This is a photo of a fellow disco 4 owner’s car. Today, this little plug popped its head and we overheated. The thread was still in there so we unscrewed it and no damage in there but what caused the head of this to pop off under such pressure that it put a hole in the insulation under the bonnet, coolant splattered everywhere. Radiator cap still in place. Car still starts so we were able to move it but has been towed to mechanic now. Thanks in advance for your input.

 

Heat over time makes ALL of plastic bits in the cooling circuit. I'm on third replacements now on many parts. For you this's a sign it's time to go through and start replacing now as going to continue this way until then.

 

Thankyou. Yes we are hopeful it’s just the part that has gone brittle as you said and not something more that made things get too hot in there. Really appreciate the reply. Cheers.

 

Unless your expansion tank cap's valve's faulty, it'll relieve any abnormal pressure from the circuit prior to anything popping off, unless of course the plastic's fatigued as in your case.

 

Thanks heaps, great thought. We might replace that as well to be safe. Really appreciate the input!