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Discussion Starter #1
Basically I put my new HK$ Radiator cap on which is a 1.1 bar item as opposed to the 0.9 bar stocker.
Worked fine whilst driving, held pressure in top hose and still allowed coolant to pass into the overflow bottle.
However upon cooling down rather than sucking coolant back into the radiator from the overlfow bottle the top radiator hose has been sucked in. In my thinking this must mean the return valve on the radiator cap must be faulty. There was an awful lot of vacuum and had to really apply alot of force to prise the cap off. I guess I know that my cooling system is completely sealed!

So the main question is seeing as its a higher pressure cap does the return valve also need higher pressure ( I would have thought not) Now it is definately the right cap, there are only two types for Toyotas and the wrong ones do not fit at al and if you forced it on do not hold any pressure at all whilst driving.

I am shying away from pointing at the overflow setup being at fault as it work fine right up until I put this cap on
 

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JZS161 Aristo TT
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881 Posts
Radiator Cap Selection

It has become popular to install high pressure radiator caps in an attempt to lower cooling system temps. While some makes and models come from the factory with and are designed for high pressure caps, raising your radiator cap's pressure above factory specs increases your cooling system's maximum operating pressure above design parameters. While raising your cooling system's maximum operating pressure does raise your cooling system's boiling point and therefore helps to avoid boil-over, it does not lower your operating temperature prior to boil-over, and simply allows your engine to continue to operate at temperatures higher than it is engineered for. Water boils at 212 degrees, and for each pound you raise the system pressure it raises the boiling point 3 degrees - Example: The 1.1 bar (16 pound) cap will have a boiling point of 260 degrees. Operating your engine at higher than acceptable temperatures can destroy your engine. We wish to point out some considerations that should be taken into account before an individual chooses this approach.

There are several possible reasons that you might be experiencing higher than normal cooling system temperatures:

An engine malfunction or cooling system problem - increasing your cooling system pressure will not correct this problem and will quite possibly add to the problem. We recommend correcting the problem instead of using a higher pressure cap.
Engine or body modifications have altered the performance of the cooling system or air flow through the radiator. Again, we recommend correcting the problem instead of using a higher pressure cap.
Engine modifications have increased the HP of the engine - creating more HP means creating more heat. We recommend increasing the heat dissipation capabilities of your cooling system to match the increase in heat generation. The best way to do this is install an All-Aluminum High Performance radiator, increasing the radiator cap pressure will not help dissipate additional heat.
Your car is being driven in a harsh (high heat) environment or is being driven hard for extended periods of time. Again, the best way to solve this problem is install a High Performance radiator, increasing the radiator cap pressure will not help dissipate heat.
Raising your cars cooling system's operating pressure increases the stress on all the other components of your cooling system. Before increasing your cooling system pressure by replacing your radiator cap with a higher rated radiator cap, check to make sure your entire cooling system is in excellent condition and all of the components are capable of handling these higher pressures. These may components include, but are not limited to:

The radiator (all of the All-Aluminum radiators that we sell are rated to handle these higher pressures)
Radiator hoses
Heater hoses
The heater core
The water pump
Freeze plugs
All gaskets that come in contact with the cooling system
Increasing your cooling system pressure by installing a radiator cap that has a pressure rating higher than factory specs increases stress on the entire system. Although some race cars use high pressure caps, their cooling systems are built to handle these higher pressures (All-Aluminum brazed and welded radiators, steel braided hoses, no heater core, high performance water pump, high performance gaskets, etc). We offer high pressure caps because some cars are designed with high pressure cooling systems. We recommend that you only use a cap with the pressure rating that your car was designed for.
Heater cores are fun to replace :1poke: use a .9 bar as recomended

Clif notes
increasing your cooling system pressure will not correct this problem and will quite possibly add to the problem. We recommend correcting the problem instead of using a higher pressure cap.
Info from http://www.absoluteradiator.com/Accessories.asp
 
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