Expansion Tank - or coolant leak

I’m still curious exactly how the expansion tank works on the V12.

Two weeks ago, I did my 2 year changing of the coolant. I disconnected bottom radiator hose, AND removed expansion tank to give it a good clean out (it was actually not bad).

I jacked up the front end of the car, with radiator bleed port (left side) up a bit higher than right side. I also opened up the breather I had installed at top of heater hose to help get air out of the system.

I put in about 4 gallons of 50/50 mix, both filling the expansion tank and the radiator. Over the last two weeks, I’ve made sure to run heater a bit and I’ve been topping off the filler tank (at radiator), only needing to add a tiny bit now and then.

Suddenly two days ago, my low coolant light came on. I checked, and the expansion tank was empty!? Yet the level was full at main filler neck. I’ve since filled up the expansion tank. I probably added a good 1/2 gallon.

So what exactly is going on!? I have no leaks and no engine issues.


Have you checked the line between header tank and atmospheric catchment tank? The hose adapter in it has been known to get plugged.

Also check your radiator caps – both of them – to make sure they seal properly.

I don’t know that much about the V12 cooling system, but I do know this, and it’s just straight physics: water doesn’t just magically disappear: somewhere there’s a leak.

I replaced that line between header tank and catch tank over a year ago. (I assume you mean what I’m calling expansion tank, and the plastic bottle located in the fender)

I find no leaks anywhere. Caps are good, and only 2 years old. I’m wondering, is it just that easy to not be able to perfectly get all the air out, and it simply sucks out what’s needed from the expansion tank (or header tank as you call it?)?

I’ll keep an eye on it over the next week. Hopefully that’s all it is. So the expansion tank is kinda like an overflow? And the catch tank is an overflow for that?

Have you checked the oil, and I would inspect the spark plugs.

It is odd that ive had no coolant loss for a year, and then i suddenly flush coolant and it happened.

I just got her up to 200F, as i figured i could have a high pressure leak due to not tightening a clamp down enough. I see no drips. Exhaust shows no sign of burnt coolant, car idles smooth. Oil and ATF look great, perfect levels. So i doubt I’ve just suddenly blown a head gasket.

I’ll keep my eye on it I guess…im checking every single place i disconnected.

Could a leak happen at hose clamp only at a high rpm?! I know last time i did flush two years ago, bottom hose clamp needed extra tightening due to leak.

As I understand it, these cars have a difficult time with air bubbles: perhaps thats it?

Greg, I would be guessing as far as your loss of coolant goes, but, thanks to making the copper crossover over the winter, I have a fair idea how the system works:
Coolant is pulled through the expansion (header) tank when the engine is running, through the small fitting which runs through the crossover down to the water pump. At the same time, coolant enters the header tank coming from the air purge tubing running crosswise across the top of the radiator. For this to happen, the banjo bolt has to not be plugged up. Any air that finds its way into the filler neck, or at the top of the radiator, should wind up collecting in the header tank. As the coolant gets hot and expands, the pressure cap allows air and coolant to escape to the atmospheric tank behind the LF wheel. As the engine cools, coolant alone should be drawn back to the header tank, keeping it constantly full.
One thing I have done, after tightening all my hose clamps several times over the last few weeks, is to replace most of them with “Norma” clamps. No more leaks!
Maybe your heater had an airlock? Guesswork, really.

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Thanks, just knowing how it works helps a lot. It is an odd system, perhaps air lock or a collection of air gaps. I guess in a week I’ll know if it’s low again! I’ll keep checking.

On a side note, I just added funnel scoops onto my 3" modified air intakes, only for looks. Now I’ve noticed when at a high rpm, I get a high-pitched whizzing sound, like a vacuum cleaner, in addition to the intake roar. ??? Could these actually change the noise of air?

I guess the two comments I would make is , not sure where your coolant is going but every time you open and inspect the overflow tank you’re loosing the vacuum (if that’s the right word) in your system and you’re probably having to start over to confirm if your environmental overflow tank is doing it’s job. You should probably raise the left front of your car and use your bleed valve on top of the radiator to ensure you have a complete system with no air locks. Also have you replaced your banjo bolts?
Part two of my comments would be let us know how that air intake works.
Were you looking for extra performance or just noise?

Take them off: if sound goes away…yes!


Thanks, I’ll do that this weekend to check. Yes, I replaced banjo bolt last year with an upgraded version from John John.

Air intake, I wasn’t looking for extra performance from the funnels, just didn’t like the black painted ABS pipes sticking out like a sore thumb. But I definitely got better performance intake and more noise when I first cut off the original trumpets and went 3".

Ha, can’t remove them to test, as I JB-Welded them in! I guess I could remove air intake boxes and tie the air filters in place? Is that safe?

I am just trying to make sure my Transmission isn’t suddenly making whining noises…it’s eerily similar to the sound our TH400’s normally make in 1st gear. This noise is happening at high RPM in 2nd gear.

Yes, the length of the air intakes will definitely change induction noise, which is why the OE snorkels are so long.(and tapered.)

I did something similar, and wound up adding silicone hose to quieten things down a little. Still sounds good, though!

Stop posting photos of your engine bay! It makes mine suddenly feel so ghetto! LOL

Regarding the noise, I’ll just start imagining mine as the sound Mad Max’s Ford Falcon made when he clicked on the Supercharger. It’s a bit similar.

If you look at SU carbs that are being used on race engines you will see these trumpets. They are there for a reason and its not looks.


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well, I guess my engine isn’t modified enough, as I don’t notice any power increase LOL

Found the leak! I guess it kept getting worse, so finally in the Safeway parking lot, there was a trail of green coolant telling me exactly from where (Embarrassing)

That good ol’ plastic cap for that T flush Joint that I put in heater hose to help fill cooling system was cracked. I always need pliers to open it. I guess last one did it.


Greg …

Ouch ! … “T flush Joint that I put in heater hose to help fill cooling system was cracked” …

You forgot the Hippocratic oath, “First, do no harm” :grinning:


So here’s old cap.

Instead of buying another $4 plastic tee for the crap cap, ive ordered a nice aluminum bleeder port. Plug uses oring. $32.

For now, i found an old garden hose cap from a sprinkler system i had in junk drawer. Much thicker plastic than original. That should keep a week?