Engine Surging

I have a 1991 Mitsubishi Mighty Max 3.0, and it is giving me some trouble. If the MAF sensor is plugged in the RPM’s surge up and down, anywhere from 600-2000. If you unplug the MAF sensor it runs smoothly around 1600 RPM. I put in a brand new MAF sensor and that didn’t change anything at all. Everybody I talk to says it must have a vacuum leak, any thoughts?

I would look for any air leaks after the MAF sensor.

The reason the engine smooths out, although at a fast idle is because on an OBDI engine management system, if you unplug an emission control component the computer goes into default value for that missing component.


@Tester an OBD 2 car will also switch to default values if you unplug a component.

It could also be a bad temp sensor or a stuck thermostat.

We just finished rebuilding the engine, it has a new thermostat.

Does this car have an idle adjustment screw? Many cars of that era do. Surging can be a symptom if the screw has been adjusted out of its nominal range to try to solve some other problem. The engine will surge to compensate. The shop manual should explain how to set the adjustment to its nominal range.

No there isn’t a screw