Have a 90 L98 with new plugs, wires, cap, rotor, ignition module, coil, fuel filter and FIC Bosch injectors. Was previously chasing a stumbling/part throttle hesitation in closed loop which has since resolved with the replacement of the above components. However, I am still faced with a part throttle hesitation in open loop when the car is cold. For the most part, the car runs well in closed loop once warmed up.

Any suggestions? Haven't checked the fuel pressure at the rail yet...

Forgot to mention, TPS reading looks ok in Diacom.

Lots of threads here about these type of issues in closed loop, but not open. Searched w/out success.

Any ideas? Now that the weather has gotten cooler, this is very noticeable.

I am now finding this issue has returned (although significantly milder) once the car is warmed up and in closed loop. Fresh out of ideas

Try a new O2 sensor. Mine did the same crazy stuff From being rich.

O2 sensor is new and was replaced with the addition of the headers. Readings look good in Diacom

Still having issues with sputtering and power loss in open loop. Car feels like it hits walls at certain RPMs and is way down on power. I have also replaced the CTS which did not seem to help this problem.

Once the car reaches closed loop and is up to temperature, drivability improves greatly.

At this point , I feel that possible causes could be a vacuum leak or bad EGR. I also plan on getting a fuel pressure guage just to rule out any fueling issues (although I would think a fueling issue would be present in both open and closed loop).

Any ideas?