Rough Idle. Catalytic Converter?

My car just came back from the mechanic and had a tonne of work done, including PCV, bunch of hoses and valve job. There has been a persistent rough idle that remains.

 

The cylinders all look good, there are no boost or vacuum leaks. Car runs perfectly when driving under load.

 

Logged the rear O2 sensor andvoltage seems to go to 0 when engine under no load.

 

Could this be a partially clogged cat or could it be something else? No codes either.

Can you post LV? If not, what are your LTFTs in the A/B/C ranges? Any misfires/roughness at idle, 2500, 4000 RPM?

You can quickly check for a clogged cat with a vacuum gauge.

I will do that tonight. Will also rom raider it. Anything besides speed, load, rpm and o2 sensors that might be useful?

Rough idle, is many times a leaking vacuum hose. Try double checking all the connections, it can be a bit of a pain.

Just had that done as well as smoke test. All good.

See attached learning view and rom raider log.

romraiderlog_20180528_224144.csv

Was the battery disconnected in the workshop? In that case the engine may have to re-learn the idle mixture.

 

 

No codes is generally a good sign, but look for pending codes as well. A clogged cat usually don't cause idle problems.

The LV and logs look great, nothing in there to suggest any performance problems. If the MAF is clean, and it and the upper O2 sensor are both known good, I wouldn't have anything to suggest other than take it to a Subaru specialist. If idle is bad and higher revs are good, a clogged cat is not a suspect. I am a little curious about the valve job. What exactly was done to the valves, and why?

Oh hey, can you log AVCS angles and target angle?

Had the same issue with mine for a while. Car ran perfect when under load, but at idle it was a persistent rough idle. No vacuum or boost leaks. Checked and cleaned MAF. Thought it was possibly spark plugs, changed those but no luck. Then, out of curiosity I figured I'd replace the fuel injectors with new ones and instantly my rough idle was gone! You may have a clogged injector, it's worth a try before taking it to a shop for diagnosis.

Was the battery disconnected in the workshop? In that case the engine may have to re-learn the idle mixture.

 

 

No codes is generally a good sign, but look for pending codes as well. A clogged cat usually don't cause idle problems.

 

I discovered that this weekend, Had my battery disconnected for about 20 hours. Turned it on and it had a rough idle, luckily it seem to correct itself after a little while.

Had the same issue with mine for a while. Car ran perfect when under load, but at idle it was a persistent rough idle. No vacuum or boost leaks. Checked and cleaned MAF. Thought it was possibly spark plugs, changed those but no luck. Then, out of curiosity I figured I'd replace the fuel injectors with new ones and instantly my rough idle was gone! You may have a clogged injector, it's worth a try before taking it to a shop for diagnosis.

Did you log roughness counts before replacing those injectors? The situation you describe would almost certainly have shown roughness counts bordering on a misfire code, and OP has zero roughness.

Did you log roughness counts before replacing those injectors? The situation you describe would almost certainly have shown roughness counts bordering on a misfire code, and OP has zero roughness.

No, unfortunately I did not log roughness counts at the time, although I should have.

Check your grounds. There's one from the headers to the frame, and another from the head to the frame. Headers to the frame is on passenger side, head to frame is driver's side. Bother are underneath the car. A severed ground can cause all kinds of nonsense.

Good call Dish! I knew I was overlooking something essential.

Is there a drawing or picture showing the location of the grounds? I assume I just need to check for resistance of zero?

I discovered that this weekend, Had my battery disconnected for about 20 hours. Turned it on and it had a rough idle, luckily it seem to correct itself after a little while.

 

 

That's the normal behavior - it just has to re-learn the correct mixture.

Here is a log with vvt angles. I couldn't find the avcs so I assumed vvt are the correct values. Also, could not find a target angle field.

romraiderlog_20180529_230210.csv

That's the normal behavior - it just has to re-learn the correct mixture.

 

Well I assumed that was what was going on after a little bit of panicking wondering what I did wrong.

Is there a drawing or picture showing the location of the grounds? I assume I just need to check for resistance of zero?

 

Once you get under the car and remove the engine splash guard, the ground straps should be obvious, going from each cylinder head to the frame rails. Just a visual inspection should be sufficient.

 

https://www.google.com/search?hl=en&authuser=0&tbm=isch&source=hp&biw=1218&bih=677&ei=PQoPW5C7NIqXjwSaz73ADg&q=legacy+ground+straps&oq=legacy+ground+straps&gs_l=img.3...1391.5628.0.5836.24.17.0.7.0.0.144.1293.13j2.15.0....0...1ac.1.64.img..2.14.1220.0..0j35i39k1j0i5i30k1j0i8i30k1j0i24k1.0.UKZ8ql-RGGU

Here is a log with vvt angles. I couldn't find the avcs so I assumed vvt are the correct values. Also, could not find a target angle field.

 

Correct, AVCS is Subaru's name for their VVT mechanism. Everybody has to have their cutesy name for it - Honda VTEC, BMW VANOS, etc.

Since the cams are always within 1 degree of each other, I think we can safely assume they're following the target angle within the acceptable range.

Is it normal for A/F Sensor #1 (AFR) to be going down to 11 and up past 20? This is the air fuel mixer (O2 sensor 1) is it not?

Yes. Rich (11) at WOT, lean (20) under compression braking.