This was my experience. I filled up a chimney starter twice for 30 minutes each and emptied it all into a steel bucket and left it for 30 maybe 40 minutes to cool down a little (it's very hot). I closed the air vents in the car. It was then placed in the back of the car, still very warm, sitting on bricks, which were in a large pan of water. The bucket wasn't touching the water, just resting on bricks, which had absorbed some water. The coals were nicely gray on top, more red in the bottom half, no smoke. All this was in the back of an estate car. I covered the top 75 percent of the bucket with aluminium foil to prevent too much heat escaping and damaging the roof lining. I taped up with gaffer tape all the door and boot joints, apart from my entry door. During this time the doors had been closed. I smoked a cigarette, downed a handful of benzos and a few glasses of pastis and got in. From closing the car doors to getting in was about 20 minutes.
I opened the car door, and got it and then remember nothing until waking up feeling very spaced out about 6 hours later, with the car door still open and the coals extinguished.
I put this failure down to two possibilities... too much alcohol and benzos knocking me out before i could close the car door. Or the concentration of co being so high that it knocked me out immediately. Bearing in mind that i took the benzos immediately before getting in i'd be surprised if it was that that ko'd me so immediately. I think it was a high concentration of co so that i didn't have time to close the door. My preferred plan now is to place the coals in the car, take the benzos and alcohol, tape up the seams.. all done in 5 minutes. Wait for the effect of the drugs to begin to kick in and get in 10 minutes after the coals, making an effort to get in and close the door in a single movement in case the co is already quite high. And if it fucks up again it will be straight in with the coals and the benzos.
Happy days.