Cadillac Repair Questions? Ask a Mechanic for Answers ASAP
Your problem is still most likely a leaking head gasket. This is very common on the 4.6 Northstar engine. The head bolts actually pull the threads out of the block. The problem may not be evident on the chemical test, but you aren't performing the chemical test while under load pulling a grade. If this is the problem, it will worsen and become more evident.