99 Legacy 2.2 (phase II). I have noticed a slight stumble at idle when I have come to stops over the last several months. It would happen every 10 seconds when idling for extended periods (stop lights, etc). Seemed like more of a hiccup if anything. Has been running great besides that with no codes being thrown. 

Today I changed my oil and was letting it idle for an extended period and noticed that the idle was much rougher with the stumble much more frequent. Enough to throw misfire codes on all cylinders. 

Parts that are newer or I was able to swap for spares: plugs, wires, coil, O2 sensor, MAF sensor, knock sensor, injectors, fuel filter, and fuel pump. Tank is only a year or so old, was recently dropped and looked spotless. 

I had somewhat recent cleaned throttle body and IAC valve, but thought I would check they were still spotless. Smoked checked intake no apparent issues.

Looking for advice or ideas on where to possibly look next. Thanks in advance. 

-Will

Pull the plugs and see what they look like. NGK copper core plugs and NGK plug wires.

 

100% what was said above....

these cars are very fussy about plugs and wires.
NGK basic copper core plugs at the proper gap are all it needs, anything else is a waste of your money.
Wires should be either OEM or NGK only... again, anything else is a waste of your money and will fail prematurely.

replacing stuff randomly is bad practice.. you need to figure out what the problem actually is before throwing parts at it.

8 hours ago, heartless said:

.. you need to figure out what the problem actually is before throwing parts at it.

sometimes easier said than done  

if throwing bits at it, at least try one component at a time,test, next, so you know what it was

14 hours ago, Steptoe's photos said:

sometimes easier said than done  

yeah, sometimes it is a challenge, but if you just throw parts at it without actually testing those parts before replacing, that is not ideal
and yes, one thing at a time for sure.

'99 2.2 is not all that complicated, really.. and there are usually only a couple of things that would cause a misfire.

coils on these cars dont fail all that often, and when they do, it is usually pretty obvious. Also quite typical to be worse when the weather is damp... I have a very short video clip showing what a bad coil does here:

 bad wires (typically aftermarket) will do something similar, but sometimes harder to see, so checking things out after dark can often help. If it is bad enough, it can often be seen in full daylight, like the bad coil above.. but sometimes it is not that bad, or in a difficult to see spot.

I have new ngk plugs and wires on it. The old plugs didn’t look bad. Had slight bit of oil on the threads. I replaced the valve cover gaskets and spark plug seals as I head been meaning to anyway.
 

I already had a spare coil that I was able to check with. Checked the fuel pressure and everything seems to be in spec.

Going to have access to a diagnostic tool that can monitor live data. Any input on some things I should check?

Thinking i might be running lean. O2 sensors at 0 volts at idle. Seem to operate through rest of throttle range as they should up to 0.9 volts. I checked fuel pressure and believe i am within spec based on what is in my factory service manual. I am at 31-32 psi with vacuum 41-42 psi without.

refresh all ground connections.

I have read, once or twice, of iron debris collecting on the face of the crank angle sensor causing misfires....

On 5/6/2023 at 6:44 PM, Willrb said:

Enough to throw misfire codes on all cylinders. 

Parts that are newer or I was able to swap for spares: plugs, wires, coil, O2 sensor, MAF sensor, knock sensor, injectors, fuel filter, and fuel pump. Tank is only a year or so old, was recently dropped and looked spotless. 

I had somewhat recent cleaned throttle body and IAC valve, but thought I would check they were still spotless. Smoked checked intake no apparent issues.
 

Check the timing belt and tensioner.

Have you tried spraying starter fluid into the intake when it's stumbling and see if that smooths it out? 

What happens if you prop the idle open by hand or drive with partial throttle like 10 mph - does it stumble then?  Or just at idle? 

If it's happening just at idle I'd swap in another idle control valve. 

Is the fuel pump aftermarket?  Pressure looks good but I don't trust aftermarket subaru pumps/alternators/starters - they're routinely garbage and fail or work poorly.