I thought this is normal for all cars unless you have a v8? I've always noticed that the rad fans kick in for my cars when you turn on the AC and there's also a definite noticable loss of power (Subarus even turn the AC off themselves when you floor it). Could it be a placebo effect? I would say if you could find another e550 and see if it performs the same that would prove whether the issue you have is normal.

he does have a v8 but merc uses the same air cond design across the board

In my first Celica (which had <100hp left) turning on the A/C had no noticeable loss of power, same thing in my wifes Matrix. OPs E550 has 4x the power of both of those so I can't imagine a properly functioning one feeling any noticeable loss from the A/C being on.

What are you changing to go from HVAC running to heater on? Are you just changing the temperature setting or is the fan speed changing too?

I find this hard to believe as well.

I also don’t believe the IAT difference. You have a radiator which is pumping huge amounts of heat into the engine bay all the time. The AC heat exchanger is not going to make that significant of a difference to your IAT. In fact, your OEM airbox probably gets its air from outside the engine bay. Edit: read you have 10 degree difference. You won’t see a major change with this.

The fact you can see the engine load change at idle with the computer is pretty neat. If it was a bunk compressor you’d think the most amount of load required would be getting it moved from a stop (just like torqueing a bolt. Breakaway torque is higher). The numbers you posted make sense in my mind to run the AC. The idle valve on a modern car should be adaptive enough to keep your idle stable (which it does). When the valve opens, the load on the engine increases but the RPM stays the same. (Idle valve essentially opens slightly increasing throttle to keep RPM stable).

I think it’s in your head. Lol. Your rad fan will turn on when the AC is turn on. There will be a voltage drop in the system. This is really reaching way out there, but if you have an aftermarket tune and they somehow messed up the injector dead times and different voltage levels it could mess with your air/fuel mixture. But it would have to be really out there. Really grasping at straws now. Edit: I see you have an aftermarket tube. Load the OEM tune. This would definitely problem solve my made up grasping at straws issue.


What was the last item on the car that was changed when this started happening?

Also, is this power loss just from a stop/idle or through the whole rpm range?