> I don't want to buy and fit a new radiator, only to have the first
> problem blow a hole through that as well,
As long as your pressure cap is relieving properly (and I can't imagine how one
could fail closed), there should be no danger of the "problem" blowing a hole in
Normally it's very obvious when a radiator has been overpressurised (as can
easily happen if you succeed in getting mains pressure on it ... mains pressure
is many times what car engines operate at). Since you didn't mention tubes
being distorted and whatnot, I think it's safe to assume that either you knocked
a hole in it or it's rotted from the inside and just chose this moment to go.
> but I cannot work out what
> that problem can be. It must be a blockage somewhere in the system,
> I'm sure, but flushing shows that water flows right the way through the
> system without any trouble. Can you help? Please?
It seems suggestive to me that it worked reasonably well without the thermostat.
Since you say you've tested the thermostat, I wonder if the seal on the water
pump is bad and allowing air to be sucked in when the engine is cool (and thus
the coolant is not under pressure). I've never seen this on a Triumph, but I
have on other cars and the symptoms are similar.
Another possibility is a combustion leak into the water jacket, like from a
leaking head gasket. That can sometimes "air lock" the water pump and cause "no
If you lived just a bit closer to me, I'd strongly recommend my local radiator
shop (Mac's Radiator in Bellflower, CA). Whether you want to trust yours with
your LBC is up to you ... but if you just knocked a hole in your radiator, they
can likely repair it for much less than the cost of new; and probably
troubleshoot your overheating problem at the same time if you take them the