Running rich and misfiring

Hi guys, first time poster here. Really need a second opinion on why my truck has problems running really rich and rough when it’s first started. It’s a 1990 with the 2.4L 4G64.

When I first start it in the morning, it will start and build rpms like normal but is misfiring. Watching the exhaust pipe, a little black smoke will puff out constantly, and even some black liquid. After waiting a few minutes, the engine will all of a sudden stop missing and rev up quite high for a second or two, and then run completely normal.

If the engine is not cold and started, it will start only if I give it a little throttle, otherwise it will start then immediately die. The engine will miss and run rough for a minute or two then just start working normal again. If I’m sitting in traffic, shit the engine off and immediately restart it, it will misfire for a minute or two. Even while driving. If I release the gas when it’s running rough and the engine is hot, the engine will rev so low it will stall. Once engine stops misfiring after it’s started, it will run perfectly until the next time it’s shut off and restarted. When it’s misfiring after started, blipping the throttle or revving the engine has no effect on the amount of time it takes for the motor to stop missing start running normally.

The problem started happening only on some cold starts, and would run rough only for a few seconds. Then it started happening very cold start, then hot starts, and then duration of time it misfires has started increasing.

Problem started about a thousand miles ago then started changing stuff. So far I have changed the fuel pump, filter, pressure regulator, all 4 injectors, spark plugs and wires, ignition coil, and throttle position sensor (set to manual’s specs). The timing belt was done about 3k miles ago. Timing was set at the same time. O2 sensor only has about 4K miles on it.

I have checked resistance values for the MAF, intake temp sensor, coolant temp sensor and motor position sensor found on the idle speed servo. Everything is in spec except the motor position sensor. It’s an open circuit as far as I can tell. I can’t seem to find a replacement online, and I can’t see why this sensor would even cause the problems I’m having in the first place. I’m probably not measuring its resistance correctly.

Timing has been checked with a timing light when the engine is missing during its warm up. Timing is good, around 12 degrees or so and advances normally with rpm.

The ECU was remanufactured about 20 years ago. Took it apart, looks fine. Checked it’s grounds.

Check engine light never turns on. I checked for codes, system normal. I replaced the check engine light in the dash with a known working light, light still never comes on. Even with ignition on before starting engine, no check engine light. Repacked the ECU with another reman unit. Check engine light now works when key is first turned on but then goes away and provides no insight into the problem.

Tried unplugging and clogging the vacuum lines from emissions charcoal canister as a potential source of excess fuel, but had no effect.

EGR valve was inspected long before any problems and was found to move but has no effect on idle speed like manual instructs. I suspected it was clogged and left it alone.

Any help would be greatly appreciated!