Tachometer fluctuates after startup

Normally when I start the car the engine would maintain at a steady RPM first. However since yesterday morning I noticed the tach would go down a bit then up then down a couple of times after startup, does this indicate some part about the fail soon?

 

I took a video (

), it's less noticeable than in the morning, however the dip at 0:07 bothers me, wonder if it's normal. Edited June 13, 2021 by seiyafan

If you start the car, let it run for 5-10s, then turn it off and start it back up again does it still do it? Seems solid while it's running - are any accessories on? AC, maybe?

Maybe the throttle body is due for a cleaning...?

 

My RPM has a dip sometimes when it drops to idle. I have a throttle body gasket on the way and it will be pulling the throttle body for a good ole cleaning this upcoming week.

The RPM bounced more in the morning, I took the video above after I drove home today.

Here's the startup this morning, I drove about 10 minutes afterwards and the engine light came on.

 

I'll second the throttle body cleaning - for the CEL, did you pull the code?

P0171

The shop cleared the code for me, but the traction light is still on and the break light is still flashing. On the way home there is a bit of noise when I give it more gas.

Bad O2 sensor?

Edited June 14, 2021 by seiyafan

Service manual says first check for leaks in the exhaust, then intake, then fuel pressure, then the coolant temperature sensor, then the MAF sensor for mass air flow and air temperature

The car makes a buzzing noise when gas pedal is pressed a bit more, this happens after driving for about 5 minutes, the noise becomes louder when the gas is pressed down more. Poured in a bottle of Techron on Monday but didn't feel any improvement.

If you can get under the car with the engine running, you might be able to track down an exhaust leak (if there is one). If this only happens when the car is up to temp, you might have to time it right so the fans aren't on (or maybe just unplug the fans and be quick), but if you wave a lit candle near anything that has a gasket, you should see it flicker if there's a leak. Not a 3.6R owner myself, but from the looks of it, I think you'd only care about the manifold gaskets right at the heads unless there's a hole or crack in the pipe upstream of the cats, since everything else would be downstream of the front O2 sensor.

 

Otherwise, maybe spray some ether near anything in the intake tract downstream of the MAF sensor that has a gasket, and see if you have a vacuum leak. The hard part there is that there are a lot of vacuum lines that end up connecting to the intake post-MAF.

Thanks for the suggestion, I will try to let a shop figure it out, not very handy for this type of task, lol.

I tried to find the part number for O2 sensor and the Subaru website lists two 22641AA53A and 22641AA68A, which one should I order?

That buzz you mentioned sounds an awful lot like a leak to me, so I'd try to chase that down before you go spending money on parts you may or may not need.

I second the possibility of a leak given the behavior of the rpm gauge. Whether it's a leak in an air line or exhaust.

Poured in a bottle of Techron on Monday but didn't feel any improvement.

Injector cleaner (if you have issues with fouling on injectors or valves) takes time - if you see any effect (I wouldn't expect it to impact your current issue based on the code) it's going to be after you run a fair portion of a treated tank of fuel through the engine.

MAF sensor was the culprit. The shop didn't find any leaks but pulled MAF from another Subaru and tried on mine, the code went away.

 

Edit: I replaced the sensor with a used one order on Ebay, the CEL went away on its own after two days.

Edited June 26, 2021 by seiyafan

Did the rpm drop go away, too? You should try cleaning your old one and see if it functions properly after that (always good to have a spare!)

Did the rpm drop go away, too? You should try cleaning your old one and see if it functions properly after that (always good to have a spare!)

 

Yes it went away too. In fact a couple of months before this issue surfaced I noticed that every morning when I shifted from P to R the rpm would drop and engine sounded like it's about to die for a second or two before returning to normal, this symptom has also been fixed since I swapped the MAF sensor.

 

I did a visual inspection of the old one but didn't see any dirt or clogged area, but I am no technician.

Edited June 29, 2021 by seiyafan