Is this coolant leak from the O-ring behind the thermostat?

Trying to track down at least one and possibly a couple of different coolant leaks. A couple of months ago I replaced the upper radiator hose as I clearly had some coolant seeping out at the hose to radiator interface. I continue to smell burning coolant and my reservoir runs low so I'm back hunting for more leaks. On first glance I saw obvious evidence of coolant at the bottom of the fins on the radiator.


I thought that was for sure the culprit but also thought why not do a pressure test to confirm, and upon pressurizing to 13 psi I actually see no evidence of leaking at the radiator, but I did find some puddling in this spot, which I can describe as behind the thermostat but I cannot assuredly see where it exits the block/housing, I can only see this slight puddle from above.




I have read several threads (https://www.tundras.com/threads/what-is-this.146390/ and https://www.tundras.com/threads/leaking-coolant.54959/) about the leak from the O-ring behind the thermostat but mine looks a lot better than those did, from the top at least.


In any case I am planning to proceed with replacing the radiator (original, 18 years 189k miles), cap, lower hose, thermostat, thermostat seal and this o-ring as well in the next week or so. Just want to make sure there might not be something else back there that I haven’t read about yet and I need to have more parts on hand.

I suspect I can get a better look at things by removing the air intake tube from the throttle body, so I'll plan to do that tomorrow.

I’m actually only about 10k miles from needing the timing belt and water pump done also, but that’s over a year away by my usage so as long as this leak isn’t from the water pump itself I’ll probably hold off on that a bit longer. There are some seeping oil leaks I might like to address when it comes time for that job and right now I just need to focus on the coolant leak at hand.

 

Replacing those things, especially the radiator, is a great step in the right direction regardless of if it's leaking or not. The radiators in ours trucks are known for going bad and leaking coolant into the built-in transmission cooler. This is a nightmare to deal with when it happens. The radiator and hoses are all super simple to remove and replace, it took me less than an hour to replace my cooling fan a couple months ago.

The thermostat and that O-ring should also be pretty straightforward. You might need to remove the fan to get good access to it, but that's not a huge challenge. Make sure to be careful when unbolting the thermostat pipe/housing from the water pump, I had one of those bolts snap on me when I changed my timing belt. Your bolts are likely in better shape then mine were, I've got a rust belt truck. There isn't a gasket between the water pump and housing from the factory. Make sure that you buy the Toyota-specific sealant when putting it back together. (Part #

$21 00295-00103 Formed-in-Place Oil Pan Gasket - 3 oz. Tube

00295-00103) It's really great stuff. Thermostat seal is also essential. I've done two timing belt jobs, and that seal has torn on both of them when removed.

I'm also noticing some evidence of a leak in the last photo you added. Right in that joint that uses FIPG (Form-In-Place-Gasket, AKA gasket maker). You can see the red/pink crusty stuff from dried up coolant. At 189k miles, the timing belt has very likely been changed before. I wonder if a previous mechanic used incorrect gasket maker when reinstalling that thermostat housing.

 

In addition to what you mentioned there are front and rear water crossover pipes that each have 2 metal/neoprene gaskets prone to leaking.

 

A good exploded of the parts associated: https://www.tundras.com/threads/05-4-7-2uzfe-overheat.135301/#post-3429269

Pics from the FSM related to thie t-stat housing etc. and instructions on re-seal: https://www.tundras.com/threads/coolant-leak-on-radiator-return.147204/#post-3689993

If you have no history on the radiator, proactive replacement at same schedule as timing belt (roughly every 100k miles) is advised to avoid bricking your transmission ("pink milkshake" is real).

 

I did see that as well, I'm curious if the fresh coolant I'm seeing might be coming directly from the bottom side of the FIPG junction. I've had the truck since about 45k miles and was admittedly late in having the timing belt done, which happened at 109k miles. Hence I'm about a year away from it needing to be done again, so I will likely do all of these things now only to shore up a few obvious spots only to have to redo a bit of it in a year with the timing belt. But that's not the end of the world to have to replace a couple of seals and the coolant again a year down the line.

I've read about those as well but haven't gotten into the details, can either/both of those gaskets be accessed without pulling the intake manifold?

 

Rears really can’t be accessed without pulling intake. Fronts probably just need to remove throttle body and thermostat housing.

 

Gotcha, I'll get a better look when I get into this and if the fronts show leakage when I get in there hopefully I can source the gaskets from a local dealer. Thanks.

About to fire the RockAuto parts cannon and have a side list going for OEM parts that I'll attempt to source at the dealer, am I forgetting anything?

RockAuto:
Denso radiator
Lower radiator hose (I replaced the upper a couple months back)
Aisin thermostat
Aisin thermostat seal
3 gallons of Aisin coolant

Local parts counter:
OEM Radiator cap 16401-20353
OEM water pipe O-ring 96761-35035
FIPG for thermostat housing
(additional gaskets if need arises to dive in a little further for front crossover pipe)

 

Looks great to me! I can't think of anything else.

 

Is there a part number for that o-ring? Can't seem to find it

 

Why yes there is.

This post will teach you how to fish, it's important to be self-reliant on your truck. You really only need to read the first 2-4 bullet points.

Once you hit that site in the 1st bullet point, and plug your VIN# into search, then click Search button,

• Click on Engine, fuel system and tools b/c we're looking for stuff related to the engine
• Click on 16-03 - Radiator & water outlet b/c we're looking for stuff related to radiator/cooling
• Click on "Open full size scheme" under the 1st exploded diagram because the pic is so damn tiny
• Hover your mouse over the number that's pointing to the part you want to see its name, and if the name looks right, click that number, and your part number will appear at the top of the next page.

​

 

I don't know how you do it, but I appreciate you!

 

The O-ring part # was also listed in my last reply outlining my parts list.

Regardless, that resource shifty shared is infinitely more valuable overall.

 

Completed this service yesterday, although I am second guessing myself that I used a bit too much FIPG after seeing how much squeezed out externally, and assuming a similar amount squeezed into the passage internally. Shouldn’t go anywhere though I don’t think.

 

Don’t kick yourself. It looks good. Did you replace any of the small coolant hoses? If so post the part numbers.

 

I wouldn't worry about it too much. A similar amount squeezed out when I did mine for the first time, no issues so far. Probably better to have too much than too little.

 

I did not, only hose replaced was the lower radiator hose since the upper got done a couple months back. When the timing belt comes due in a year or two I’ll likely replace the crossover gaskets all the way around as well.

 

I wish I had replaced those gaskets when I did mine as few years back. I’ve been procrastinating taking the intake off lately. It seems every job I do recently has been fighting me.