Here in Brazil we have a traditional meal called Arroz de Carreteiro - basically Truck driver’s rice. The truck driver, who would not have so much time to waste on stops for food would cook rice and beef jerky (and some ocasional spices) on a pan with water that was “installed” on the truck’s engine. I have seen Jamie Oliver cook over a car engine on a show but… well, it is Jamie Oliver and he might not have the best hygiene habits of this world. Is it actually safe to cook over a car’s engine? Wrapping the food properly with tin foil or a pan, of course… Would the food taste like fuel, engine and oil? Is this method actually used around the world? The Jerky rice story is in fact quite anecdotical.
People in Alaska cook small meals on snow machine engines. As long as the food is wrapped well and the engine is in good condition it should be safe.
Exhaust goes out the pipe and engine fluids stay in the engine. The heat cooks the meal. I haven’t tried this and YRMV.
I’ve seen the method recommended in an army survival manual that my Dad had. No particular reason why it should wind up oily unless the engine compartment is really gross, and as long as the food’s well wrapped it won’t get dirty. Of course under wartime conditions the remote possibility of tainted food was low on your list of priorities. 
The only danger I can think of is the difficulty of controlling cooking temperatures. Put it in too cool a place, and your food may cook incompletely, or slowly, resulting in the potential for foodborne illness. Put it in too hot a place, and there’s a risk of fire that could consume the whole vehicle.
In the military (or in commercial fleets), there are often multiple copies of the same vehicle in use, so with experience a standard procedure could be developed that says for vehicle X, place food in location Y for acceptable results.
If your food is poorly wrapped, a vehicle with fluid leaks might put some interesting smells into it, but there’d have to be some horrendous leaks to actually get engine working fluids (oil, fuel, coolant) onto your dinner plate.
The problem these days is that they tend to tuck everything that’s hot enough to cook on away in some nook of the engine compartment (and then usually put one of those cheesy plastic covers over the whole thing.) Gone are the days of nice flat exhaust manifolds looking you in the face when you open the hood.
The catalytic converter would probably be the best thing to cook on since much of the emissions control equipment is devoted to keeping it working within a relatively narrow range of temperatures. Where they are on most cars, though, your food might get a little road grime-y.
IIRC, Mythbusters and Alton Brown (!) did a special episode about this, where they teamed up and cooked an entire meal as they drove from one place to another.
They carefully considered the various parts of the engine and the temp with respect to the drive time and placed their dishes accordingly (well, tin foil packets and sealed metal canisters).
It worked pretty well from what they said, which makes sense- a temp of 200 will cook anything if given enough time. Your car engine is basically like a big relatively low temp heat source that you can secret packets and canisters around Mythbuster-style if you so wish.
You can also use the dishwasher as well for things like fish, if you seal it up tightly.
About 25 years ago, someone published a book called “Manifold Destiny”, about cooking on a car engine. There were two updates to the book since then.
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Back when I was fighting fire we used to cook all sorts of things on the engine. The one we did most often was frozen burritos. Buy a few frozen burritos at the gas station in the morning, wrap them in tin foil and by lunch they would be hot and ready to eat. We experimented with all sorts of things, ribs, chicken breasts, everything came out great but it cooks more like a slow cooker. You need to plan ahead and need to be running the engine for a long time.
ETA: The newer the truck the harder it was to do.
We regularly cook burritos on the exhaust manifold when we go on 4wd trips in the mountains. A straight 6 motor has more cooking space than a V8. There are some accesories designed for backyard grill use that can be adapted for holding manifold chow.
And yes, there is an electric fridge in the back too. The cig lighter sockets are pretty lame and should be replaced with some better quality sockets for reliable operation. Ever have ice cream when you’re camping?
Seal it up so engine compartment gases are kept out. Have you seen an engine compartment? They’re not typically very clean.
If it’s food that need to be heated to a certain temperature (like chicken or ground beef) use a food thermometer to test it. If you’re handy enough, you could use a typical inexpensive digital thermometer and pass the cable through the firewall, so you can monitor the temperature while you drive! (Don’t do this on my say-so. There are reasons for that firewall … but there are fittings that pass wires through, which you might be able to use. The thermometer would be a pretty-much permanent installation.)
Happy eating!
Cool. Thanks.
Slightly off-topic, but I have friends who are drivers and firemen on steam locomotives in the rail preservation scene. They continue the old tradition of cooking on the shovel.
Now, they assure me it tastes great. I am a little leery though, as the firebox temperatures are as hot as Hades, and I can kinda see stuff that’s black on the outside and still frozen inside. Still, they swear it’s good.
On the other hand, there are tales of using the same shovel for… well, there was no toilet on a steam loco…
My recollection of the mythbusters attempt is that while everything was cooked it was all tainted with an engine-y taste presumably due to oil and fuel fumes. Is my recollection wrong?
Actually, answer is yes. My recollection is wrong. Though that isn’t what I remember. So maybe they’ve done it twice or maybe I’m thinking of someone else, or maybe my memory is failing.
Forklift drivers have been doing this for as long as I can rememeber. When I worked in the shop they would sometimes ask us to improvise some kind of adaptors that would make better use of the engine heat. We never seemed to find time.
Stick it in the hot coals for a while and then wipe off the grit, and it wouldn’t bother me a bit. I don’t think even tardigrades would survive. But I wouldn’t be against reserving one shovel for cooking duty!