Sounds like your going to have to pressurize your system and look where it's coming from. Keep in mind that when you use a radiator pressure tester, a little leak will be a bigger one. But sometimes this has to be done to find leaks. Just be prepared financially to fix problem/ problems with your cooling system. Cracked intakes are a problem with all 4.6/5.4 engines. Why it's fluctuating at idle could be an intake gasket or some intake hose leak. Injectors are sealed so coolant can't mix with fuel. Enough coolant getting into any electrical connector can cause problems. Is the coolant getting on coil on plug wires? You need to dry up the area fix leak, and apply dielectric grease to all connectors that have gotten wet. You'll need to first clean connectors with electrical cleaner first. I'm getting ahead of myself. Diagnose coolant leak and possible engine condition first.
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