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Old 13th September 2020, 19:05
cbjroms cbjroms is offline
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Default Cooling System Problem

So I am running a Herald 13/60 engine in my (Sammio) kit car. The top of the radiator sits lower than the thermostat housing due to the shape of the bonnet. So, in the hose which connects the bottom of the radiator to the thermostat housing I have added a T piece, one limb of which is connected to an expansion tank. The expansion tank sits slightly higher than the top of the thermostat housing.

I was out in the car this afternoon and noticed that the temp guage was reading very high. First thought was that the temp controlled radiator fan had failed but that all checked out fine. Then I noticed that the radiator felt cooler than anything else and that the expansion tank was bubbling. I carefully removed the pressure cap from the expansion tank (it was full to the top) and lifted it higher above the engine. There was a gurgling noise, the level in the tank dropped back to normal, the radiator heated up and the fan kicked in.

I had presumed that with the exapnsion tank being higher than both the thermostat housing and radiator, any trapped air would make its way to the expansion tank. But today, the air seems to have collected in the radiator. What am I missing?

Last edited by cbjroms; 14th September 2020 at 17:31..
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Old 14th September 2020, 18:49
cbjroms cbjroms is offline
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I have put some photos onto piston heads:
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Old 14th September 2020, 20:02
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Mister Towed Mister Towed is offline
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Originally Posted by cbjroms View Post the hose which connects the bottom of the radiator to the thermostat housing I have added a T piece, one limb of which is connected to an expansion tank.

What am I missing?
There's your problem, the thermostat housing should be connected to the top connector on the radiator, and the pipe to the expansion tank should connect in as high as possible in the top hose.

Otherwise, any air trapped in the cooling system would have to travel all the way to the bottom of the circuit before it could enter the pipe and escape into the expansion tank and air doesn't like travelling downwards in a liquid.

One way of avoiding trapped air in the first place is to disconnect the top hose from the thermostat housing and fill the system through that hose until coolant comes out of the housing. Also don't forget to pre-fill the heater matrix if you have one fitted.

Finally, the symptoms you describe could be due to head-gasket failure allowing compression gases to overpressurise the system. Check for 'mayonaise' stuck to the inside of the oil filler, it's not always present but it's a giveaway if it is.

Hope that sorts it.
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Old 15th September 2020, 14:16
cbjroms cbjroms is offline
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Thanks Mister Towed,

My description was incorrect. The top radiator hose does go to the thermostat housing. Bottom radiator hose feeds from the water pump.

On Pistonheads the suggestion is that I need to change:
a) Expansion tank should feed into bottom radiator hose.
b) Add connection from radiator filler drain (always open on Honda radiator I think?) to the expansion tank top (but not vent).

Does this make sense?
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Old 16th September 2020, 10:26
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Grey V8 Pete Grey V8 Pete is offline
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Hi! I would offer the following which I hope will solve your problems.
Yes a coolant hose should ideally go uphill from the thermostat so natural convection aids the flow. However you should be ok provided the water pump is in good condition and the radiator and engine are free from sediment build up.

On your pistonheads photo there should be a plain sealing cap on the radiator at 4 so it seals on the top face of the rad boss with no spring or pressure relief. There should only be a pressure cap on top of the expansion bottle 7.

The hose 6 needs to be Teed off immediately after the thermostat housing which is where any air will tend to collect.

Hose 6 needs to be small bore (mine is 8mm) so that as the coolant heats up and expands it will expel any air up into the expansion bottle.

The small bore hose attached to the top neck of your expansion bottle 7 is an “overflow” drain and so should be open ended and finish down under the chassis.

The expansion bottle should not be filled up! Leave about 1” of coolant above the lower connection which should be ample. Keep a check on this after the first few start and cool down cycles and top up as required to replace if any air has been expelled from the system.

Finally I would fill your coolant system from empty as follows:
Remove the plain seal cap from the radiator.
Remove thermostat housing and thermostat.
(I would fit a new thermostat anyway at this time as a precaution.)
Start filling system through the thermostat housing keeping an eye on the open radiator filler hole!
When the coolant level reaches the radiator neck replace the seal cap.
Important! Check now in the workshop manual to see if there are any bleed vents on the engine that need opening to remove air locks.
Continue filling until the coolant level reaches nearly to the top of the thermostat housing.
Fit the (new!) thermostat, new gasket and thermostat housing cover.
Continue adding coolant through the neck of the expansion bottle until it is about 1” above the bottom inlet.
I would then unbolt the expansion bottle from the bulkhead and raise it up for a while so any air bubbles can escape.
Rebolt the expansion bottle in place.
Recheck there is still 1” coolant above the lower connection.
Replace the pressure cap on the top of the expansion bottle.
Start the engine and allow it to warm up, keeping an eye on the temp gauge and check the top hose gets hot when the thermostat opens.

Finally recheck the coolant level in the expansion bottle when the engine is cold.

Hope this rather lengthy post helps. Let us know how you get on. Peter.
P.S I have this layout on my V8 engine and it works well!

Last edited by Grey V8 Pete; 16th September 2020 at 10:28.. Reason: P.S. added.
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