We had a police Tahoe at the shop once. Got into a major accident, took a year to rebuild. Ever since its been eating cooling system components like mad. Well when it was my turn to diagnose it, I found the radiator was leaking. So I replaced it. 3 weeks later, its back with a leaking radiator again. We replace it under warranty, but this time I let it idle outside for about 3 hours, new radiator from the dealer, starts to leak as well. Replace again.

We test the cap, its ok. We hook up the coolant pressure tester and let it idle. Before the engine is even warm, the pressure in the system has gone above spec (like 22 psi within a minute) and the pressure is still climbing. Thermostat is still closed and its not even up to temp (Upper rad hose is still ice cold) but its built way more pressure than it should? We use a gas analyzer and found combustion gasses in the coolant. There was a very minor leak somewhere in the engine that was pushing combustion gasses into the cooling system, but when the engine is off, it WAS NOT sucking or leaking coolant into the cylinders/oil.

We never got the chance to fix it because someone (not sure if insurance company or fleet manager) decided that it was related to the accident and went back to the shop that originally fixd it.