Coolant bypass to thermostat housing

neil123 said:

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My tls is a 97 and the thermostat housing was changed to house the coolant temp sensor which I know is a common mod.
But I'm thinking the bypass to the housing would make the bike run a cooler as when I'm riding it barely gets above 75 so is always on cold map.

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The thermostat bypass hose is part of the updated thermostat housing modification. It is a good thing.
The bypass hose allows the coolant to circulate through the head and cylinder passageways to allow for better heat distribution during warm-up before the thermostat valve opens - reducing the possibility of hotspots developing.

The bypass hose is not the cause of the cooler than optimum running temperature.
Rather, the thermostat itself is the problem. It is the wrong heat range. The OEM thermostat is 180F (82C) for the '98 to '01 TLS, and the '97 TLS may be even cooler.

When the thermostat opens and the ambient temperature is too cool, or the engine is not pushed hard enough, the coolant temp remains below the 176F (80C) threshold point where the ECM switches to the "warm map."

Therefore, a simple change to a hotter thermostat (190F; 87C) will cure the problem and keep the ECM on the warm map. This results in better throttle response and improved fuel economy.

For reference, see page 5-2 in both TLS and TLR service manuals. The TLS manual I have only shows the '97 version, so the bypass hose is missing. However, the TLR manual shows the updated coolant flow path, and it also presents a clearer image.

neil123 said:

.... the thermostat I have is rated at 88 degrees, I have ordered another incase this one isn't functioning correctly.

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So, being in the UK, I suppose your thermostat is 88C. Correct?

For the sake of comparison, I run a 190F thermostat in my TLR. It's service is 80% commuting, 20% sport touring.
Here in Phoenix, AZ, USA, the winters are likely warmer than your summers.
In the winter, the TLR coolant temp will run around 189F during my commute. No fans required.
In the summer (105F ambient temp) the coolant temp will run around 198F if I keep moving. When traffic is slow, I turn on the fans to keep it below 205F.

I can't swear to it, but I think my TLS still has the OEM (182F) thermostat installed. Its coolant profile is generally a little cooler than the TLR, but it still manages to stay on the warm map due the smaller radiator and generally warm weather here. On cool mornings, the coolant temp will float around a mid 170s but not long enough to be a problem.