The main plot device is the lack of petrol. The Doc has an ice generating machine and lamp oil. The ice-generating machine is steam driven and works at ambient temperature. Therefore it is fair to assume that it uses pressure transformation to achieve freezing.   It should be fairly simple to rebuild the compression equipment from the icebox into a small scale cracking unit. The required technology is similar.  Cracking is the process of breaking down heavy oils like lamp to light ones like petrol. At this point in the film they have several days which should be plenty of time to do this. This would be a much safer course of action. [It take a considerable amount of pressure and heat to perform cracking, over 9000kpa and 2000 degrees Centigrade, and even after that the product has to be filtered to remove any remaining inpurities. Also they had no technology to test the "petrol" to see if it was the right compound to be used as fuel for a car.]

If the 1955 Doc saw his tomb and sent Marty to 1885 he shouldn't be surprised to see him in 1885, since the Doc that's in 1885 is the same one who sent him there in 55, and hence has already seen his tomb. Some people argue it's not the same Doc, but time travelling doesn't magically create a new human being! The 1955 Doc sends Marty back to 1885, ages 30 years, invents a time machine, travels around, and ends up in 1885. 30 years may dull the memory a bit, but I think you'd remember sending someone into the past to rescue yourself... [Actually, the Doc that is in 1885 is the Doc from 1985. The letter that Marty receives in 1955 is from the Doc who was from 1985 not 1955. Therefore, when Marty goes back to rescue Doc, he's actually rescueing the Doc from 1985. The 1955 Doc saw his tomb but when Marty goes back, the Doc really isn't suprised to see Marty. All he said was that "he gave him specific instructions not to come for him" and what was it. He also said that "it was good to see him", not "what are you doing here" I think he knew Marty would come for him.]

The fuel problem for the DeLorean is a non-issue. Internal combustion engines can and do run on alcohol, and there was plenty of alcohol available in the Old West. [The problem wasn't the fuel; it was the damage done to the car from their attempt to use alcohol as fuel. As you know, they tried using alcohol (some booze of some kind), and it caused an explosion that Doc said would take a month to fix. Since the photo of the headstone said Doc was going to be shot on Monday, they didn't have time to fix it. Perhaps if they would have used the right kind of alcohol, the explosion wouldn't have happened...]

In the last scene in 1885 where Marty, Doc, and Clara are on the train racing toward the bridge, Marty keeps yelling at Doc on his walkie talkie to tell him the gauge readings. But after Doc sees Clara on the train, he tries to rescue her and Doc is no longer holding his walkie-talkie. Doc is holding on to the train with one hand while reaching for Clara with the other. Doc's walkie-talkie is no longer anywhere in sight. Marty sees all this but he nevertheless keeps yelling into his walkie-talkie right to the end as if that is how Doc could hear him. Marty does this even when he is facing forward all by himself in the DeLorean. Who is Marty talking to on the radio? It can't be Doc. [Would it not be possible for Doc to have put his radio in his pocket?]

When Marty asks the Doc if they can take Clara with them to the future, the Doc says no, because you mustn't tamper with the time continuum for your own benefits. However, in the second film, he does change the continuum for Marty and Jennifer's benefits, by going with Marty and Jennifer to change the future, so their son wouldn't go to jail. [The reason that he says that the space/time continuum shouldn't be tampered with is because of the repurcussions of what he did in BTTF II. He is learning from his mistakes, and even says so later on in BTTF III.]

During the final train sequence, how does that gauge in the Delorean measure the pressure inside the train, when nothing is physically attached between it and the Delorean to measure that? [It could be more like a watch, telling the time needed for the 3 things to blow up. Doc could have measured that time.]

Doc's tombstone was placed by his "beloved Clara". However, that was discovered (and carved) before Marty went back and changed the past by him and Doc rescuing Clara. So how did Doc meet Clara without Marty being there, given that the ravine was already called Clayton ravine, meaning that by definition she must have fallen in? Bit of a paradox, methinks. Even assuming he met her at the station, then after that her carriage fell into the ravine, the timings just don't work at all - she couldn't have fallen for him in minutes, and/or couldn't have had her name on the gravestone given that she fell in the ravine before Doc was shot. [There are 3 different timelines: 1) Doc never goes in 1885, Clara hires a carriage, she falls into the ravine and it's named after her. 2)Doc goes to 1885, he is sent to the station to pick up Clara and they fall in love (notice that the ravine could have another name in this timeline, we know the "Clayton" name by Marty when he is already in 1885). 3)Doc does go to 1885 and is asked to pick up Clara, but he forgets about it trying to find a way back to 1985; Clara hires a carriage and is about to fall into the ravine, but she is saved by Doc and Marty, Doc falls in love, etc... Notice the "Eastwood ravine" sign in 1985.]

In the scene where Doc explains to Marty the idea of pushing the DeLorean across the bridge, Marty points out that the bridge is not completed yet (in 1885). Doc says that Marty is just not thinking 4th dimensionally, and once they return to 1985, the bridge is complete, safe and still in use. Unfortunately, the whole idea of returning to the exact same point in space, but a different time, is fundamentally flawed. If the DeLorean was to return to the same point in SPACE, only 100 years later, it would have to account not only for the completion of the bridge, but to all of the other changes to the physical environment. For example, in 100 years, the North American continent would have moved because of continental drift, the Earth would have rotated on its axis, so that point in space would no longer be Hill Vally, CA; the whole planet is revolving around the sun, therefore, the planet Earth wouldn't be in that exact point in SPACE, the sun is revolving around the galaxy, the galaxy moves through the universe, and on and on. Anyway, the DeLorean couldn't have shown up on the completed bridge 100 years later because Earth is moving through space in several different ways. [This entire arguement assumes that the time machine does not maintain or compensate for momentum. Since every time they travel through time they end up in the same location we can assume Doc designed the machine to compensate for plate tectonics. planetary orbit, and galactic drift.]

When the 1955 Doc sends Marty back to 1885 he points to a cave where to hide the DeLorian left of the track that Marty takes (directly towards that cinema screen). In 1885 then, Marty takes a U-Turn to flee from the Native Americans. Assuming a 180 degree turn, the cave to hide the car must be on the right hand side. However, the cave appears directly in front of him. [It is very possible (being way back in the 1800's) that it was a cave-filled area at the time and there was a cave where the Doc pointed out in addition to the one Marty hid the Delorean in. It is also possible, since the Doc hadn't been to the old west yet, that his calculations of where the cave actually was were slightly off.]

In 1885 Marty uses the alias "Clint Eastwood." But at the end when Tannen threatens to shoot Doc if Marty doesn't come out to fight, Doc calls Marty by his real name, and no one seems to notice. [They'd have guessed it's some sort of nickname - like "Doc" instead of "Emmett L. Brown".]

Marty smashes his head on the fence not long after he arrives in 1885 at 8:00 am. When he wakes up Maggie tells him he's been asleep for nearly 6 hours, so it could only be about 2pm at the latest, yet it is dark outside and they eat dinner soon after that. [It is possible that Maggie and her family did not have an accurate timepiece and thus could only guess at how long Marty was out cold. Also, farm families, true then as it often is now, wake at or before dawn, so 8 a.m. wouldn't seem as early in the morning as it would to us, which could also make it difficult for her to judge the time.]

Just before the train hits the inside of the ravine, the train explodes. It shouldn't just explode in midair. [Remember when Doc says when the boiler reaches 2000 degrees the whole thing will blow? That's what it was doing.]

Doc said to Marty, after they rescued Clara 'I wish I had never invented this infernal machine, it causes nothing but disaster' and he never wanted to change the future (the space time continuum) Why then did he make another time machine with the locomotive? [Because he had to tell Marty he was all right and had to come back for Einstein.]

As per http://www.bartleby.com/65/in/intern-co.html, J. J. É. Lenoir built the first gasoline powered internal combustion in 1859. de Rochas patented a design in 1862, and Otto built an engine in 1878. Gottlieb Daimler built what is considered the ancestor of today's Internal Combustion Engine in 1885. All this is by way of saying that gasoline would not have been impossible to find in 1885...especially considering that Edmund Drake drilled America's first oil well in 1859. [1) Internal combustion engines were not running rampant, so there wouldn't be any places to get gasoline. 2) Engines evolved a ton from 1885-1985. The fuel used 100 years ago was completely different and wouldn't have worked in a Delorean.]

At the end of the film, Jennifer has the fax from the second movie saying 'You're Fired'. Then, because Marty stood up to Needles, the message erased. However, Jennifer still has the piece of paper. If Marty didn't get fired in 2015, then there would be no fax, and therefore no piece of paper at all. Jennifer shouldn't have had the piece of paper. [Saying this would mean that the photograph in the first movie would disappear as it would have never been taken, and Doc and Marty never picked up the papers in the second movie, therefore the headlines wouldn't change, rather they'd disappear. But they *do* change, because they did pick up the paper themselves, as a sort of artifact from the future. Jennifer therefore would have brought back the fax paper from the future, but since the fax didn't print when events changed, there would be no message on the paper.]

After Doc went back to 1885, he put the Delorean in an old mine shaft. When Marty went to 1885 to find him he got a hole in his gas tank, and need gas. Well, all they had to do was get some gas out of the Delorean that Doc came back in. Would had saved a lot of time. [Oil and petrol are normally removed from a car for long-term storage. Although Doc could have kept the petrol he removed... This issue was also dealt with in the DVD commentary, in which it was speculated Doc didn't want to risk reopening the cave, potentially causing a cave-in and thus a paradox if the car can't be retrieved in 1955.]

Okay, we're going to get a bit deep here. Doc and Marty found Doc's grave in 1955. Doc sends Marty back to 1885 to rescue Doc. Now.....remember....all hinges around the image on the picture. Once the grave stone is broken in the fight with Mad Dog Tannen the image of the grave disappears in the photo. Wouldn't the picture disappear too? No grave in 1885 means no grave in 1955 which means no reason to take a picture in the first place. But then again....once the grave is broken then there would have been no grave in 1955 for Doc to see. Therefore he would have never sent Marty back in the first place. HENCE...once the grave was broken....the timeline is fixed and Doc of 1885 would live happily ever after with Clara, Doc of 1955 would just work on getting Marty home, and Marty would have disappeared from the 1885 timeline and things would have continued from when Doc and Marty are digging out the old DeLorean. Told you it was deep. [Marty took the picture of the grave BEFORE he went back to 1885 and changed the past, so there's no logical reason why the picture itself would disappear. Obviously, he wouldn't take a picture of nothing, but he had already taken the picture, which is why only the grave disappears.]

In the scene at the start of the movie, the time machine is flying 50 feet or so in the air when it is struck by lightning and sent back to 1885. Later in the movie, we learn that the lightning destroyed the flying circuits. Assuming this is true, as soon as the time machine appeared in 1885, it would have fallen to the ground and been destroyed. [We can assume a flying car from a future where everybody has a flying car would have some kind of device to get it down safely in event of malfunction. Otherwise, cars would fall out of the sky everytime anyone had a minor problem.]

When the Doc from 1955 is loading up the DeLorian after fitting the new time circuit controls, he hands Marty the walkie talkies. However, in at the end of Back to the Future II the Doc has the other walkie talkie with him. How can Marty have both of them? [Doc probably put his walkie talkie in the car before he sealed it up in the mine shaft and Marty found it in there.]

When Doc and Marty find the Delorean in the mine, the tires have rotted off with time. It seems that all they had to do to run the car again was replace the tires. However, as any mechanic will know, in that amount of time not just the tires would have rotted. Every rubber hose and gasket, every piece of wire insulation, every bushing and grommet, and many other parts in the car would also need to be replaced. There would be no way to run the car short of completely rebuilding it. [There were 4 or 5 days between the lightning strike and the trip to 1885. If any car parts needed replacing, Doc could have done it easily. Notice that the DeLorean even sat higher off the ground in the drive-in, suggesting that Doc had to replace the axles. The DeLorean went through a lot of refit to make up for the 70-year storage.]