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This is a well maintained 2007 Subaru with 140k km on the 3.0 litre six cylinder EZ30R engine. She's running a little scratchy most noticeable when cruising although no codes or anything.

I decided to take a look at the airflow situation and noticed bank two seems to be running richer with less responsive data for the pre-cat wideband AFR and post-cat oxygen sensor.

Couple of screenshots attached we can see AFR on bank two is less responsive and also the O2 sensor reaction on bank 2 can be a second or two delayed compared to the bank 1 signal.

I understand the two banks won't be perfectly in sync but more than a second difference seems long with EFI. Could be a normal situation too I guess. Perhaps this is an airflow problem internal to the engine.

Some tests I've done;

  • Switched sensors between banks, no change
  • Smoke tested exhaust and intake, no leaks
  • Replaced exhaust with a junkyard system as mine was rusty, no change
  • The removed exhaust didn't have signs of oil or or coolant cleaning
  • Checked fuel trims, all 3% or lower

The graphs below show data sampled at 20 rows per second;

Screenshot One Screenshot Two

Appreciate your help. I'll post responses and comments, hate to see a question die here.

EDIT 2019/06/16 Removed previous propane graph to save space. Adding results of my next propane test from @Zaid's help. The propane bottle may have been running a little low but the results tie in with @Zaid's image regarding a bad cat. Also added a chart showing AF & O2 sensors whilst driving steady and gentle on the highway, bank #1 O2 sensor is all over the place!

Propane test, second attempt Highway cruising

EDIT 2019/06/20 Now logging STFT to find bank 1 is cycling additional STFT at cruise, would this mean an air leak someplace ?

Bank One Bank Two

ajayel
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  • Great detail in the question! When you replaced the exhaust did it including replacing the catalytic converters? – Zaid Jun 11 '19 at 09:55
  • Thanks @Zaid, I replaced the exhaust from the manifolds to the middle of the car, this included both left & right catalytic converters and the Y piece that joins the banks together under the middle of the car. I was surprised to see the same symptoms although it's likley the replacement parts were from a high mileage car. – ajayel Jun 12 '19 at 03:06
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    I would use the propane test to confirm if the Bank 1 cat is bad. – Zaid Jun 12 '19 at 07:43
  • OK I read the propane test Q&A it looks very interesting and will try it over the weekend – ajayel Jun 13 '19 at 08:04
  • Just did the propane test described the results are at the end of the question – ajayel Jun 13 '19 at 09:37
  • Hmm... this is interesting. Why does it look like both post-cat sensors read rich (as they should normally) in the 47-56 second range? Could you repeat the test by feeding propane for longer (about 15 seconds)? I'd also like to see about 30 seconds' worth of data for the sensors before adding the propane. Remember to make sure that the engine is up to normal operating temperature. – Zaid Jun 13 '19 at 10:29
  • I need to try this test over again, I think it was too soon after starting the car, will report back soon – ajayel Jun 15 '19 at 07:24
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    I think the chart you removed is clearer than the current one in showing the erratic behaviour of the Bank 1 cat. It does look like the Bank 1 cat is on its way out. It looks fine for the most part at idle, but even at part loads it doesn't seem to be able to function well. – Zaid Jun 16 '19 at 20:42
  • Something to note regarding whether Bank 1 STFT is a cause or an effect, there's a bit of a chicken-and-egg situation going on there. Is the cyclical STFT causing the cyclical post-cat O2 voltage (via a bad cat that can't consume oxygen) or is causality the other way round? I can't tell just by looking at the graphs which case it is. You'll have to rule out the presence of intake/exhaust leaks (if you can rule this out it would support the latter scenario - that the O2 sensor measurement alternating between rich and lean is inducing fluctuations in STFT). – Zaid Jun 20 '19 at 08:33
  • Thanks @Zaid, I've tried two catalytic converters now and re-checked for leaks on the intake and exhaust. I'm fairly sure now that there is an issue internal to the engine causing lean spikes. – ajayel Jun 24 '19 at 22:05

1 Answers1

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It looks like there is an issue with Bank 1, not Bank 2.

Given the stellar job you've done with ruling out false readings due to sensors or leaks, it looks like the catalytic converter on Bank 1 is not doing a good job scrubbing out the oxygen, which is why the Bank 1 post-cat O2 is cycling between rich and lean.

Lambda probe test

You can see more information in my answer to this Q&A:

How can I tell if I have bad cats?

Zaid
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