hello

alright lately my 1989 240 has been giving me a stumble and rough start for about 3-5 seconds on start up. It also triggers the check engin light while this is happening. If i give it gas it clears its throat and the light goes off right away. if i wait for it to resolve on its own it takes a few seconds longer and then the engin runs smooth and the light goes off.it has done this for about a month now with more and more frequency, but not every time i start the car. It also doesnt seem to matter if the engine is warm or cold.

Is this one of those things that i would need to have a code reader to diagnose and it would probably tell me something like engine running lean / rich
and then i need to suspect ECU or oxygen sensor or AMM ? my guess is something with the computer itself or the sensors that send the computer its information to control fuel injection, my 89 has the older ECU should i swap it for a 91 model (i forget the numbers 561 ECU for a 961 ECU upgrande..or something like that.. ) or just have the codes read first and then see ..

any help is appreciated, i want to get this cleared up before winter so i
thanks ..