Hi I'm Larry new to site. Im having some trouble with my Cadillac. I have a 4 door 1990 Cadillac deville with a 4.5. fwd, I've had this problem for over a yr now and it seems to be getting worst...Ive had the cat almost 3 yrs now and it ran a1 when I first got it. When I drive it it shuts off on me. I have to put it in nuetral rq and restart it. It won't go past 70mph..it spits and sputters and last nite it backfired on me. Now I've been a mechanic for 15 years. So far I've replaced the map sensor, tps sensor, idle control sensor, waterpump and gasket, temp control sensor, replaced the injectors, fuel filter, egr valve, fuel pressure regulator, plugs and wires, distributor cap rotor, the thermastat, heater core, the ignition module on top of the distributor cap, the little chip inside the distributar, I've checked the wires firing order and that'* all correct. I've replaced the first o2 sensor, inspected all the vacume lines for leakes n cracks I've even drilled holes in my cat thinkin the cat mite be clogged up. I've cleaned the throttle body several times, I've looked everywhere for the crankshaft position sensor and I can't seem to locate it. I was told it has 2 a upper and lower but still not seeing it...ive looked behind the starter with a mirror but didn see it and no wires run behind the starter. I've been under it on top of it on both sides and even around it and I can't find it. Shows codes for e41 e48 I've looked em up and tried every possible to fix em. The service engine soon light stays lite for most the time but every so often it goes out and comes back on. Does anyone put there have any idea of what I can do next

No crankshaft position sensor or camshaft position sensor on the 4.5L Deville. As it has a distributor, and inside the distributor is a Hall Effect switch, the computer is able to sense the position of the engine to fire the injectors sequentially. Completely unique in the GM family, although the same Hall Effect switch was used in Chevy 3.8 V6s with computer control. It was not for determining TDC in this application, but rather a better way for the computer to interpret even firing in an engine that was odd/even fire.

Drive it with the front oxygen sensor removed and see if these issues improve. It won't fix it, but if the cat is clogged, there should be a noticeable improvement. If there isn't a noticeable improvement then the issue is likely somewhere else.

Put a vacuum gauge on it.