My thoughts are this - when cold, the water pump spins as normal (has no choice really!). However, it cannot pump through the rad from the bottom, as the thermostat is closed. This means the warm coolant goes through the bypass hose and to the block and heater, until such a time as the temperature is up to 88 degrees, when the thermostat opens and allows full circulation of the coolant. Before the thermostat opens, the top hose at the thermostat housing gets warmer, but I suspect just due to heat transfer.

The main hose from the water pump to the bottom of the rad near the water pump does get warm before the thermostat opens as the coolant is being mixed by the pump spinning.

Once the thermostat is open, the bottom hose gets hotter as the hot coolant is now allowed to be pushed through the radiator. The radiator then warms up as the hot coolant is circulated.

If this all makes sense so far, riddle me this: Once the thermostat is open, the rad only gets warm up the side where the hoses are, and not across the bottom. Presumably, the coolant flow would be across the rad, as the longest route to the top is that way, giving the coolant plenty of time to cool. The top hose gets very hot near the thermostat housing, but not at the rad, suggesting back flow of the coolant through the thermostat now it is open.
I read somewhere that the 300tdi rad allows the coolant to flow direct rather than across the matrix - is this correct?
I suspect a radiator blockage, especially as the engine gets up to running temp at idle, with no viscous fan on. Am I right?

your thinking is quite right , water pump circulates the coolant through the bypass prior to thermostat opening as thermostat opens on a 300tdi it physically closes the bypass so coolant must go through top hose to rad, other engines like 2.25s and 2.5s use a restricted bypass, if its only getting hot on the side hoses are id fit anew rad its a common failing with the 300,could be blocked or baffles have failed, a hot top hose indicates thermostat is open

jamesmartin said:

your thinking is quite right , water pump circulates the coolant through the bypass prior to thermostat opening as thermostat opens on a 300tdi it physically closes the bypass so coolant must go through top hose to rad, other engines like 2.25s and 2.5s use a restricted bypass, if its only getting hot on the side hoses are id fit anew rad its a common failing with the 300,could be blocked or baffles have failed, a hot top hose indicates thermostat is open

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cheers James - its a swine, as this has only come to light since the water pump was changed, and just after I changed the oil cooler pipes! That would have been a perfect time to change the rad!

jamesmartin said:

your thinking is quite right , water pump circulates the coolant through the bypass prior to thermostat opening as thermostat opens on a 300tdi it physically closes the bypass so coolant must go through top hose to rad, other engines like 2.25s and 2.5s use a restricted bypass, if its only getting hot on the side hoses are id fit anew rad its a common failing with the 300,could be blocked or baffles have failed, a hot top hose indicates thermostat is open

Click to expand...

Hang on though, the coolant flow from the water pump is towards the bottom of the rad, so the top hose can only get hot on the thermostat end if the flow through the bottom hose is restricted, or did `i say that already?

steveo said:

Hang on though, the coolant flow from the water pump is towards the bottom of the rad, so the top hose can only get hot on the thermostat end if the flow through the bottom hose is restricted, or did `i say that already?

Click to expand...

current flow is into the block through the rear of the pump then out through head via themostat housing, water pump draws from bottom hose as well as bypass

Just drained coolant and flushed rad - seems a bit less hot on the top hose, and god knows where the piece of foam came from that ended up in the bucket.

Maybe it was part of the packaging from the new water pump.

So , getting technical on the sorry excuse for a radiator, I borrowed a Snap On temperature tester today. With the needle sat just under halfway as normal, both the top and bottom hoses were within a few degrees of each other, meaning that the coolant is not being cooled....

The rad had a hot spot between the pipes half way down, and was cooler everywhere else - the only temperature reading of any note was down the side between the pipes.

Radiator, your time is up!