my belts and found the cap wedged behind the water pump pulley. Now I have a spare. Bet two cheapos wont outlast an OEM thats for sure.