What would make a 1994 Cadillac DeVille backfire and sputter?

Answer 1

My first reaction is that water or dirt in the fuel would cause a car to backfire and sputter.

It is a symptom that indicates the car is not getting enough fuel. You may have gotten some bad gas.

Easiest fix, pour some STP gas treatment in the tank to absorb any extra condensation or water and drive the car to use up the gas.

When you're down to less than half a tank, fill the tank with new gas at a reputable high volume gas station. If you see an improvement, just keep driving and add new gas when you can. The situation will probably clear up on it's own after a couple tankfuls.

In extreme cases, the bad fuel could be drained and the fuel filter could be replaced. On most GM cars the external fuel filter is usually pretty cheap -- like under $40 and is located just in front of the back wheels near a frame rail. The labor is pretty minimal too. It's like a ten minute job for most mechanics. Not sure about the specifics of your 94 Cadillac though.

Also, there is also usually a filter like sock inside the gas tank, where the fuel pump is.

If that is clogged up, or if the fuel pump is going bad, you might be looking at a lot of money for repairs. (Hundreds of dollars)

Tip: Never let your gas tank go lower than half a tank. The gasoline in the tank actually helps cool the electric fuel pump and prevent it from overheating and wearing out.

One other possibility is that you may have a bad spark -- spark plugs overdue, distributor cap might be cracked, (crack is really hard to see, doesn't look like an actual crack but more like a dark, fine human hair on the inside of the distributor cap) or rotor worn out....something related to that.

These are all typical replacement items that should be serviced from time to time.

At some point GM stopped using typical Distributor caps... not sure if your 94 has one.

My 92 Caddy does.

All in all, I'm betting you just got a big batch of bad gasoline (dirt or water in it) somewhere, and I wouldn't buy gas there again...at least for a while. Good luck.