I have a '91 Sedan Deville that gets poor mileage (14) and has sluggish acceleration

An old '90s Caddy- check the Catalytic converter- other than that, you do not say how many miles are on the engine- you have already tried guessing, and you see how much that can cost- my advice is to start dreaming about your next car before you put even more money into that thing

If you have already swapped out the cat conv and, I presume, had the rest of the exhaust system checked for blockages, you might want to check to see if your fuel injectors (I think 91's had them) are clean and distributing sufficient fuel to your engine.

Have you changed the injectors? The original Rochester injectors that came with that car WILL fail due their design and the effects of 90/10 gasoline. My 92 Allante was similar, but not as severe as your problems. You will be amazed at the difference in mileage, performance and etc. Make sure that you replace ALL the injectors. My recommendation would be to buy from injectors4u.com . You want Bosch injectors - do not get OEM Rochester. They are fairly easy to change - but if you are not comfortable doing - find a good independent mechanic. Also, rebuilt injectors are very common on the internet. Its not that much more - buy new ones.

All the work done so far was at my local GM dealer. I guess they were "guessing" even though their best mechanic worked on the car. They replaced all the injectors but I assumed they used GM replacements. Two of the replacements failed but were covered under warranty. The engine has 165k on it. They did a compression test on it and said it was up to spec on all 8 cyls. It doesn't use oil. A separate mechanic replaced the fuel pump/sending unit but he did not use a GM unit and ever since the gas remaining reading on the computer has been inaccurate. Also when he replaced it the stalling stopped but fuel mileage continued to be poor. The stalling resumed about 6 months after he replaced the sending unit/fuel pump and I took it back to the dealer who replaced the cap/rotor/plugs/rotor. The last set of plugs/cap/rotor only lasted 10k miles even though the plugs were platinum GM. I'm beginning to wonder if the fuel pump is failing again since the ignition fix didn't stop the stalling? This thing is sending me to the insane asylum!!

I would agree - there is an issue there. Normally, injectors, plugs, rotors, etc last for many years and maybe 100k miles. Do you have a history on the car. Could someone before you have replaced the central command unit with something other than OEM or similar unit? You might want to consider talking with Dick Hussey at allantesource.com about your engine - I think both cars used the same one. He knows more about these engines than GM. Send him a copy of the posts you have made here.

you took a 20 year old Caddy to the dealer?! Wow- what is the attraction with this old thing? The world is full of cars- get something else- I already told ya- this much discussion about a 91 Caddy- (not their finest hour)- (and not even an Allante)- is just kinda absurd

No - my Allante was never taken to a Caddy dealer for service. I would definitely try to avoid that on any car that is out of warranty. As for older cars...... someday you may find that some of them (but not all) are just fun to keep and drive.