header tank problem

[Fortyfiver]

Just wondering if anyone has had the same problem, when reving the car the header tank fills up via the top hose which is tee’d to the top of the radiator, when at idle the water drops back down, the bottom hose from the tank is plumbed to the U hose, the water doesn’t seem to be being drawn into the engine from the bottom, the top hose only has a 4mm ID and the bottom one has a 12mm ID
thanks Alan

[Swifty75]

This is a problem with the header tank set up. I had this and it was fine until the car ran at higher revs and the water pump pushed more water through the top hose. It returned it to the header tank and bypassed the radiator meaning less cooling. I know Richard at Church house classics (watch his video on YouTube) had this same problem.

[Fortyfiver]

I expected a small amount to bypass the radiator but it doesn’t seem to be going back into the engine via the bottom hose
Alan

[sujitroy]

i have the stav weber header tank With his setup there is a small plastic restrictor in the small hose going back to the top of the header tank? this is needed to prevent what seems that your set up is doing. the restrictor is about a few mill and no more

[Fortyfiver]

Perhaps I should restrict it less than the 4mm it is
Alan

[KOY 23]

Is this happening on a home made solution or on a proprietary kit fitted as per the suppliers instructions?

[jbuckl]

Many photos of these kits show them with the return plumbed back to the hi pressure side / top hose.
That means they are not working as a header thank is intended to.
it should be a small bleed off the high pressure side; an opportunity to vent within the tank then a return to the low pressure side .
Bottom of the tank (return) should (must) not go to the top hose.
Header tanks on a stag are not essential.
Header tanks on a stag that are incorrectly fitted are not useful.
Header tanks on a stag can provide extra coolant capacity / low coolant warning.
Header tanks on a stag can assist with venting trapped gas (air & or combustion gasses)
Header tanks on a stag can mask another fault....e.g. small coolant losses & small head gasket leaks.
Apologies in advance for any upset caused by stating my understanding based on a few decades in powertrain design.

[Fortyfiver]

It’s a Mercedes tank with the top pipe fitted to the top of the radiator via a 90 degree elbow and the bottom pipe fitted to the U hose via a tee piece as most seem to be, I can understand why the water flows from the top of the radiator into the header tank but can’t understand why it doesn’t continue through the header tank and back into the engine as I’d of thought the pump would suck it through, it fills up the header tank until I drop the revs then the level drops back down
Alan

[KOY 23]

I also have the Merc tank but plumbed differently. The small dia top pipe goes into a stub soldered into side of rad filler.
Tank lower pipe enters rad via a 22mm stub soldered low down into N/S. Maybe not technically ideal but works well and H/T does not bypass the rad. and compromise the cooling system.

[Swifty75]

Itâs a Mercedes tank with the top pipe fitted to the top of the radiator via a 90 degree elbow and the bottom pipe fitted to the U hose via a tee piece as most seem to be, I can understand why the water flows from the top of the radiator into the header tank but canât understand why it doesnât continue through the header tank and back into the engine as Iâd of thought the pump would suck it through, it fills up the header tank until I drop the revs then the level drops back down
Alan

Surely though when the level drops back down the water is exiting the header tank somewhere. If it is not going back through the top hose to the radiator (unlikely), its got to be going through the bottom hose to the water pump draw and cool side of the system?

[Fortyfiver]

On the bottom hose from the header tank I’ve fitted a reducing insert as the header tank is 26mm and the tee piece at the U hose is 16mm the hole through the reducer is 12mm, I’m wondering if the pump is drawing water from the least resistance and none from the header tank, there’s only a couple of mm in it so not sure
Alan

[NeilR]

I still maintain your return feed is too high. As it is on some of the so called purpose made options.
Having to fit restrictors in the upper pipe shows that to be the case.

Only place for the lower one is low down, nearer the bottom of the rad. Then it won't happen.

Just my personal opinion of course.

Neil.

[jbuckl]

Itâs a Mercedes tank with the top pipe fitted to the top of the radiator via a 90 degree elbow and the bottom pipe fitted to the U hose via a tee piece as most seem to be, I can understand why the water flows from the top of the radiator into the header tank but canât understand why it doesnât continue through the header tank and back into the engine as Iâd of thought the pump would suck it through, it fills up the header tank until I drop the revs then the level drops back down
Alan

Seems odd for sure. Is the feed pipe hard work to pump through because it’s uphill?
maybe the pump has difficulty at low speed.
Ive seen some pump heater return spigots with 3/8 holes only. Seemed a bit tiny to me. Might work for the heater but not for a tank?

[Fortyfiver]

Will probably have to re plumb as suggested, was advised to try turning the heater to cold and it worked, no more filling of the header tank, turned it back to hot started filling again, can’t work out why bypassing the heater matrix would make any difference, used to think I was logical until the stag came along

[NeilR]

That is wierd !, wouldn't have expected that.

Neil.