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1992 Cadillac Running Rough

I have a 1992 Cadillac DeVille with 25,000 miles on it. It’s never been driven in the winter and garaged when not being driven.
Last year, it was running rough, so I took it to the dealer. They changed the spark plugs and wires. There seemed to be some improvement after that. After about 500 miles, it started to rn rough again. I returned to the dealer who changed the spark plugs again and replaced two of the fuel injectors. Still no improvement. A few hundred miles later, I took it to a local mechanic. He replaced four more fuel injectors and there seemed to be some improvement.
After another few hundred miles, the engine started to run rough again, the fuel dropped down from about 20 mpg to 10 mpg, and it stalls at traffic stops or if the brake is applied. After 30 minutes of idling it will use 5 gallons of gas. There are no strange odors or visible leakage.
Any suggestions on the cause and how to correct this problem?

I can only post a wild guess, but you may want to check the cat converter to see if it’s plugged.
Unless I miss my guess, this is a distributor-based ignition system. That too presents possibilities, but it sounds like the dealer tech isn’t well versed in distributors.
Another possibility is that the temperature sensor is telling the ECU that the engine is running cold, making it constantly run rich. Its signal can be checked.
And you definitely want to change the last two injectors.

I’d also strongly recommend that you find a competent shop that can actually do diagnosis. Clearly you dealer is either unable or unwilling to do so. Without an OBD2 system and standards for the work, you clearly aren’t going to get the problem resolved there.

Summary: I’d start with the temp sensor, the exhaust and the distributor.
Or I’d seek out an independently owned and operated shop.

The problem might be caused by a leaking fuel pressure regulator.

This is located on the fuel rail.

Start the engine and let idle for a few seconds and then shut off.

Remove the vacuum hose from the regulator. If gas runs out of this connection, the regulator is leaking and requires replacement.