The 75 and ZT Owners Club Forums - View Single Post

T-Cut · 11th May 2019, 10:36

The small bore pipe into the header tank is primarily an air vent to clear any bubbles from the head and the upper corner of the radiator. It's not a formal part of the cooling circuit - at 8mm bore, it's too small for that. Indeed when the pressure cap's in place, the small drilling you see in the tank neck is blanked off by the O-rings on the cap. Interestingly, the vent has a separate outlet underneath the neck and out of sight. That exit is much wider and will remain clear, even if the small neck drilling gets silted up. The silting is due to spending 99% of the time blanked off, so it never gets flushed. The drilling only gets used in abnormal heat soak situations, which rarely if ever happen in the diesel.

The question of coolant flow from the neck vents is an interesting one. Of course it can only be observed with the cap off, so this in itself is abnormal for the running engine. In the KV6, flow through the vent is frequently mentioned, but I'm unclear whether all KV6s do it. It would be useful to have a survey on this. In my 1.8T I've never seen it and I'm unable to make any coolant come from the vent by applying high revs. The pressure drop across this small bore pipework is much higher than anywhere else in the cooling circuit, so it's not a given that any flow will be seen in all engines. It will vary with the location of the pump in relation to the vent pipework and overall layout of the circuits. I'd expect the diesel to behave more like the 1.8 engines than the V6, so wouldn't be surprised if little or no coolant is seen. Again a survey on this would add some understanding.

TC

11th May 2019, 10:36	#8
T-Cut This is my second home Rover75 and Mreg Corsa. Join Date: Nov 2006 Location: Sumweer onat mote o'dust (Sagin) Posts: 21,753 Thanks: 341 Thanked 3,660 Times in 2,924 Posts	The small bore pipe into the header tank is primarily an air vent to clear any bubbles from the head and the upper corner of the radiator. It's not a formal part of the cooling circuit - at 8mm bore, it's too small for that. Indeed when the pressure cap's in place, the small drilling you see in the tank neck is blanked off by the O-rings on the cap. Interestingly, the vent has a separate outlet underneath the neck and out of sight. That exit is much wider and will remain clear, even if the small neck drilling gets silted up. The silting is due to spending 99% of the time blanked off, so it never gets flushed. The drilling only gets used in abnormal heat soak situations, which rarely if ever happen in the diesel. The question of coolant flow from the neck vents is an interesting one. Of course it can only be observed with the cap off, so this in itself is abnormal for the running engine. In the KV6, flow through the vent is frequently mentioned, but I'm unclear whether all KV6s do it. It would be useful to have a survey on this. In my 1.8T I've never seen it and I'm unable to make any coolant come from the vent by applying high revs. The pressure drop across this small bore pipework is much higher than anywhere else in the cooling circuit, so it's not a given that any flow will be seen in all engines. It will vary with the location of the pump in relation to the vent pipework and overall layout of the circuits. I'd expect the diesel to behave more like the 1.8 engines than the V6, so wouldn't be surprised if little or no coolant is seen. Again a survey on this would add some understanding. TC Last edited by T-Cut; 11th May 2019 at 10:41..