I have a 2004 SL55 with about 110k miles, and my engine will run at 80* C for the beginning of any of my drives, but pretty soon it starts ticking up and once it goes to about 95* C I'll flip on the heater for fear of the engine overheating. Doing that will keep it around 90-95ish. Sometimes if I'm cruising it will start to go back down to near 80*C but anytime I stop in traffic it climbs back up. It almost seems like the fan never wants to kick on, or there's possibly air in the system?

I did recently replace the water pump, thermostat, and radiator, but not radiator fan or coolant temp sensor. I also added a larger heat exchanger, and swapped the secondary IC pump to the Bosch, but I didn't split the system. There's no coolant leaks that I've seen so far, but I would still think the car should always be running at about 80* C? I don't know how to check for air other than running it with the heater at full blast for a while. It doesn't have the bleed nipple that the E55 has. Is there a better way to bleed the system? Is there anything else I can check or do to get this running right? Currently I don't want to drive it over a few mules for fear of it overheating. Thanks.

 

The fan may be bad. I bleed the system by taking the top intercooler line off where it goes I under the supercharger. and bleeding the air out. I would check the fan operation with a good diagnostic tool. You can turn it on. Next step is checking the thermostat temperature and function. Maybe you got a 95 stat or it’s not functioning as it should.

 

I have an 07 SL550 and had similar issues. I found the fan was intermittent. You can always pull over when the car temp is creeping up and see if the fan is working.

 

You can have an air bubble trapped in a hose, thermostat neck, or at the top of the heat exchanger, you just gotta drive and jostle the car sometimes and use the cap very carefully, with a towel to vent it. I do it barehanded at the shop but I care much more about you than me. I doubt there are anything wrong with the parts though. If it's only overheating 10C it's usually air. If it was the fan the temp spike would be a bit worse than that I reckon.