This question is inspired from this question earlier today.

In Back to the Future, we see the DeLorean famously exiting the van (see below), but there's a lot of smoke, or at least a whitish gas of some form exiting the van. Why was this gas there in the first place (in-universe)?

I reason that if it was just for effect (in-universe that is), why didn't Doc get Marty to tape the DeLorean exiting the van?

  • Doc Brown was an alias. One of many. In the future he was known as Heisenberg. Say his name... Jun 16, 2015 at 5:08
  • @MajorStackings - I'm unsure what that's got to do with the question, although it's quite interesting! Jun 16, 2015 at 5:09
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    @WadCheber yeah, but he might've told Marty to start filming when he got there. I don't think it was (in-universe at least) just for show; out of universe it was definitely for show Jun 16, 2015 at 5:28
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    The steam or smoke is apparently the result of the coolant system doing its job, but it is unclear why it would be working so hard before it has been used for the first time. I also don't know why it needs cooling at all, considering the fact that when the Doc touches the car after the first trial run, he says "It's cold! Damn cold!" Maybe he didn't expect time travel to make stuff cold.
    – Wad Cheber
    Jun 16, 2015 at 5:33
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    Doesn't the car run on plutonium? This stuff works all the time, not only when needed. And the way it works it definitely needs cooling all the time... Also - didn't the car's engine started before driving out? Every engine when starting will make a little smoke. Or not so little... Depending on the condition?
    – AcePL
    Jun 16, 2015 at 9:34

2 Answers 2


It was likely steam from the DeLorean's nuclear power core. You can actually see some steam being emitted from these vents in the clip (at 0:47 onwards).

enter image description here

These "Steam Vents" were apparently part of the control system used to keep the time machine's nuclear reactor from overheating and would therefore have already been engaged prior to a new set of fuel rods being inserted (assuming, of course that the Doc has tested that the reactor actually works).

enter image description here
Popular Mechanics Aug 1985

Out of universe, we learn from the same article that the gas was in fact CO2

Since the car now sported a bank of flared exhaust vents behind the nuclear reactor (which lies a Dodge Polaris hubcap for a base). Pike's staff rerouted the vehicle's original exhaust system to vent in front of the rear wheels. Two CO2 extinguishers were installed in the passenger compartment and the gas was piped to the exhaust bank, where a trumpet with a special built-in baffle expelled the gas, simulating a nuclear reactor's exhaust.

Popular Mechanics Aug 1985


Probably water

What we know:

According to historical records, on October 26 1985, the minimum temperature was 48.9F (about 9.4C). So, we could assume that at 1AM in the morning on the day in question, the temperature was around 50F. That's reasonably cold.

Now for speculation

You will note that the ground on the car-park seems to be wet; it is totally possible that the DeLorean had been rained on and so was wet. We could therefore conclude then that when the DeLorean's ignition was turned on, and if it was left for a while, the internal temperature of the van would have been greater than the external temperature. Pair this with the water being inside and you have water molecules heated to above the external temperature.

Consequently, when the door of the van was opened, the water was exposed to a cooler temperature outside. So, I would say that the opaque gas was simply water vapor that had accumulated inside the van (on the DeLorean) and was simply reacting to change in temperatures.

  • But the vapor completely fills the interior of the DeLorean, as well.
    – user31178
    Dec 15, 2015 at 5:52
  • @CreationEdge Doc may have left the door open? Dec 15, 2015 at 5:56

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