It is very important to check fuel pressure with a pressure tester to confirm it is around 45 psi. If that is so, then we need to check engine vacuum to rule out the possibility of a partially clogged exhaust system. Here is what a vacuum test tells us:
Normal Engine Operation
At idling speed, an engine at sea level should show a steady vacuum reading between 14 in. and 22 in. Hg. A quick opening and closing of the throttle should cause the vacuum to drop below 5 in., then rebound to 23 in. or more.
General Ignition Trouble or Sticking Valves
With the engine at idle, the continued fluctuation of 1 to 2 inches may indicate an ignition problem. You should check things like spark-plug gap, primary ignition circuit, high-tension cables, distributor cap or ignition coil. Fluctuations of 3 to 4 inches may point to sticking valves.
Intake System Leakage, Valve Timing or Low Compression
A vacuum reading at idle that is much lower than normal might indicate leakage through the intake manifold gaskets, manifold to carburetor gaskets, vacuum brake booster or the vacuum modulator. Low readings could also be caused by very late valve timing or worn piston rings.
Exhaust Back Pressure
Starting with the engine at idle, slowly increase engine speed to 3,000 rpm. Engine vacuum should be equal to or higher than vacuum at curb idle. If vacuum decreases at higher rpm, an excessive amount of back pressure is probably present due to a restriction in the exhaust system.
Cylinder Head Gasket Leakage
With the engine at idle, the vacuum gauge pointer will drop sharply every time the leak occurs. The drop will be from the steady reading shown by the pointer to a reading of 10 in. to 12 in. Hg. or less. If the leak is between two cylinders, the drop will be much greater. You can determine the location of the leak by doing a compression test.
Remember, engine problems can affect transmission performance. If you suspect an engine problem, connect a vacuum gauge to the intake manifold. Note the location and action of the vacuum gauge needle, and use that information to determine the engine problem. Correct the engine problem before doing extensive calibration work on the transmission.
Please perform the vacuum tests described above; then share the various readings with me along with the actual fuel pressure in psi so I can get a better idea of what is exactly happening with your engine.