07LegacySE

While I agree that the heat and O2 data is still suspicious, I find it strange that changing the O2 sensor made no change, so I can only suspect the wiring, pins and ECU, etc. Not the sensor itself. I was able to take a log with Torque, just not with Inj pulse width, for anyones' viewing pleasure, formatted into a CSV. RPM is high at first with the AC on (It's very hot in southeast TN at the moment). Basically everything goes haywire, not just O2 sensor readings, but this is to be expected as the engine is dying and the ECU does whatever it can to try to compensate. However, as I saw before The O2 sensor (and calculated A/F ratio) goes lean right before RPMs drop, then I'm assuming pulse width goes high to compensate, then RPM goes up eventually, then it's rich as a result, then the cycle continues. I can confirm pulse width width with FreeSSM again, just can't log with it (That I know of). Anyway, if anyone has any ideas from this data, let me know. https://drive.google.com/file/d/1ZaO46GIZBrr8L1AjnaRVLghow0x6aT0e/view?usp=sharing

I installed a Denso sensor, who's probably the OEM supplier (and also my employer, so I do trust it).

What's strange is it was doing exactly as you described, injector pulse width was going up as it was receiving strange values from the primary O2 sensor. Unfortunately I haven't found a way to record data with a fast refresh rate yet to be able to share it here.I don't see an option with FreeSSM and with the Torque app I only saw an option to export a screenshot of a graph of data but I didn't spend much time investigating that avenue. Also I would mention that the catalytic converter is dead, not sure how that could be ruining a primary O2 at a long idle though. Yes fuel pressure is another thing on my agenda to check along with general engine health with a compression test, plus I'll reconfirm with FreeSSM that the injector pulse width and primary O2 sensor readings are going crazy again, even with the new sensor. The TB was something else I was considering, that's why I initially pulled it off and cleaned it but short of going to a junkyard and pulling one I'm not sure what else I can really do (a new one is like $600).

Actually, I must not have let it idle long enough when I tested it originally, so it's not resolved with the new primary O2 sensor. Nothing has changed......

New primary O2 sensor resolved the issue, thanks to everyone for their suggestions. A big thanks to JmP6889928 for the ultimate solution!

I suspect you are correct.... Thanks to more parameters from FreeSSM I was able to see the Injector pulse width go crazy high each "cough" and go higher and higher as it progressed until it died. It also looked like it was giving crazy current values (positive and negative. therefore lean and rich). It looked to be telling the computer it was lean when it was adding pulse width. So I'll be ordering a primary O2 and hopefully that'll be the issue, I'll post an update for sure! Thank you very much for your suggestion! So this was an initial suspicion due to my strange issue with the coil last year when losing cylinders 3 and 4 on the highway at steady state cruising, that was resolved with a new NGK coil.

Quick update: New MAF had no impact. I watched the MAP sensor readings this time and noted that it lost vacuum for a snap second every time it missed but it returned to normal when the ECU tried to correct and raised the engine speed. To me I would expect the vacuum to go down if the engine rpm went down because of a miss, so that makes sense. But if it were a vacuum leak because of an intake manifold gasket leak, I would expect engine speed to increase and vacuum to decrease, but I saw the opposite. To me this seems to be more electrical still. But that's just my opinion.

Yeah it does not, but it still does have that block, and the upper and lower gaskets. I only replaced the lower gaskets (I believe the ones you are talking about) when I did the head gaskets @130K with OEM ones. It's interesting because I didn't think about a vacuum leak because it felt like a miss and the idle up was the ECU correcting and trying to keep the engine running as a result though. And the fact that that this only happens after a long hot idle. Yeah so I just replaced the TB gasket when I cleaned it last weekend, the upper and lower gaskets I can replace if the MAF doesn't fix it. I appreciate everyone's suggestions. If the MAF doesn't work, I'll swap out the upper and lower intake manifold gaskets as well and see what happens. And I'll get the compression test results too when I get the chance to do it as well.

But why only after a long hot idle? It's fine all the time otherwise and fine just off idle. Further investigation has shown me that if it starts to get crazy I can shut the engine down for 5-10 minutes and start it back up and it'll idle fine for a further 5 minutes or so before starting to rough idle and die again.

I have tried cleaning it with just a general MAF cleaner, but I will try a parts store replacement MAF sensor (I am thankful for generous parts store return policies if it does not fix it).

2007 Subaru Legacy Special Edition 5MT 220k Miles This has been going on for about 2 months now: If the vehicle sits in traffic moving slow, or a drive-thru or in a hot idle for a while (~15 minutes) it will start to develop a rough idle. It will begin to feel like it is noticing a miss and catching that and idling up, increasing fuel trims as a result. Then as it tries to get back to a steady state it will miss, RPM's will dip, idle up, fuel trim up, and the cycle continues. This cycle continues to go at an increasing rate until eventually the computer can't keep up and the engine dies. If you restart it it can't idle on it's own and you have to keep it >1500 rpm to keep it running. Some investigation I've done while hooked up watching live data just shows the fuel trims going crazy showing the computer trying to keep the engine going. (No codes btw, just get an EVAP code often after I replaced the gas tank and filler pipe a few years ago, I know where the evap leak is from a smoke test, just too lazy to pull it all out and replace it again, thanks Rock Auto). I have also replaced the coil (Was losing Cylinders 3 and 4 randomly and getting misfire codes for them) for a few months before this issue and that resolved that issue. I also removed and cleaned the throttle body. Plenty of plugs, wires, etc. This weekend will probably be another compression test. Previously it had around 130 psi of compression on my cheap harbor freight compression tester with a weak battery and not all the spark plugs removed at the same time so probably not a high rpm either. The only thing I have noticed watching the data is that it seems to be about when the intake air temps reach about 140-150°F the issues start to happen. STFT and LTFT otherwise aren't too crazy beforehand, everything seems normal and no codes. If anyone has any ideas of anything to check, they would be greatly appreciated!