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Coolant leak from rear thermostat housing. Please see picture.

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Hi guys, hoping to tap into your knowledge here!

So I recently developed a minor misfire and did an obd scan, with O2 sensor heater faults, P0135, P0155, P0161. Research here suggested that it could also be the engine temperature sensor behind thermostat. So time to go digging.

I’ve noticed a minor loss in coolant of late, but just thought it was down to poor bleeding after a water pump change. However this seems to not be the case. There’s a bit of pooling of coolant behind the thermostat. It looks like a gasket, but what I would like to know is if anybody knows what part this is?


Some clues will be helpful to narrow down things when on the phone for parts. 

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Thank you steve! Amazing for the quick info.

Just wondering, I found the O ring fine. I noticed that the thermostat seal pointed to a LS400 phase2 non VVTi 1UZ. Some digging, now I know what the component was called, I found part number 16341-50020 as the equivalent for the 3UZ LS430, I wonder if this correct.

Sorry for the formatting, weird copy paste thing. 

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Steve is the expert here.

If you can: I’d strip down and re-seal using new gaskets and seals. Not mechanically complex, rather time consuming.

That pink residue looks like some leaks but could be over spill.

Do replace the coolant reservoir cap if it’s it’s worn or old. A little leak of pressure over time will drop the coolant level. 

Even carefully filling and bleeding the system needs a few miles and recheck/top up to stabilise the coolant level: after that it doesn’t move.


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I would also recommend changing the thermostat while you are in there, I noticed my engine was struggling to reach mid range on the needle and found the rubber seal on the stat itself was distorted allowing coolant to pass even when fully closed.

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Thanks for all your help here so far. Unfortunately I seem to be having some problems. The water bypass gasket (16341-50020) is meant to go somewhere else. The join right below the engine air intake appears to be sealed without gasket, can somebody confirm this?

In regards to the misfire, I have noticed it’s a cold misfire only, and the temperature gauge seems to read fine, however I understand that this car has two different sensors, one sending to the dashboard and another communicating with the ecu, is this information also correct?

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Hey, thanks Steve, those diagrams are a helpful and along with Toyodiy searches I think I can better describe things. I’ve found a picture of the joint as described, hopefully I can more accurately describe where the leak is coming from.

In the picture the red circle is where the leak is, and from what I can tell from the diagram, there is no gasket for the join. It seems similar to the thermostat housing, the gasket there fits around the thermostat, rather than to seal the join. So please tell me if I’m wrong to conclude that there is no gasket there!

The blue circles are where the gaskets, 16355A (pn. 16341-50020) reside, which is the coolant bypass. 


Final thing, if somebody has access to Fig 16-01 which refers to the water pump assembly, that would be really handy for more answers. 

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