The solution I've seen is to run 2x19mm hose under the fuel-tank and bypass the hard pipes entirely.

Beats dropping the tank.

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but you're doing that anyway! So maybe run the hoses above the tank? I'm not so sure about chopping out part of a hardline and creating an inaccessible joint above the tank

- sounds like a bad idea to me.

Just spitballing here, but I have been considering another idea on this too, although I'm unlikely to try it myself.

For background, I'm currently thinking about a coolant system simplification when I reshell which would see me fitting a Davis Craig Electric Water Pump, removing the thermostat

(because the EWP controls the flow

/ temperature), teeing the heater matrix off the radiator at the front of the car and then using the heater pipes for my chargecooler pipework

(plus a number of other things that aren't relevant to this post).

Now, teeing both sides of the heater off the coolant rad makes no sense for a car with a thermostat because flow through the heater would be limited by the stat in exactly the opposite way to what you'd want.

However it did make me think that you could tee the input to the heater matrix off the rad's input and send the matrix's output through the remaining good hardline

(AFAIK only one pipe goes bad) and plumb that back into the engine as normal.

I suspect it would have a slightly detrimental effect on heater performance, but might be a path of less even resistance than running pipework under the car.

Well, other than the need to partially drain the main coolant system to avoid dropping about 5L of coolant into the frunk when you break into the radiator pipework.
