[Fot] Radiator burping

[Tony Drews]

As someone mentioned, any overflow should go to the overflow 
tank.  The seal between the top of the rad cap and the outer flange 
is supposed to be an "infinite" pressure seal.  The spring and inner 
seal is where the 7 lbs or 21 lbs or whatever the cap is rated for 
applies.  It lets the water in between the inner seal and the outer 
seal to go to the overflow tank.  When it cools down it's supposed to 
suck water from the overflow tank back into the radiator.

As mentioned, two things will cause the water not to go to the overflow:
1. blocked line to the overflow
2. crappy seal between the radiator cap and the radiator

I've had issues in the past where after a 30 min session the overflow 
tank (1 - 2 qt size) was almost full and it wouldn't pull back into 
the radiator.  This was caused by a minor leak between the combustion 
chamber into the water jacket.  There was still enough pressure in 
the radiator that the water wouldn't pull back in.

A hard run at low speed will likely put more heat into the system 
than it handles well, and then shutting down the car would cause more 
heat soak where you may see some localized boiling and would 
certainly see the water expanding.

I have a little bleeder on the top of my thermostat housing to get 
the last little bit of air out of the block, but having a little air 
in the system would give it some room to expand too so I don't think 
that's the issue.

One other issue I'll mention is that the stock radiators need a cap 
with a longer than usual distance between the inner seal and top of 
the cap (the spring part needs to be longer).  I run an aftermarket 
radiator so it uses normal caps.

21 psi is certainly too high - you risk blowing the figure 8 seals 
with that.  I'd be nervous going much over 10 psi.

- Tony Drews

At 01:00 PM 5/7/2008, Mark Vaden wrote:
>Hi All,
>
>Last weekend I was running my TR4A at VARA's British Extravaganza. The
>car ran great, but after each run it was burping water around the edge
>of the radiator cap, and not into the overlfow. On the second day I
>bought a 21lb cap, and it did not burp out the cap during my first 2
>runs, but it did burp out on my 3rd run. I think that 21lbs is too
>high, but it was all they had available at the track . The car never
>ran hot, and it is a new engine rebuild. The head was pressure tested
>and magna fluxed. Any ideas on what would cause all of this pressure?
>
>TIA,
>Mark
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