I have a 1990 3.1L, multiport fuel injection (NOT sequential FI), which runs very well most of the time. However, when I cold start it runs rough (misfire) for about 1 minute (no throttle) or until acclerated. The is no misfire when running normal, but it might be running a little rough when stopped at a traffic light (in Drive, foot on brake).
I put new plugs and wires on, but it did not affect this problem.
I am not using coolant, or oil, the plugs that were removed were somewhat worn, enough that I expected to notice a difference after they were changed.
I have not yet (1) cleaned the MAF, (2) checked carefully for vacuum leaks, (3) checked for leaking injectors.
Since this is a 1990, OBDI, I can't say which cylinder is misfiring or how the fuel trims look etc.