The oddest surge I've seen.

[Dolzell]

I really don't like asking the Internet for automotive advice because it's like being asked over the phone or when you're nowhere near the persons car and I know all too well how that is. However, I haven't been successful in tracking down this issue so here I am.

I have a 2002 Subaru Legacy GT (non-limited) and it's developed this annoying little idle surge. Most of the time it's just annoying but there are times it feels like it will stall but that is not that frequent. Driving along seems fine with the exception under heavy load (i.e. In 5th gear at 2k or less rpm and give it moderate gas) you can feel a "hiccup."

Some details:

Plugs and wires are all new.

3 step fuel/injection service has been done.

No pending, history, or current codes.

The iac, tps, map, and other sensors seem to be functioning when monitored in the live data stream.

I can't help but think it's a fuel issue. I've filled up 3 times since it started but it still persists. I've only seen a clogged fuel filter give issues in hight rpm which is not the case.

I'm stumped.

To be clear the the surge is a drop to about 500 then bounces to 700 or so rpm and back. Sometimes it will level out. Nothing drastic but enough to feel annoying

Edited March 15, 2016 by Dolzell

[Rooster2]

Even after 3 gas fill ups, it is still possible that you have some water in your gas tank from an earlier fill. Suggest at your next fill up, that you buy 1 gallon of E-85. The ethanol in E-85 will absorb any moisture in your tank. I have done this before with good results.

 

Changing a fuel filter is a good idea. Even though my car seemed to run good, I could not believer how much better it ran with a new fuel filter. I guess even some filter blockage is not a good thing.

[Fairtax4me]

Most of the time idle issues on engines from those years are due to a dirty Idle control solenoid. If it has the cone style valve, (round can that screws to the side of the throttle body) those are known to be problematic and often simply cleaning them does not fix the problem. Those generally require replacement. 

The type that bolts to the top of the throttle body is generally more robust and cleaning usually will restore them to normal operation. You will need a new o-ring gasket when re-installing that type of IAC. 

 

This could also be due to a bad O2 sensor. 

When watching Live data at idle (engine at operating temp) your fuel trims should be around 0%. Anything more than about 5-8% consistently (+/-) indicates the ECU is adding or removing fuel in order to compensate for a perceived fuel mixture problem. A high positive fuel trim number indicates a lean mixture reading from the upstream O2 sensor. A negative number indicates the ECU is reading a rich fuel mixture. 

[Dolzell]

Rooster2, I was thinking along those lines as well.

Fairtax, I'm not proficient when it comes to fuel trims and of course when I hook up my scanner and start the car it doesn't do it (even though I just shut it off 5 minutes ago after driving two towns over and it was doing it). It was running a little rough and my

short term fuel trim was 13%

Long term fuel trim was -22%

However after I've sat and watched it the last ten minutes the car has smoothed out more and now they're

Short term .8%

Long term 0 to 1.5%

I think you may be onto something, sir.

Edited March 15, 2016 by Dolzell

[Fairtax4me]

Your idle trims look ok then. Try taking it for a drive and see what the trims change to, then once you stop see if they come back to those low numbers at idle. 

 

Look at the fuel trims while driving at a constant speed. (careful quick glance, or have someone else drive while you watch) 

 

Under heavy acceleration or going uphill you may see them jump up. When going down hill or decelerating they may go down. A steady speed gives an idea for a the baseline trim that the ECU is following while the engine is under steady load. 

[Dolzell]

Ok here's what I have (averages)

 

Idle:

st; 0 to 10%

lt; -15 to -20%

 

Load(4th gear, 20mph, 1.5rpm, full throttle):

st; 0 to 10% (sometimes negative)

lt; -8 to -22%

 

Wide open (2nd gear, 4krpm):

St; 0 to 8%

Lt; 0 to -17%

 

These are from quick glances because I had to do this alone as I'm on vacation and everyone is at work and that's the last place I want to be haha.

 

Edit: I stopped at a park to post this and when I restarted the car it was idling moderately rough so I looked and they were

St 20

Lt -26

Like I said I'm not familiar with fuel trims and such and I didn't know if this would help. I just hadn't seen them that high

Edited March 16, 2016 by Dolzell

[Caboobaroo]

I had one recently that would hunt for idle every once and awhile. Had similar irradic fuel trim readings and once it would smooth out, they were normal. Turned out to be a bad rear oxygen sensor. Also did the front one as that was my original thought.

[Fairtax4me]

Does this car have a MAF sensor?

[Caboobaroo]

Being a 2002 LGT, it's probably a MAP based system, like the 2001 Outback I had issues with.

[Dolzell]

Sorry for the late response. I had a bjj tournament this weekend and during the traveling to and from the last couple of days I haven't really noticed an issue. I wanted to be sure before I posted and sure enough today its back.

It doesn't have a MAF sensor to answer your question. And thanks everyone for the help so far.