1988 GP Service Engine Light

I have a 1988 Pontiac Grand Prix LE. The car runs fine for a while, then will begin to idle roughly. When this happens, the service engine light comes on and the engine idling smoothes out. I checked the codes from the OBD and am getting both high and low for the MAF sensor. I’ve replaced the MAF sensor with both an aftermarket rebuild and a factory new unit, cleared the codes by disconnecting the negative battery cable for a few minutes, but the problem still exists. A mechanic told me that the MAP sensor is bad and causing a false read for the MAF, so I replaced the MAP sensor but this didn’t fix the issue either. Additionally, the light for high-temp comes on and goes off whenever it feels like it, but the operating temperature never goes above the normal range. (I’ve replaced the temperature sensor too).

check both the vacuum line (it runs from the junction by the throttle body, under the upper plenum, and then to the MAP) and the wiring harness going to the map . Either of these would cause the readings you are getting. Bad vacuum to the MAP will cause false readings and corroded connections can do the same.