This is a 43 km loop that first goes about 3.5 km uphill to the rim of the crater on a dirt road, then drops 9.5 km into the crater bottom. When you get to the bottom, go left, and when the road forks about 200 meters further along, stay left. The downhill continues for about another 6 or 7 km, not incredibly technical, but fast, and good shocks are a must. On your right, a ravine will open up, which you'll follow for a little while, and then the hillside you're following will open to the left (south) as well. Soon after this, look for a not to main road to your left, with a water tower or tank on the corner, where there's a Calacal --> sign painted not to clearly. Follow this road; if you don't you can keep going downhill for a few more km, but it will eventually end, and you'll have to retrace your steps. The first few km out of the valley is tough, but then you get to the top of the first ridge, and there is about 15 of long, slow climb to where the road will meet a main one coming in from the left, at the crest of the ridge. Right is another riding area, but go left to head back down to the highway, which you will eventually hit, and then have a 5 km or climb back to the Texaco station, which will seem an oasis like no other you could imagine. Drinks are sold.
Drive north from Quito past Mitad del Mundo monument, around the bend and *past* the first signed entrance to the Pululahua crater, all the way to the top of the hill, then down towards the coast. 1 km further is a Texaco station on your right, leave your vehicle there. Ride downhill from the station about 500 meters to a dirt road (next to a concrete-built house) that heads straight north. I believe there is a sign on this road too, indicating it's the entrance to the Pululahua park.