I have a 1994 3.8 V6 convertible that I purchased this year. I changed the air filter, cleaned the MAF, installed a new EGR valve and tube, and have ran a fuel system cleaner. I have a check engine light for a P0156 O2 sensor Bank 2 Sensor 2. I replaced the O2 sensor, but it came back on immediately. It was running overall fine, but recently it started to surge in rpm while I am driving it and during the drive I believe the idle is inconsistent. What it will do is I will be driving at 55-60 and it will surge in rpm and then drop, then it will go back to normal. Any ideas of where to start trying to fix the problem? Thanks!
SN95 V6 1994 occasional rpm surge while driving---where should I start?
- Thread starter DPP
- Start date
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.
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.
Sorry I never replied back till now. I replaced the fuel filter and that improved acceleration and I don't have the surge that I use to get--so that helped.
Heater works great and when I installed the new O2 sensor I didn't notice any wire issues.
I don't have a scanner so I have not tried that.
Still have the same error check engine light, but I will continue to work on it as I get time.
Heater works great and when I installed the new O2 sensor I didn't notice any wire issues.
I don't have a scanner so I have not tried that.
Still have the same error check engine light, but I will continue to work on it as I get time.
So I am having issues with surging again. I had previously changed the fuel filter and everything improved. Now surging occurs usually in the afternoon after I have run it and restart it and like before at constant speeds.
I did a smoke test to check the vacuum lines and see spoke coming out around the EGR valve and my son says it is from the holes in the bottom of the EGR valve top--is it supposed to do that? This was a brand new valve and and I was wonder if these holes are for venting from the manifold?
I bought a new egr valve gasket, but don't want to re-install if that is not the problem. All help is appreciated.
The good news is that the rest of the system seems food and sealed by smoke test.
I did a smoke test to check the vacuum lines and see spoke coming out around the EGR valve and my son says it is from the holes in the bottom of the EGR valve top--is it supposed to do that? This was a brand new valve and and I was wonder if these holes are for venting from the manifold?
I bought a new egr valve gasket, but don't want to re-install if that is not the problem. All help is appreciated.
The good news is that the rest of the system seems food and sealed by smoke test.