1991 cadillac deville fwd 4.9 engine shudders hard under slight acceleration. replaced plugs,wires,cap,rotor button,ignition module,egr valve, cleaned throttle body, still have same problem

Fuel pressure regulator I guarantee you

Hello! when you say you checked the MAP sensor what do you mean? Did you take voltage readings at idle ? A vacuum leak is the likely culprit, especially to the MAP...The minor shake at idle being the major clue...Send a comment...guru..saailer

I’m happy to help further over the phone at https://www.6ya.com/expert/david_6df67de3b14de867

The combination of heat and missing tells me that you have an ignition problem. Warm the engine as much as possible and take a water mister and mist the exhaust manifold with the water mist at each cylinder where the exhaust comes out of the cylinder. The cylinder(s) that doesn't evaporate as quickly as all the others is the one that's missing (no combustion = no heat out the exhaust for that cylinder). The next thing to do is to pull the spark plug. examine it for dirt built up around the electrode. If it's dirty then just replace all your plugs. If the plugs look good ( not worn - no contaminates around the electrode) then suspect plug wires. If all the cylinders evaporated the water pretty much the same, then your coil wire to the distributor cap or the cap and/or rotor itself is faulty. Examine the rotor cap and rotor for thin black lines running along the inside. If you find evidence of arcing (the lines inside) then replace the cap and rotor at the same time. The data you gave though sounds more like plugs or plug wires if I were to guess... For best results you should plan on replacing plugs/wires/rotor/distributor cap at the maintenance intervals suggested in the owners manual. But now that you're having these issues you should have some kind of concrete evidence before replacing these items - that is unless you've never replaced them ever before. In which case it's high time to have replaced them.

bad plug or wire, dist cap, rotor button,vacumn leak or rocker arm off

check the egr valve it mat be stuck closed ..... code PO404

Not fuel related...
The engine is losing ignition timing.
It sounds like the ECM (Electronic control module). It is responsible for ignition timing.
Inspect it / replac eit.

Your camshafts may have some or all of the lobes worn down.....sounds like time to do a rebuild.

Btw, I’m available to help over the phone in case u need at https://www.6ya.com/expert/sean_54c3a3dc48a7773c

Had a computer on my 1990 Cadillac Deville bad, after changing plugs, wires, cap, fuel injectors, egr valve, etc.

The computer was not firing an injector and i had a miss and rough running mostly at 35 mph on the roads. It bucked like a bull.in a rodeo.

My 1996 Astro does the same thing... It has new fuel pump, fuel filter, plugs, wires, cap, rotor, maf sensor, ignition coil, alt., battery, cat converter, im stumped...

check for a clogged cat/converter. Exhaust sounds like "whoosh". NO exhaust exit, NO go