Fast Idle - Rear O2 Sensor issues

[ArcV70R]

After a couple of weeks of ownership of my 2006 V70R, I dropped it into the local Volvo specialist for a check over. They discovered an issue holding a fast idle. When revved to about 2500 and held, it drops the revs to 2100 quickly, then returns. This fluctuation continues every second or two. They diagnosed a possible issue with the Cat.

There are no relevant codes in VIDA, just a couple related to the A/C not working, which I know about. There only other symptom I have is it smells a bit 'fuelly', but I don't know what a 16 year old V70 should smell like.

I've tried to test it using VIDA, but don't know enough about fuel trims and O2 sensors to diagnose reliably.


https://youtu.be/NmrOjE5Ie7Y

The initial sharp fluctuations in the rear sensor (blue line) show what's happening with the symptoms in the video. The second half of the trace is under normal idle. I think this is normal behaviour, but not 100%. Can anyone help explain what's going on here? Should the reading from the front O2 sensor be so flat?

33117

[uqj83l]

Hi
I can’t comment on the datalogging as I haven’t got my car plugged in to compare. What I will say is that Vida fault trace is very reliable. When a part is beginning to fail it will flag up as an amber light. And when it has failed it will flag up red. No faults indicates there may not be anything wrong with your O2 sensor. I’ve owned one of these cars for over 13 years now and in my experience Hunting or erratic idles are normally caused by vacuum or boost leaks. My best investment apart from Vida was a smoke testing machine. It has helped me pin point a whole host of issues over the years. Hope that helps rather than hinders!

[ArcV70R]

Thanks for the response. I have this afternoon begun the task of working round all the boost and vacuum hoses to check for leaks. I tried a quick propane test a couple of weeks ago to see if I could find anything. That did lead me to a completely knackered breather tube. I will try again more methodically taking each of them off to inspect them if necessary.


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[uqj83l]

There’s also a one way valve built into the PCV banjo bolt on the corner of the inlet manifold. Probably not the cause of your idle issues but they can clog up and leak a small amount of manifold pressure into the PCV system. That same bolt has been redesigned by volvo a few times as it has been known for the pin to break off and then enter the cylinders of the car. I recall you mentioning that you changed the hardline that feeds into this so you may already have tackled this issue.

[ArcV70R]

I did read up about the PCV, so I think I know where the banjo bolt is that your talking about. I didn’t take on the whole PCV job, I just replaced the tube that runs from the top of the PCV box, behind the inlet manifold and into the top of the rocker cover.

[Harvey]

Unplug the maf sensor and see what happens .
possibility the throttlebody needs a clean if sticking .

[ArcV70R]

Unplug the maf sensor and see what happens .
possibility the throttlebody needs a clean if sticking .

I’ve investigated the MAF already. Even swapped it out for a new one to test, and no effect.

[stribo]

As above check for boost leaks, as well as the hoses check both intercoolers, and the small hoses that link them. When i changed our S60R intercooler, the main one was starting to bow at the bottom. Also there are 2 one way valves, one under the inlet manifold, and one to the right of it, these have a habit of failing which cause a boost leak.

[ArcV70R]

So a bit of an update in case it’s useful for someone in the future. I’ve had all the pipes out to check for vacuum leaks, and changed the small coupler host that connects the turbo to the charge pipe that goes over the engine. The pipe was showing some oil saturation so that was a precaution more than anything.

It’s been into two local garages and neither could diagnose it, so I bit the bullet and it’s been with the master tech at the Volvo dealership.

They’ve diagnosed it as a failing throttle body based on previous experience with similar issues with V70R specifically. I had stripped the throttle body last week and cleaned it, everything looked fine but they’re pretty confident that’s what it is.

They got the car through the MOT today, so I’ll swap out the throttle body at some point in the next few months, but seeing as it has no real impact on day to day driving, I won’t rush.

Volvo were great. They had the car for three days over two separate visits and only charged me for the MOT, no diagnostics fee.