Solved why my Coolant Reservoir was boiling over 5.7L

So I had recently completed a full flush of my Coolant system, even used correct Toyota Pink Coolant.

I bled the air out of the system with my heat on high, topped off Coolant reservoir to correct full mark...later after a test drive I had coolant splashed under my passenger fender well as well as inside the engine bay. Coolant reservoir was empty...

So my first instinct was that I had "over-filled" the reservoir but after a few fender well/engine bay splatters and reservoir top-offs I had to look further for my issue.

Zero leaks from Water pump, zero leaks from Radiator cap (that I could see), zero leaks from reservoir hoses, zero leaks/holes in radiator hoses.

My assumption was the old cap wasn't holding cooling system at correct pressure...
I took a gamble and ordered a brand new OEM radiator cap from @Roman and THANK YOU!

Upon inspection of my old (12 year old) radiator cap, I found the bottom seal MISSING. Coolant was not being pressurized correctly to keep temp down and therefore was boiling over into reservoir. New on the left, old on the right.

Thankfully Toyota Tundra cooling systems are good enough to work even with this small loss of coolant. Once I refilled the rad and reservoir and bled the air I haven't had any issues further.

Sometimes its the simple things that fix the issues: Check your Radiator cap when you change your Coolant!!

 

Good info. Any concern that the seal (or pieces of the seal) is floating around somewhere it's not supposed to be?

 

There is a video where that piece was causing an obstruction...

 

I know aluminum radiators have super small holes so I have zero worries on it falling down into the radiator further.

As far as it entering the engine, I think I've seen that video. I want to say this piece has probably been broken off for a while since my reservoir has been low for a long time, I finally got around to fixing that issue. I never overheated so I never noticed how low reservoir was.

I will have to go check that video again in case something has gotten in the thermostat or further but I haven't seen any issues thus far.

All the more reason to change your Radiator cap early rather than once it becomes problem

The full coolant flush was what alerted me to the issue of reservoir being low

 

I got the TRD bug last year and ordered a TRD radiator cap and TRD engine oil cap even though the original ones were still working well.

 

Honestly thought about getting the TRD radiator cap but at 236k miles on the original Radiator, water pump, etc. I'd rather not introduce a higher pressure cap or anything that might mess with the current powerplant

I know they're high quality stuff though and look good!

 

Just had this identical thing happen on my 2011. To make it worse, I had just recently finished R&R on valley cover plate, water pump, thermostat, and wiring harness (vermin ate it), so I wasn't sure which part had failed! Just ordered a new radiator cap today, so fingers are crossed that tomorrow it'll be back to normal.

 

Do you mean the huge plate that sits under the intake? I think it has some o rings on a tube that go into the back of the water pump?

I think my wife’s is leaking from there

 

Yup, that plate. It's a nesting area for rats, mice, and chipmunks. It's a pain in the ass to get to.

 

This was what I found when I dug in. There was also the common coolant leak right behind the water pump.

 

I only replaced my water pump. I didn’t know bout the valley plate o ring leaks or plate silicone leaks until afterwards.

The vehicle did sit without coolant for 5 days while I replaced everything working off and on in my spare time.

I figured the o rings dried up,especially if I used Emory cloth, brake cleaner,and compressed air to clean and dry the water pump gasket area.

It still uses about 1 cup of coolant a month to go top off the overflow jug.

When I changed the oil last month I didn’t see any coolant leaks on the front of the motor,but did see something that looked like it was running in the valley and dripping down the back of the block. It was dry residue,nothing wet.

I can’t see underneath the intake without taking it off….so I’m just assuming that’s where it’s coming from

 

Upon inspection of my old (12 year old) radiator cap, I found the bottom seal MISSING. Coolant was not being pressurized correctly to keep temp down and therefore was boiling over into reservoir. New on the left, old on the right.

Thankfully Toyota Tundra cooling systems are good enough to work even with this small loss of coolant. Once I refilled the rad and reservoir and bled the air I haven't had any issues further.

This same thing happened to me after doing a starter on a 4.7L. Went to top off radiator and the cap broke as I was taking it off, and had to pull the radiator off to get the pieces out from the bottom of the tank. No sure why these caps are breaking. My theory is the seal is sticking to the radiator inlet and causing it to break when removed. Also my other thought is that I need to find some lube that is heat resistant to put on the seals of the cap and with the heat from the water not melt, and dissolve over time.

 

I posted about this in another thread recently, but I also had the same problem except the broken part got stuck in the upper coolant hose neck and was restricting flow. I was able to fish out the larger pieces and the truck seems to be ok.

That same cap was/is used on a ton of other asian vehicles and they all have the same problem. If you have another toyota or honda I would replace the oem cap. It's a shit design, a regular radiator cap works fine and cant get sucked into the engine...