Yeah, these engines have very complex coolant circuits. All possible leaking spots that come to mind:
-Front and rear crossover pipes (sure you replaced those already)
-Water pump housing to engine block (bolts get corroded very easily here, need to tap the holes again when replacing these gaskets and install new bolts)
-Oil cooler pipe
-Thermostat housing
-Engine coolant sensor 2 housing
-Supercharger electric coolant pump
-Leaky bleed plugs
-An of course all the hoses
-Transmission fluid cooler
-Engine oil cooler
And then since we don´t have a dipstick we can´t check that easily if coolant if mixing with engine oil, you can vacuum out a sample every now and then.
My front crossover pipe started to fail at about 10k miles.
Vacuum test for the coolant system should be -0.8 bar or -12 psi for 1 minute.
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