SN95 V6 1994 occasional rpm surge while driving---where should I start?

Pull KOEO and KOER codes again — don’t just chase the single P0156. See if anything else is hiding.

Inspect wiring for Bank 2 Sensor 2 (passenger side downstream). Check for pinched/melted harness near the exhaust. Verify heater power and signal return with a meter. Get a good look at the connector contacts.

Fuel delivery check — replace fuel filter if it hasn’t been done; consider fuel pressure test. Surging at highway speeds often points here.

Vacuum leak test — especially around your recent EGR install. Spray/brake clean or smoke test.


If all of that checks:

Ignition check — plugs, wires, coil pack. If they’re old, it’s a cheap maintenance step anyway.



My suspicions:

New EGR + tube install = prime chance for vacuum leak, cracked gasket, or loose connection.
That would explain both the inconsistent idle and the surging cruise.
The O2 code may just be fallout — PCM sees weird trims, flags the downstream sensor.


Notes:

The EGR only gets commanded on during steady-state cruise or light throttle (never at WOT, never at idle). That matches his symptom exactly:

“At 55–60 mph it surges up and down” → that’s right where the PCM would be commanding EGR flow.
If there’s a leak, stuck pintle, or misrouted vacuum line from the fresh install, the PCM will be fighting airflow/fuel trim changes, causing the surge.
The inconsistent idle? Could be the EGR not fully seating closed, or a leak at the EGR tube flange.

EGR system is both timing-wise and symptom-wise the perfect suspect.