Rough idle, sudden power loss - MAF, intake leak, O2 sensor?

[JanvS]

First of all, these forums have already been a great help w DIY fixes and diagnosing problems However, I've gotten as far as I could using the topics already available and was hoping to rely on the knowledge of those here to help me further.

About 6 months ago, my V70 (2.5T, MY2005) started having problems with idling when at operating temperature: it would feel like the engine was stalling, but then the RPM immediately shot back up before settling back to around 700RPM. This problem intermittently occured. Then, one time I had a sudden loss of power when accelerating, which again lasted only momentarily. This problem started getting more frequent but still intermittent.

This week, however, when sitting idle for a while with a warm engine the RPM was suddenly all over the place, 'hunting' without settling, and after about 30 seconds the check engine light came up. Drove home without (further) problems.

Read out with VIDA, gave a bunch of error codes relating to the MAF:
CCM-0070 Configuration fault Fault Identification - (active) (probably unrelated, although not present one year ago)
ECM-120D Air mass - Signal too low (active)
ECM-121B Mass air flow (MAF) sensor - Flow too low (active)
ECM-121D Mass air flow (MAF) sensor - Signal too low (not active)

Replaced MAF yesterday. MAF error codes gone, and stay gone after driving some 20 miles. NO error codes pop up (except the CCM issue). However, the problem has not gone away: in these 20 miles, there was a hiccup while accelerating, where there is a sudden loss of power, twice. Idle at operating temperature still not stable: the RPM dips as if engine stalling about once every 2 minutes or so. Reset ECU this morning (removed fuses 6 and 23, then turned ignition, replaced fuses) and went for a drive again but no change.

VIDA Vehicle Communication has yielded the following:
- Mass air flow is about 9kg/h at operating temp, without A/C engaged. (Don't have a reading for the old MAF, unfortunately. New MAF is Bosch, correct part number. However, ordered online and box was not sealed upon arrival so not entirely unsuspect).
- Air mass, correction factor varies a bit, I'd say from 0.86 - 0.94 although I haven't monitored closely while driving.
- Oxygen front sensor more or less stable at 1,00 when idling. VIDA log shows it changes value very quickly when driving.
- Oxygen rear sensor varies wildly even at idle; not a neat sinus curve. Doesn't sit stable and doesn't fluctuate between two different values but 'hangs' in between.

I'm currently suspecting the rear O2 sensor, although I gather from these forums that the low reading for Mass air flow + the air mass correction factor suggest an air intake leak. Haven't examined the throttle body. Anyone has any comments/ideas?

[abscate]

Look at your fuel trim values at idle and at 2500 rpm to diagnose vacuum leak. At higher mass flow the leak won’t matter and the trim will tend back to a few percent. At idle , it will float to 15-20%, if you have a leak.

[oragex]

The rear O2 has nothing to do with engine operation. Simply measures cat efficiency. Front O2 will vary from 1 under acceleration but not sure if Vida can read it? Not saying it would do such behavior (better not replacing more parts by guessing, it's gets expensive)

See as above. You may also want to access the throttle body from one side and clean the valve on both sides and the body as well

[JanvS]

Thanks! Immediately went back and checked. Does it matter if engine is at operating temperature? It was still warm, but probably not at operating temperature. Readouts changed after running for a while, although I don't know what any of these mean. These are the results.

The following readouts do not change when revving to about 2500rpm, but do change slightly after having run the engine for a while

Long-term fuel trim idle, bank 1: starts at -0,09% then 1,22%
Long-term fuel trim lower part load, bank 1: starts at 1,01 then 1,03
Long-term fuel trim upper part load, bank 1: 1,00

Short-term fuel trim, bank 1: starts at 1,18, then oscillating from 0,95 to 1,05. Behavior changes when revving, although the value does not really change. Does not return to this oscillating behavior immediately after revving; see the attached photo.

Throttle body cleaning is something I'd rather not do on the street, and have no access to workshop or garage, so that would require taking it to a local mechanic who knows his way around Volvo's.

[abscate]

See how your fuel trims converge to 0% ( value of 1) at revs?

At idle the ECU is hunting the fuel trim trying to compensate for air that the MAF doesn’t see.

Vacuum leak. Beautiful data.

[JanvS]

Thanks a bunch! Never would have guessed that such small deviations could be indicative of this problem. Called the local Volvo indy this morning, but they won't have time to look at it until 2 weeks from now. I'll report back once I know more.

[abscate]

You can build a smoke tester in about an hour and test your hoses for leaks, or spend an hour pulling off each vacuum line and replacing them. There should be a sticker on your hood outlining the vacuum hose positions.

[JanvS]

Well, I dropped it off at my local Volvo Indy mechanic, picked it up this afternoon but said they couldn't find anything. According to them, the short term fuel trim is supposed to fluctuate slightly. They dropped in a can of fuel injector cleaner, told me to drive 150 miles and see if anything changed... Somehow I'm not too confident that this will do the job, but I guess you never know.

[XC70Rider]

Hi JawS, did you solve the rough idle issue? Sorry to hack your thread but I'm dealing with the ECM-121B code right now. Until I got my VIDA finally hooked up yesterday I was getting P0101 with my OBDII scanner. Now I presume it's a minor vacuum leak because she's been running smooth. I installed a new Bosch MAF, replaced all vacuum hoses and clamps, put in BG44K fuel injector cleaner but the code still appears.

How do you get VIDA to read the Air Fuel Ratio graphs? With VM player 12 all I could read yesterday were the DTCs.

Thanks in advance!