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Discussion Starter #1
So I've got a coolant leak that started off minor and has started to get worse over time. Initially, I thought it was a loose hose from a previous water pump fix, but I couldn't find any obvious leaks down there, so I decided to take a look at the thermostat housing (thermostat replaced about a year ago) and I can see coolant above the transmission (see pictured, zoomed out and in).

Now, I've touched around the thermostat housing and the top, larger radiator hose attached to the thermostat housing and can't identify any obvious leaks (I tightened the top radiator hose a while ago because there looks like there might have been a bit of a weep, but I can't see/feel any liquid now). In fact, I've checked a few times now - even with the car warm and running after a 30 minute drive, but nothing. No visible drip, no coolant visible on the floor, but obvious coolant pooling on top of the transmission. So, it appears the leak only happens when driving.
leak1.jpg leak2.jpg
Is this likely to be an issue with the thermostat/thermostat housing leaking? Any ideas?
 

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From the look of the pink residue on the t’stat spigot which the top hose clamps to, it seems thats the leak. I’d take the pipe of, ensure the spigot and inside of the pipe are smooth, and clamp it back up.

Maybe the rubber has a tiny split. I don’t know whether plumbers’ PTFE tape helps in situations like this, but the worst case scenario would be a new top hose.

Is there any pink stain higher up, around the t’stat housing joint to the block? If so, seal that joint. Or around the temp. sensor joint?

There are more complicated and expensive ways to be losing coolant!
 

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Yea its definetly that hose as Doug said. I've had the same one go on me on my way to work one morning while I wasn't near any parts store.. only a gas station and dunkin donuts for about 10mi. My service managaer thought it was funny... it wasn't (he liked SAAB's, but knew the issues. All mine were amusing for some reason :/)

The hose split right on the aluminum jut (I guess it is technically a spigot) that the hose clamps onto. I cut the hose back beyond the split and put an adjustable hose clamp on just to get to work. I remember ordering a new hose when I got to work and what the parts guy brought in had a plastic adapter in it on the end that fits over the spigot. Wound up turning into a bigger problem than it should have been.

That hose is a common failure point. Lots of pressure combined with crap thinish hose tightly clamped on a very hot piece of aluminium. Coolant coming out at that spot has expanded more due to the added heat and pressure, a good quality replacement and you should be good..
(being in NZ bcb may not be economical, but you get the idea)

And what's with the pink coolant..? Looks like the latest Lexus/Toyota HOAT stuff..
 

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Discussion Starter #4
Alright, so it's been a couple of months now... I removed the hose, inspected it for splits and leaks and the hose appeared fine, so I cleaned the connection to the thermostat, reseated it and topped up the coolant. All appeared well until a couple of days ago when the 'Fill Coolant' messaged popped up again. The leak appears worse, and I'm pretty sure it's the thermostat housing where it meets the block. Maybe the gasket has failed...

Now, the thermostat was replaced within the last couple of years so I checked the two 13mm bolts with a torque wrench and it's tightened correctly, and so is one of the two 10mm bolts (see green ticks). However, the second 10mm bolt (red cross) turns indefinitely and won't tighten.

thermostat leak.jpg

Is this bolt the likely cause of the coolant leak?
Does this mean the thread into the block is stripped and I'm screwed?
Should I take the thermostat housing off, replace the gasket and try to seal it? Any tips on this?

Cheers.
 

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Is this bolt the likely cause of the coolant leak?
Probably.

Does this mean the thread into the block is stripped and I'm screwed?
Yes, but not necessarily.

Should I take the thermostat housing off, replace the gasket and try to seal it? Any tips on this?
Yes, yes, but maybe you can re-tap the hole. Be careful and only go as deep or slightly less than the original depth.
 

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Discussion Starter #6
Is this bolt the likely cause of the coolant leak?
Probably.

Does this mean the thread into the block is stripped and I'm screwed?
Yes, but not necessarily.

Should I take the thermostat housing off, replace the gasket and try to seal it? Any tips on this?
Yes, yes, but maybe you can re-tap the hole. Be careful and only go as deep or slightly less than the original depth.
Bugger. I always get a bit wary about retapping threads...especially when the visibility and getting the right angle might be difficult...

Could I use some anaerobic sealant like this?

If the anaerobic sealant is a reasonable bet, would I use it with the thermostat gasket still present?
 

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Don’t know - I suppose you can try it. While it fills a gap, .015” is about the thickness of three sheets of copy paper.

Be sure to clean the mating surfaces really well.
 

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So I've got a coolant leak that started off minor and has started to get worse over time. Initially, I thought it was a loose hose from a previous water pump fix, but I couldn't find any obvious leaks down there, so I decided to take a look at the thermostat housing (thermostat replaced about a year ago) and I can see coolant above the transmission (see pictured, zoomed out and in).

Now, I've touched around the thermostat housing and the top, larger radiator hose attached to the thermostat housing and can't identify any obvious leaks (I tightened the top radiator hose a while ago because there looks like there might have been a bit of a weep, but I can't see/feel any liquid now). In fact, I've checked a few times now - even with the car warm and running after a 30 minute drive, but nothing. No visible drip, no coolant visible on the floor, but obvious coolant pooling on top of the transmission. So, it appears the leak only happens when driving.
View attachment 271992 View attachment 271993
Is this likely to be an issue with the thermostat/thermostat housing leaking? Any ideas?
Recently had same problem in my 2002 Aero.
Replace hose, (Rock Auto Parts- $15) problem solved.
 

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Discussion Starter #9
Recently had same problem in my 2002 Aero.
Replace hose, (Rock Auto Parts- $15) problem solved.
I've posted an update since then. Unfortunately, it appears to be from where the thermostat meets the block.
 

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Pressure test the system to verify its coming from that stripped bolt.
If so, or if you can already say for sure its coming from there:
It's going to take some skill but you can get stubby 1/4 inch drivers these days that will accept drill bits.. you're going to have to clear out a lot of space but with enough skill you will be able to drill that out.
Before all else get some brake clean and spray that area down to clean it up. Then run it (if you dont have a pressure tester) and watch carefully to see where its dripping from. A flashlight and telescoping mirror from a parts store will be your best friends.
 

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The bolt under the red cross is (normally) not connected to the cooling flow (how should it seal with a simple bolt?).
Why dont you replace the thermostat housing, maybe it has a crack?
Check the picture:
thermostat.JPG
 

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I'd also be looking at the throttle body hoses. They are in that area and a small split will shoot coolant around. I had to deal with that one a few years ago. Right pain to figure it out until the pinhole in the hose got big enough to see it when it was under pressure. They are small and cheap to replace with standard "heater hose".
 

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The best thing you can do is pressure test the cooling system. I found it easy to rig up a piece of hose from my compressor that I connected to the small hose on the coolant reservoir and put a bolt into the hose that usually goes there.

I found my leaking hose in 30 seconds when I got teh pressure going. Not much, about 10 PSI will do it. Everything else at this point is a guess since you've checked the obvious stuff.
 

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Discussion Starter #14
Pressure test the system to verify its coming from that stripped bolt.
If so, or if you can already say for sure its coming from there:
It's going to take some skill but you can get stubby 1/4 inch drivers these days that will accept drill bits.. you're going to have to clear out a lot of space but with enough skill you will be able to drill that out.
Before all else get some brake clean and spray that area down to clean it up. Then run it (if you dont have a pressure tester) and watch carefully to see where its dripping from. A flashlight and telescoping mirror from a parts store will be your best friends.
Thanks for the replies all.

I've cleaned it up, pressurised the system and used the telescopic mirror (great idea!) to locate the leak.

I can now say with certainty that it's coming from where the thermostat housing meets the block. I'll take the thermostat housing off and inspect it and while I'm there, replace the thermostat o-ring. This might be the easiest and best approach at this time but...

What would be great to get further advice on is whether or not only the two larger 13mm bolts (top and bottom) serve to seal the thermostat housing to the block, or if all four, including the two 10mm bolts do this. Happy to replace the thermostat o-ring and see if it leaks, but if need be, it'd be great to know whether I need to re-tap the 10mm bolt marked with the red cross in the image (with the stripped thread) and/or use any anaerobic sealant so I sort it first go.
 

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The bolts have nothing to do with the sealing, the O-ring around the thermostat does that job. you either have a bad o-ring or a crack somewhere.
 

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Only the two 13mm bolts attach the thermostat housing to the head. The two smaller bolts are only there to hold other things, like ground wires and a coolant pipe. They don't go through the thermostat cover and into the head.
 

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Discussion Starter #18
Alright, so it wouldn't be a DIY job without a few hiccups and a bit of a coolant mess!...I've got the car dismantled and the thermostat housing almost off - the 10mm bolt won't come out. I'm glad to read it doesn't seal anything!

I've just bent the bracket backwards with the 10mm bolt attached as this is the only way I can see the mating surfaces and access the thermostat. O ring looks like it's in poor condition and the thermostat housing looks quite filthy. See pic.

thermostat housing.jpg

Is a plastic scraper enough to clean the mating surfaces?

I've seen elsewhere that it's possible to replace the thermostat without draining the coolant. I haven't drained the coolant and wiggled the thermostat a little and coolant started gushing out - is much going to come out if I remove it? Will I need to drain the coolant properly first?
 

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It is not possible to replace the thermostat without draining the coolant

It is possible to replace the temp sensor without draining the coolant but you have to move quickly so you don't lose a lot of coolant

That thing is a mess.... you really need to get the bolt out and get it good and clean. The surface of the head as well

But I suspect you've found your leak.
 

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I agree that the thermostat cover probably is the source of your leak. It should be perfectly clean and smooth, since it mates right up against the cylinder head.

You can replace the thermostat without draining the coolant, though. I've done it several times. You just need to have your new thermostat ready, and act fast. You won't even drain the expansion tank if you're quick enough.
 
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