First, Bill is right about using the Mercedes degreaser first, but he left out a "0" at the beginning of the part number for the "Lemon Acid" as Mercedes calls it.
I also seriously doubt that using a supermarket citric acid would be the same as using the Mercedes product. First, the supermarket/health food store product is designed for human consumption. The Mercedes product is not. For instance, the Mercedes Lemon Acid clearly states on the label that you shouldn't allow any to get on your skin. As for putting "a non-foaming automatic dishwasher compound" in my car's cooling system, I simply wouldn't do it. It simply wasn't designed to do that specific job.
Finally, a local dealer just told me that you should thoroughly flush the system with water after running the Lemon Acid through it, and remove the thermostat to be sure it's rinsed thoroughly as well.
"We drive into the future using only our rearview mirror."
- Marshall McLuhan -
Northern California Wine Country...
"Turbos whistle, grapes wine..."