Fuel injectors not pulsing; have fuel and spark

Was the same fault there before you changed the cylinder head? If so, that would point to piston problems on cylinder No4.

Are you able to do a compression test on the engine? If there's piston/ring trouble on the 4th cylinder that could cause a misfire because of poor compression.

The other thing that springs to mind is that there's something amiss with the valve clearances on cylinder No 4.

I'm not familiar with your car - does it have an overhead camshaft? Unfortunately when refitting an overhead camshaft after a rebuild, some owners have managed to bend a valve. This usually happens because the engine has been turned during disassembly/rebuild and the timing marks have not been aligned when refitting the camshaft.

This results in a valve in contact with a piston and when the camshaft is fitted and torqued down the valve becomes bent.

I think your problem may lie in either a bent valve or incorrect valve clearances.

With engine cranking, do you have spark at the spark plugs? Do you have fuel injector pulse? I can't tell if that fuel pressure is initial prime or not? The initial prime doesn't need rpm signal, no cranking, after that the computer needs to see rpm signal. I saw what you replaced, I'd have to check wiring circuits before replacing anything.

Causes A code P0303 may mean that one or more of the following has happened: Faulty spark plug or wire, Faulty coil (pack), Faulty oxygen sensor(s), Faulty fuel injector, Burned exhaust valve, Faulty catalytic converter(s), Running out of fuel, Poor compression, Defective computer... Possible Solutions If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back. If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

could be a fault in the #3 injector wiring, with the engine running back probe the wiring at the injector with a led trouble light it should glow at each pulse. a pulse can be felt by holding the injector if you have no light. compare with # I&2.

If doing the valve seals the head would most certainly need to come off and upon replacement a new gasket would always need to be used but whilst the head is off the engine check the bores turn the engine by hand untill the pistons go up and down to see what wear is on the bores unless whilst the head is off you go the whole hog and do the piston rings at the same time and the oil rings

How the computer determines a misfire is through a lagging effect in the crankshaft turning-when one cylinder is not producing power, the crankshaft doesn't turn as fast, and the computer senses it through the crankshaft sensor.
Misfires are usually a fuel problem, spark problem, or a low compression problem. Compare the spark at #4 with the other cylinders-all should have a blue, snapping spark. If good spark, check the spark plug -it might be defective. Compare the spark plug in #4 with the others. If it has a black sooty appearance, it is running rich-injector may be leaking. If the lower insulator on #4 is whiter than the other plugs, it is running lean, and the injector may be partially clogged, or not pulsing well.
If gas and spark is good, do an engine compression test. If #4 has low compression, it could be a valve problem, or worn piston rings, or a head gasket failure.

If it isn't pulsing right, maybe it is staying on all that time -and spraying fuel like crazy into your cylinders! Come on, Joel, a weak spark and injectors not pulsing right? Get a different ecm and plug it in. Then you can learn the circuits on your old unit and fix it in your spare time.

fuel lines hook up backwards problem solved...

Sounds like either a blown motor (rings need replaced) or a bad head gasket or cracked head.
If the rings are bad your car would have felt like it was losing power because the engine will no longer have the same high compression that it's required to turn the wheels. You may have heard your engine knocking or seen a light blue colored smoke coming from your exhaust as a result of your engine burning up oil in the combustion chamber from the rings leaking.
If your head gasket is leaking or your head is cracked you'd likely find milky looking oil when you drain your oil from water/antifreeze being mixed in and noticed a white smoke coming from your exhaust that is actually water vapor coming through the exhaust.
If your car was running rough when you last shut it off try pulling all of your spark plugs and either cleaning them or replacing them all. A bad head gasket or bad rings can both foul spark plugs. A car will run rough with 1 or 2 fouled plugs, but once enough foul it will no longer run at all just sit and crank over. If changing the plugs get's it started for ya, check your tailpipe first for water, if nothing, change your oil. If no signs of water/antifreeze or milky colored oil then it's probably rings.

broken/worn piston ring(s). Each piston has 2 rings. These can be replaced by the DIY mechanic by
1) removing head of engine (remove head gasket - you'll replace it), then
2) manually wind the engine (with a ratchet wrench & socket) til the piston in question pops up
3) remove both rings (with ring pliers - available from any auto parts store)
4) install 2 new rings (with a ring installation collar - available at any auto parts store)
--- choose ring installation tool to the correct size for your pistons.
5) re-install head of engine (replacing head gasket)

Note that this procedure preserves the engine timing, so you need not reset it.