2004 Avalon with mystery coolant leak

Car is slowly leaking anti-freeze… fine around town when I’m slowly filling it back up… but can’t do long journeys/hills/mountains all that well. I’ll stop after a long hill and the system is boiling enough to spray the anti-freeze into the reserve tank and then out of the reserve tank. When I wasn’t noticing before, the temp gauge would then go and I would notice. But now, I am proactive about stopping and checking. I let things cool, refill, bleed, and can move along. Mechanics I’ve taken it to say it’s ‘probably’ a head gasket, but my gut just doesn’t feel right as it’s only overheated twice as I was discovering the issue earlier this Spring. And the previous owner was a retiree that never drove it and took it to the same shop for it’s entirety. I finally found a rogue drip today… something the mechanics missed or wasn’t as big of an issue then? (see picture). Can anyone help? I live in a rural town and I don’t want to risk driving it to a bigger town with a toyota dealership without some more answers or strategies on this particular situation. I am aware there may be more than one leak. I am also considering the possibility the car had more issues with it than I knew a year ago, despite a very reliable mechanic shop and care routine from the previous owner. And lastly, I am aware it took me a minute to realize the issue and there may be added damage because I wasn’t consistently refilling it once the leak started.

Your mechanics are, shall we say, less than competent if they did not suggest a pressure check of your cooling system to pinpoint a leak as well as using something like this to determine a head gasket problem.

This test is easy enough for you to do it on your own. Then you can decide how you want to proceed. I’d stay away from those (multiple?) mechanics you’ve taken it to unless they wanted to charge you a diagnostic fee you did not want to pay.

If they did want to charge you a fee, that is only fair, they don’t work for free nor is this test kit. Better to KNOW in person than expect internet folks, not next to the car, GUESS what might be wrong from a photo.

Good Luck!


You’re describing an engine with a blown head gasket.



Yeah, there’s no “wishing” this away. It is what it is, no matter how well a little old man took care of it.

And the fact that it’s been overheated at least twice now isn’t a good sign either. I’d take it to another mechanic for a through inspection of the engine and cooling system. Most newer engines don’t take too well to overheating.


They did test once and it came back all green… and the pressure failed instantly… but the oil test came back negative from the next mechanic (first one didn’t check the oil mix) .

Any idea what the average cost of that fix is? And the oil test came back negative… would that still be an indicator of a blown head gasket?

If you’re car came into my shop with a blown head gasket, and at this age, I’d try sodium silicate to stop the leak.

A sodium silicate repair can last two years or longer. The repair occurs rapidly, and symptoms disappear instantly. This repair works only when the sodium silicate reaches its “conversion” temperature at 100–105 °C. Contamination of engine oil is a serious possibility in situations in which a coolant-to-oil leak is present. Sodium silicate (glass particulate) contamination of lubricants is detrimental to their function.


The first mechanic recommended this solution… But I’m still not sure why you think it’s definitely the head gasket. Just a vibe or was there a definite indicator ? I’ll prob get the test on my own

Yes. The gasket can fail from the coolant passage into the cylinder such that the heat of combustion sends it right out the exhaust. It makes the piston and the spark plug for that cylinder really, really clean!


do you have smoke coming out the exhaust? if so I will also say a head gasket. if not and the oil came back good it might not be. I cant tell from the picture the location of your leak. is it possible the water is coming out the weep hole of a faulty water pump or the gasket? just a thought.

You said ‘they did test once and it came back all green…and the pressure failed instantly’. What does ‘all green’ mean? Good? For which test? If the pressure ‘failed instantly’ the mechanic should have seen where the failure was - did he?


What??? You wouldn’t offer to repair the engine correctly, even if the rest of the car is in good condition and relatively low miles? I can’t believe you would suggest a band-aid fix such as this on anything but the most beat-up rolling wrecks. This product is designed to squeeze a few more months out of a car which is destined for the junkyard, not to be a substitute for a proper repair on a decent car.

A 2004 Avalon is still considered a good car, even by today’s standards. Even if it is correct that a proper engine repair costs more than the car is worth–which is doubtful in today’s market–what’s really important is repair cost versus the alternative. If it costs $2500 to have a proper head gasket repair done, including both heads being reconditioned by a professional machine shop, the question becomes “can you get a better car for less?” and the answer is most likely no.

As long as the rest of the car is still in decent condition, meaning no rust or body damage, it makes sense to have this correctly diagnosed and repaired.

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I do not… and the last test for oil in the anti-freeze came back negative… The liquid is definitely anti-freeze from that spot… and I have no idea what part of the car that is, hence why I posted it… when you say faulty water pump or “the gasket”… you mean “gasket” as in the water pump gasket? This is so helpful! And new information. There is no white smoke from the exhaust and the car only leaks when running…

I called and got specifics… First mechanic did a block test (came back green, but I really don’t trust those guys) and the other suggested it’s the head gasket becuase after filling and re-pressuring the system… there a fast climb of excess/combustion/force getting into the system. Some kind of air from somewhere, but they assume it’s the head gasket… and they didn’t do anything else to ascertain (they did check for oil in the water though, no contamination). They didn’t even mention a block test and they admitted not really knowing that it was the head gasket in the first place. I also suspect they didn’t see the leak that is present now… either it wasn’t there or it got worse, I’m not sure.

I do have an appointment with a garage in town with a person that has been recommended to me lots. He doesn’t repair head gaskets, but he’s willing to diagnose… but ideally I’d love more insights from you all. Any indicator of a type of leak that would be unnoticeable and get worse with time? I am aware the engine has been overheating…

It’s a phenomenal car… I know it’s a quick fix… but I’d rather sell it for what it’s truly worth before I put something like that into it… let someone else make that call. Thank for this info!!!

Does anyone have any insights how climbing a hill would impact the leaking? besides the obvious increased work on the engine…

or using the AC?

Combustion gases come from the combustion chamber.
That should be your answer. Why not try Testers suggestion.

As for appearing when climbing hills, your engine is working harder.

This is pretty conclusive to me: BAD HEAD GASKET. Why do you doubt it?

Why are you not looking for a shop that can actually determine the problem and fix it ?

I would not be too quick to jump on the head gasket. People do this too often. Based on your post, IF it is the head gasket, it is not too late to put in some Subaru Cooling System Conditioner.

Unlike the Sodium Silicate solutions, this can be used with your antifreeze. You just add a bottle, no need to drain the system. Also unlike the Sodium Silicate and many other anti-leaks, it will not clog up your cooling system or do any damage. It is installed at the factory in Subarus and other manufacturers. In fact it was Toyota’s engineering that recommended to Subaru to use this when Toyota bought 20% of Subaru’s parent company.

You can get it at a Subaru dealer or on Amazon.


I have this in my Subaru. I also used it in my Silverado that was loosing a small amount of coolant (cup/week, qt/month) due to head gasket leak. Been in for a couple of years now and have not lost a drop since. Just one bottle, 5.3l engine 18qt cooling system.

But, have you tried a new radiator cap yet? When a vehicle overheats with extended highway driving or going uphill on a highway, that is the first thing I would do. It works almost all the time, almost. Its cheap.

Edit: BTW, the Subaru conditioner only works on very small head gasket leaks. If your head gasket is truly leaking and you are getting into the range of having to top off the system daily, then you can try the Sodium Silicate, but also has limits on how much it can repair. It can work on a larger leak than the Subaru conditioner though. After that, only new head gaskets will work. If you are due for a new timing belt, include that to save some money. If you have a timing chain and you have over 200k on hte engine, you might have that replaced when you do head gaskets.

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