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Old 10th May 2019, 19:10   #1
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Default Water Flow

Simple question,

The Header Tank on a 75 diesel has a small pipe that goes into the header tank just under the cap, When the engine is running(cap off) should water come out of it into the header tank in a continuous stream?.

If it doesnt what does that mean?
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Old 10th May 2019, 21:58   #2
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it should trickle out once the stat is open.
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Old 11th May 2019, 05:58   #3
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Quote:
Originally Posted by Number 6 View Post
Simple question,

The Header Tank on a 75 diesel has a small pipe that goes into the header tank just under the cap, When the engine is running(cap off) should water come out of it into the header tank in a continuous stream?.

If it doesnt what does that mean?
They can and do block up, and if that happens you get air locks in the system when changing the coolant, which prevent the full volume of coolant going back in.
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Old 11th May 2019, 06:17   #4
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Thanks, Colvert and Trykey,

I will check again when the stat opens and check to see if there is a flow, If it is UNBLOCKED and there is NO water Flow what would that suggest ?
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Old 11th May 2019, 06:51   #5
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I had this issue on Debs diesel, it took a good backflushing if the heater matrix then two lots of radiator flush before it cleared.
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Old 11th May 2019, 07:18   #6
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I had this issue on Debs diesel, it took a good backflushing if the heater matrix then two lots of radiator flush before it cleared.
Thanks for that,,,How did you achieve the Back Flushing.

I am trying to find the reason for the loss of water, I can see no obvious leaks, I did a recent Antifreeze change and also later a heater matrix flush.

I have never had to top the water up ever in all the years I have had it, But now it is taking a lot of topping up every 50 miles.

There is no cross contamination of oil and water and no bubbles in the header tank when the engine is running

Just trying a process of elimination.
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Old 11th May 2019, 07:21   #7
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Hi John
Great link here from Tcut

https://www.the75andztclub.co.uk/for...d.php?t=245285
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Old 11th May 2019, 10:36   #8
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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.

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