Hi there,

I need some helping solving an overheating issue on my 420. Which has 126,000mi on it and is... jet black and damn beautiful I have read a lot and done a decent amount and the engine cooling system still seems sensitive and... non-functional.

Problem: The radiator hose blew off the top of the radiator a few months back. I stopped immediately and added water to the coolant mix to make sure I didnt crack the head. The car had never had a similar issue. It was sudden-onset.

So, I replaced the hose, replaced the thermostat (also tested it before installation and it pops just perfect at 80C or 176F), flushed the coolant (it was clean) and replaced it. I also checked the pressure actuator on the AC system that activates the auxiliary fan for radiator cooling when AC system is running. It works. The auxiliary fan motor also works... although I have not had the AC system serviced recently so low PSI could be contributing to the auxiliary fan possibly not kicking in at the right PSI. The main radiator fan, its motor and temp-switch also work. When I unplug the fan at idle when car is at normal temp it defaults to 'high' blowing mode, and goes back to 'low' when plugged back in... which should indicate that it is functional. The top radiator hose that originally blew off does not remain hard when the car has sat overnight (which supposedly indicates a non-cracked head). ANYWAY... Here is what the car does now:

At idle it does not overheat. It hovers just above 80C. Once I begin slowly driving around the neighborhood it creeps up toward 90 and beyond... at that point I get paranoid about another blow-off and turn the heat on full blast, whence the temp drops immediately and I retreat to my garage.

Anyone have any ideas?! Thanks for your help.