4-cyl with emission code 23

[JimR]

I have a 4-cyl 1988 that displays a code 23. According to my manual this is for a Manifold Air Temperature sensor for the 6-cyl engine only which doesn't fit my engine since it doesn't have a MAT sensor. The vacuum hoses are all connected and seem sound.
The engine tends to surge RPM or just balk sometimes, so there is still something that need fixing. Any ideas?

 

[Oreif]

It could be that the ECM is bad or someone tried swapping PROM's in it. Code 23 should be turned off in the programming code for the 4-cyl's. Has anyone swapped out or messed with the ECM? It could be also that someone swapped in a "performance" chip and the PROM is going bad.