Hi!

I have a Volvo S60 I 2001 (eng B5244S2). The problem is a hunting idle when the car is in closed loop mode (CL). The car then goes into an open loop (OL) when I start to drive it on the road at higher RPM and do not return at idle to CL when stopping. When it's in OL the car idles perfectly and runs fine, steady idle and with good acceleration and no higher fuel consumption.

I have checked the MAF and MAP, for air leaks in tubes and manifold, plugged the EVAP hose temporary (for unwanted fuel mixtures), checked fuel pressure, checked injectors for leakages, checked injectors spray patterns, changed the oil trap assembly under the intake, changed plugs and coils . I have change the upstream O2 sensor to a new one (and it is oscillating as it should between rich and lean in CL mode). After an MOT (in Sweden in OL mode), the car got a failed inspection due to high CO values at idle and at 2500 RPM.

Any tips for further testing I could do is appreciated.

/Stefan

 

Has your ETM ever been replaced? Check out Xemodex's web page for symptoms/diagnostics on common throttle body issue.

 

Hello and thanks for your suggestion! I have assumed that the ETM is OK because the car runs perfectly when the computer runs in open loop, but I will check this one more time!
/Stefan

 

I did one more test today. I hit the brake several times when the car idled uneven and when the oxygen sensor after the CAT measured a lean value the oscillations of the RPM stopped.
Well, it indicates a rich mixture in the fuel mix and the oscillation follows when the engine is at higher RPM's too, until the computer change to open loop and then the oscillations stops (at all RPM's). Tricky!! The fuel pressure seems OK and steady. No leaking injectors and the spray pattern seems OK (same from all injectors).

 

The rear O2 sensor is there to check the cat's efficiency. The ECU compares the front/rear sensor signals to determine if the cat is converting any remaining HCs. The front sensor is the lamba which controls fuel trim. The "lamba" cycle is a feedback loop so there will be oscillations depending on the time it takes for the feedback to correct the input. When the ECU is in closed loop mode, its listening to the front O2 sensor and adjusting the fuel trim accordingly. In open loop mode, the ECU is ignoring the O2 sensor and is just running off stored program instructions.

 

Yes I know that it is the upstream oxygen sensor that controls the ECU. But I begin to wonder if I should change the rather new lambda sensor one more time. I bought a 'pirate' version instead of a genuine Denso sensor and though it seems to function (switching between rich and lean) it could be that the computer not liking this particular sensor.

 

curious what led you to replace the O2 sensor in the first place?

 

The first sensor died, and forced the car into OL. Then I changed to a new sensor and got bad idle (car stopped at red lights). I thought it was a dirty Maf sensor and changed that also and the car did'nt stop any more but I still had hunting idle (not missfires). Then I have diven a long time in OL mode before reurning to the idle problem.

 

OK so before replacing the sensor, I'd check the connector/wiring for the new sensor. Also check the exhaust system and connector for ground.