Since I have a bunch of time off to use before the end of the year, I took the day off work. Drained the radiator, the header tank, and half the coolant in the recovery bottle. Then I filled the system (correctly this time). Previously, I had filled the header tank, then moved on to the filler neck. Turns out, if you’re patient, you can fill most of the system through the header tank. I’m much more confident that it is actually full this time. Then I put one of the caps on the header tank and the pressure tester on the filler neck. It got to about 2psi, maybe 3, before coolant started dribbling out around the seal. Then I tried the other cap. It didn’t make it to 1 psi. One pump on the pressure tester pump and it started leaking. Neither of these really feel like they’re tight when they’re on all the way, so I’m not surprised they leak. Now I need to get two more.

 

Well done on all counts.

Yes, coolant OUT = air IN, there be your problem.

We have Tridon brand caps down here, CRAP and thats polite. Their upper seal is a glorified rubber washer, and will not seal, even on Aussie cars. Son chased the exact same issue for 2 years on his Commodore, before my spare V12 CPC brand cap fixed it.

The CPC caps have a purpose formed seal at the top, and they actually seal to the outside world.

Not easy to find out side OZ I am told.Thought I had a snap of the underside of a CPC cap, but I thought wrong, and Google is being unfriendly at the moment.

 

Gareth, I have found that Stant Lev R Vents caps are excelent.

 

I keep thinking this issue is solved, but it isn't. I replaced the caps with Stant Lev R Vents and they give a much better seal. But that didn't fix it. I replaced the bleed plug on the top of the radiator, and it gave a much better seal, but that didn't fix it either. What do I do now? I'm going to let the car cool off, add more coolant, and the pressure test it. Is there something else I should do? I'm getting frustrated and running out of time before winter, and the car can't stay where it is...

To restate the problem, when freshly filled with coolant everything seems fine. The engine runs up to a normal temperature and seems to stay there, although I haven't run it for all that long. However, after being allowed to fully cool down, when run a second time the temperature rises higher than before and coolant starts overflowing from the expansion tank. Air is being sucked in somewhere, rather than pulling coolant from the expansion tank.

 

Gareth
As a checklist - and I have not re-read the thread - have you done these things:

• Verified the overflow bottle in the wing is working OK and that the tube to is it unblocked and not leaking
• Changed the thermostats (if not these are in my views the most likely cause)
• checked that the radtop purge system is working and all tubes unblocked
• had the radiator out and cleaned out the fins and the rubbish that accumulates between it and the aircon condenser/oilcooler stack, and cleaned out the condenser fins ?
• had the radiator checked for blocked fins?

If you had a leak I doubt it would cause the tank to overflow. This sounds more to me like a blocked rad or thermostats than a leak. If all this has been done, then beg or buy one of these:

See what it shows up.

 

To clarify....

Do you mean the atmospheric tank hidden inside the fender or the metal tank in the engine bay?

Cheers
DD

 

Yes. Coolant starts pooling behind the front left wheel. That tank has filled up and is over flowing.

And Greg, Everything on the cooling system is new.

 

Although there's a bit of apples-vs-oranges, I'm reminded of something from my XJ6 days.

For years there was a common complaint among owners that about coolant loss and a seemingly endless need to top-off. Going right to the point, some were over-filling the system to the point where the atmospheric tank was over-filled. Normal expansion would then push coolant out of the tank and onto the ground. Seeing the coolant loss owners would add more coolant.

The fix was to do exactly......nothing. Stop adding coolant. Drive the car for a couple days and see if the discharge stopped which, in all cases that I recall, it did. It was a matter of coolant in the atmospheric tank seeking its own level, so to speak.

You mention that you "....haven't run it for all that long".

I'd be tempted to drive the car and see what happens.

Cheers
DD

 

Well I could try that, but until pretty recently I did have an actual air leak. The radiator bleed plug I had was incorrect and a bit loose. The new one is a much more secure fit. Previously coolant has been leaving the radiator and not returning, so that a previously full radiator would now have several inches of air at the top. That doesn't sound like its natural level to me. The last time I added coolant the expansion tank was between 1/3 and 1/2 full. I took coolant out of it and put it back in the header tank, since it wasn't doing that on its own.

 

Is the line between the header tank and expansion tank open? Common for them to be plugged. Disconnect from header take and see if you can blow air thru it. If so, apply a vacuum to the line and ensure you can pull fluid from the expansion tank.

​​​

 

I'll test it. But if coolant is flowing through it one way I'd assume it's not plugged. Coolant is definitely going to the expansion tank.

 

Gareth
I am in full agreement with Doug on driving it.
Having done that expansion tank hose test, I suggest:
Ensure the radiator purge system is unblocked by blowing through all pipes and tubes. Be SURE the thermostats have the bleed holes upwards, are opening in hot water, and all purge tubes are connected properly; post photos if not sure. Refill all (sequence: both caps off, radiator bleed plug out, header tank full to brim filling it slowly; cap on header, fill central spout SLOWLY until bleed plug running bubble free, bleed plug on, fill spout until full). Have car pointing uphill if possible.
Put the heater on defrost, start car, loosen bleed plug and continuously top up, spilling plenty no doubt but do not worry. Replace bleed plug.
Heater still on defrost take car for 10 mile run. let it completely cool and do not open anything. Then before opening any caps undo bleed plug and top up if needed. Replace bleed plug, remove expansion tank cap and top up tank, replace and do spout top up. repeat 10 mile run and let cool as before.
Then repeat the top up sequence and report back.
If loads of coolant loss on second go, pressurise system cold and see what happens.

 

Hello All... Great discussion here, learned a bunch of things and, one gave me a bit of concern which leads me to question my set up. So, just for clarity...

Two caps. One on the filler, one header... Are they the same cap? My caps are the same!

What I THINK I got in all them exchanges is:

Header tank should not have a cap with the lower primary seal (16psi) found on the filler kneck? This, so as the heat and pressures build, and the fluid expands - fluid is free to pass into and out of the expansion tank in the fender through that spout found on the neck of the header tank...? And then the opposite, as things cool and pressure decreases, fluid will be drawn back up from the expansion tank back into the primary"system"...

In my minds eye,,, that header outlet spout is BETWEEN the lower primary seal (on my caps) and the upper cap seal in the header tank?

Are there 2 different caps?

 

Early cars had two different caps, and could cause confusion. Later ones used the same cap. The pressure release function just doesn't do anything on the second cap. All is explained in Kirby Palm's Book found in the How-to sticky at the top of this forum.

 

JJJ
The cap which controls the system pressure is the header tank one.
The spout cap DOES NOT control system pressure because BOTH SIDES of the cap release valve are within the system.
When the spout cap blows off the coolant is released into the radiator purge pipe system, so no system pressure change; unless faulty, no coolant exits from the TOP seal of the spout cap under any circumstances.
When the header tank cap blows off, the system pressure drops because the coolant exits the system to the catch tank. Therefore the pressure rating of the header tank cap is important.
I believe that the best thing to do with the spout cap is to fit a solid cap to it. I ran my car like this for years until I remodelled the cooling system.
Look carefully at these two diagrams and you will see that the above is correct.
Anything you do not follow, please do not hesitate to ask. Understanding the V12 cooling system is right up there with the most misunderstood things about the car, and all sorts of people still hold that the spout cap does do something to control system pressure!

 

As usual, thanks Greg!

I will definitely be looking at these documents. Cool.

My question, what is leaving me puzzled, with the header tank and it's cap (the one I have) was/is this. Ok,,, pressure reaches the cap pop point, ok, fluid/extra pressure runs off into the expansion tank in the wing via hose... But then (engine OFF), as things cool, pressure decreases how does fluid get drawn back and reenter the "system" with a 16psi rubber spring loaded seal, that opens in the opposite direction, in the way?

I think I need a different cap...?

 

These are EXCELLENT diagrams!
Thanks!

I removed and replaced my no longer functional header tank and I have been itching to cut the thing open to see what's going on in THAT thing. I don't think it's as simple as fluid in fluid out. Seems there's passages in that thing. Theses cars are full of lil secrets...

 

If your header tank looks anything like my old one, it will be full of rust scale. A while back, someone made stainless tanks, but I haven't seen them advertised in a long time.
Some of the tubing inside the tank is extended from the outside, but nothing really magic is in there.

 

JJJ
If you look at a pressure cap of the type that fits the header tank, you will see:

1. that the lower portion has a large spring holding down, which takes (for example) 16 psi to overcome, when this level is reached the spring gives and fluid escapes.
2. BUT, look closer and you will see an INNER portion of the sprung bottom part, that will easily open DOWNWARDS. This allows fluid to be sucked back from the atmospheric tank under the partial vacuum created when the coolant cools off.

https://www.google.com/search?q=how+...C5HjUpuTuLgD26

 

Hollllly ****... Lol, ok.
And allllll these sleepless nights!
Honestly, I didn't know!
Thanks, Greg.