MY 1991 BUICK REGAL CUSTOM 3.1L WONT START turns over but dosent fire. GOT SPARK, FUEL PRESSURE IN FUEL LINE. LIST OF REPLACED PARTS. NEW FUEL PUMP, FUEL FILTER. THROTLE CONTROL SENSOR. MAP SENSER, SPARK PLUGS, FUEL PRESSURE REGULATOR. WHAT IS MY PROBLEM OR WHERE SHOULD I GO NEXT.

You need the exhaust converter so the O2 sensor can
work properly

It sounds as though the converter is blocked,as well
as the fuel trim, if that is correct term for an OBD1 System,
is not able to report back the oxygen in the failed converter

So the ecm can not provide the correct fuel mixture by
pulsing the injectors

I would also expect the timing chain to have a lot of slop
& possibly be off a couple teeth, from worm plastic gear
on cam

Almost fires? Do you mean it does fire some, that you know the plugs are getting spark? If so, then you probably need to have your fuel pressure tested to see if pump is working properly. Can you hear the pump buzz on briefly when you turn key to on? It should come on for a couple of seconds to pressurize the lines. If you don't hear it, then your pump may be bad, or something not turning pump on. Good luck.

Check your fuel pressure at the schraeder valve that is on the fuel rail. If you have fuel pressure, check for spark.If no spark, then ignition diagnosis is needed.If no fuel pressure, check for diconnected, or pinched wires, as this can easily happen during an engine swap.

do you have spark if not your could be bad if you have no spark your timing chain could have jumped or is broken

Before I went too much further throwing parts at it, I'd have fuel pressure tested. Not uncommon for low fuel pressure to starve a cylinder or 2 and have a misfire code come up. Should only take mechanic about 5-10 minutes to verify low or o.k. , then you can eliminate that possibility.

how's the fuel pressure?

replace the ignition module,under the coil pack.

Then it has to be fuel, compression, or air. Check for fuel pressure first, if it has fuel pressure check for compression, must have a minimum of 100 pounds to run. Last check for air supply being restricted(very very very rare). if it has all of these and sprk it has to run as long as its timed right which is impossible to get out of time on a fuel injected engine like this unless someone mixed the plug wires up on the coil pack at which point it will be back firing out the intake and exhaust.

car wont start i had the started check put a new battery in what should be my next step?

if it runs on starting fluid its a fuel problem.. check fuel pressure even if its a couple of pounds off, the vehicle will not run. but using starting fluid may give you the extra poundage you need to make it run . also check vacuum hose at pressure regulator on fuel rail