Having worked in Glen Canyon, just up-river from Grand Canyon; I can tell you that GC in August is HOT. 90's on the rims and 100's down in the canyon. But it's a dry heat.
John backpacked a 50 mile, rim-to-rim-to-rim in the Grand Canyon with the Boy Scouts in August some years back. It was 113 degrees the day they hiked in. One day was enough to teach them to stop moving and seek shade by 10 am. They then started hiking by 4:00 each morning and it was still evil hot. A ranger told them that once the outside temp gets over your body temp, there is no way your body can regulate without outside help. He advised them to take off their shirts, dip them in the streams and put them back on soaking wet to gain evaporative cooling. Better, but still wicked hot. If you plan to see the North Rim, check seasonal closings. It is at a higher elevation and they close due to winter snow. John felt the north rim was more beautiful than the south rim but it is a long drive from nowhere to get to it. Side note with more Ranger info - they airlift more people out of the canyon for drinking TOO MUCH water. This doesn't mean don't drink water, it means EAT. When you are hot, you lose your appetite. If you drink and drink and drink and do not eat, you effectively flush all the salts and minerals out of your system and it will crash. While you may not be planning a 50 miler in the canyon, you can still get sick and woozy from even a day of this severe inbalance.