1994 Chevy Lumina

Check fuel pressure
do an injector leak test
check intake manifold for vacuum leaks
smoke test when cold would be best
remove check and clean IAC valve

If a new IAC didn't help, and all still the same, than should be NO need for cleaning an old IAC
if spark plugs getting wet after long stop do an injector leak test

You need a fuel pressure gauge and a scanner

Fuel Pressure Regulator (PFI)
Install fuel pressure gauge to fuel rail fuel pressure test fitting. Remove vacuum hose from fuel pressure regulator. Turn ignition on and note fuel pressure on gauge.
Start engine. Check for manifold vacuum at pressure regulator vacuum hose. Repair as necessary. Reconnect vacuum hose to pressure regulator and note fuel pressure on gauge. Compare first and second reading. Fuel pressure reading should be 4-7 psi (.28-.49 kg/cm2 ) less with vacuum hose installed. Fuel pressure should decrease as vacuum increases. If results are unsatisfactory, replace fuel pressure regulator.
PFI Injector Balance Test (C-2)
The injector balance test is used to pulse the injector for a precise amount of time, spraying a measured amount of fuel in the intake manifold. As each injector is pulsed, a drop in fuel rail pressure occurs. This pressure drop can be recorded and compared to other injectors. An injector with a pressure drop of 1.5 psi (.11 kg/cm2 ) or more, greater than or less than other injectors, should be considered faulty.
NOTE:Allow engine to cool down to avoid irregular readings due to "Hot Soak" fuel boiling. To prevent flooding, the INJECTOR BALANCE TEST should not be repeated more than once without starting and running engine.

CAUTION:To avoid possible vehicle fire, wrap a shop towel around fitting to avoid fuel spillage.

With ignition off, connect Fuel Pressure Gauge (J-34730-1) to pressure tap. Unplug harness connector at all injectors. Connect Injector Tester (J-34730-3) to one of the injectors. You can use the scanner to cycle injector by # off on without disconnecting the injectors or the wireing harness
Follow manufacturer's instructions when installing adapter harness. Ignition should be turned off at least 10 seconds to complete ECM shutdown cycle.
Turn ignition on. Fuel pump should run at least 2 seconds after ignition is turned on. Bleed air from gauge and hose to ensure accurate gauge reading. Repeat this procedure until all air is bled from system. Turn ignition off for at least 10 seconds.
Turn ignition on again to bring fuel pressure to maximum. Record initial pressure reading. Energize tester one time and note pressure drop at lowest point.
Disregard any slight pressure drop after low point is reached. Subtracting second pressure reading from initial reading indicates amount of injector pressure drop.
Repeat step 4) on each injector and compare pressure drop. Recheck injectors not within pressure drop range. Replace injector(s) failing second check.

Am not in for replacing parts to check
but for sure if don't have the tools that would be cheaper if it works
Did you consider renting the tools?
Replacing the injectors should be relatively OK upper manifold out and you are there

Also I would add coolant sensor may cause a lean or a rich condition