I experienced an overheating situation and I know the head gasket now has a leak (yes, I know it'll need a reman engine to fix correctly). I'm trying to figure out the root cause. Using the GAP IID tool and letting the car idle, within about 30 minutes I noticed coolant temperature sensor 1 will rise to 240F and eventually overheat. Temperature sensor 2 remains close to ambient at 86F.

Based on the extreme temperature differential is this an indication the thermostat is not opening? I have no fault codes, which I would assume I'd have with a thermostat failure in a 2016 with the electronic thermostat if it failed closed.

 

Replying to my own post here. It’s too early to say conclusively, but if anyone is trying to diagnose a large variation between both coolant temperature sensors, this may be your root cause.

I believe I purchased this LR4 with a leaky head gasket because, even stone cold after sitting overnight, there would be pressure in the coolant reservoir that would release when the cap was opened. The reservoir would then refill on its own with coolant.

I used Blue Devil Pour n Go head gasket sealer by pouring it into the top radiator hose and letting the car idle for about an hour. I then let it cool for about an hour. I then went on a lengthy drive with 86F ambient temperature, air conditioning running, and my coolant temps are 205F and 167F. I’m going to keep an eye on it, but its entirely possible the head gasket putting gas and pressure into the coolant system had an effect on the thermostat, or didn’t allow the flow of coolant to the radiator.

Something to consider if you’re troubleshooting an overheating condition or you find a big difference in temperature between the two sensors. Check for pressure into the coolant reservoir after sitting overnight. If so, you may have a small head gasket leak.

 

Temp B (T-stat sensor) will typically be lower than A (rear manifold). Extended driving or idling will see B rise, sometimes 120-160F. Prolonged idling will see it rise to the same temp as A as the radiator isn't receiving the full benefit of cooling whilst in motion.