Quite a number of overheated engines we have carried out repairs on all have come in with the heater pipes removed and the hoses looped, though this is correct the one important problem we have found with this is the incorrect looping of hoses has either been done incorrectly or the hose has been kinked over so much that it hardly allows for the heated coolant to flow to the thermostat for correct temperature regulation of the engine, in effect causing the engine to overheat.
It is very important that if you carry out the heater core bypass method that there is full flow of the coolant.
As pictured below you can see that the pipes have been arranged so there is full flow of the coolant
Another mistake we have noted is the incorrect connecting of these pipes, remember that coolant also flows back out the reservoir bottle too, if the pipe coming from the engine is opposing the flow from the reservoir due incorrect bypassing then this action will cause a mild overheat situation at irregular intervals.
The below picture is based on the GEMs engine.
On the Thor (Bosch injection) looping the heater pipes involves a bit more but the effect of making sure the flow of coolant is the same as the GEMs, get this wrong and you are 100% guaranteed to blow your engine