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Help diagnosing erratic idle


keeweechris
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6 hours ago, Loren said:

 

Yes you can check just by pulling the side covers back a bit... you just need to be able to see all the alignment marks.

Off by one tooth doesn't mean it will run bad through the whole range... but could make the idle a bit s***.

 

 

You really need a proper vacuum gauge... not a boost gauge. 20 inHg is a healthy engine... less might be badly sealing valves... or cam belt off a bit. Do the compression test too.

 

 

Cambelt all good still... Well, all cams line up. Can't see crank, but seems unlikely that it skipped a tooth, being wrapped in cambelt 270degs.

 

Will focus now on coils, plugs, injectors, comp/leakdown, then fuel pump.

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6 hours ago, keeweechris said:

 

I haven't tried starting it without MAF though, but was curious how to force it into Open Loop. Are there map values for lower revs? I'll try this anyway, because if the bad idling (misfiring) sticks around, I guess it proves it's mechanical, fuel, or sparks, not sensor-related..?

Yea no harm in trying, it only takes two seconds to unplug it and fire it up. Its possible that there could be a problem with the wiring/grounding to said sensors too, just swapping sensors out sometimes neglects the wires that go to the sensors. 

The issue you have described really does sound like an intake leak even though you have ruled that out.. 

 

Do you have an aftermarket blow-off valve at all? 

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14 hours ago, A_J_T said:

Yea no harm in trying, it only takes two seconds to unplug it and fire it up. Its possible that there could be a problem with the wiring/grounding to said sensors too, just swapping sensors out sometimes neglects the wires that go to the sensors. 

The issue you have described really does sound like an intake leak even though you have ruled that out.. 

 

Do you have an aftermarket blow-off valve at all? 

Nah, stock BOV.

 

Yeah, that's why I logged data from the ECU. I seemed to be getting realistic data from the MAF sensor, but keen to hear your opinion.

 

Here is MAF data on cold idle:

image.png.1ae9caa2ed1b133ccf9ac5ce8dfa140d.png

 

And here is the O2 sensor & RPM. It's all over the place, and shows the idle speed flapping around too.

image.thumb.png.2687d74bf435c246475cc10c982c0c55.png

 

I logged heaps of data, but it's hard to show all on one graph. Let me know if you want to see any other data.

 

Cheers

Chris

image.png

Edited by keeweechris
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13 hours ago, A_J_T said:

Got a photo of your engine bay? 

The odd miss on idle is not uncommon on these. The stumble is the interesting thing 

It could be detecting knock on tip in. can you log knock and see if you got any feedback?

Just a regular stock GTB twin turbo engine bay... stock BOV, stock airbox (K&N panel), everything clean a tidy, replaced intake pipe 3 months ago, and all vacuum lines. Was running very nicely after that.... smooth and 11/15psi boost.

 

Yeah, all subaru's I've owned have had that slightly irregular idle, where it shakes ever so slightly every so often. But this is not that. It shakes constantly, changes exhaust tone, and misfires/stumbles on partial throttle anywhere between idle and 0psi. Then under boost it takes off and has no noticeable problems.

 

I'm wondering now if it has some headgasket or valve issue. Leakdown will reveal that I'm hoping.

 

Doesn't seem to be knocking. Data for "Knock Correction Advance (degrees)" and "Knocking Signal #1 (On/Off)" was all zero's for the time I logged it on cold idle the other day.

 

Gotta find time to swap out the coils and plugs and injectors systematically, to rule them out. And fuel pump. Hoping it's one of those!

Edited by keeweechris
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On 6/11/2020 at 6:34 PM, Bugle said:

Alternator? Alternator/battery combo?

 

I would have looked at plugs/coils before doing some of the mahi you've done, but at least when you sort it you know there is nothing else wrong 

 

Keep at it, you'll get there!

 

 

 

 

Thanks man, yeah, it seems like a bit of mahi, but hopefully I crack it.

 

Got cylinder 2&3 done (compression 150psi, no change when swapping out coils and plugs), then got bored and tried the fuel pump, haha.

 

Will get onto 1&4 tonight.

 

Tried removing alt belt, but it still ran bad without the alt. Thanks for the idea though.

 

Thinking about it this morn, it seems like something not right with closed loop system. It gets going well on boost and above 2000rpm. So I've got to understand whats involved in closed loop running... O2 + TPS + MAF I guess. But I've swapped all those... scratching head a bit.

Edited by keeweechris
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4 hours ago, keeweechris said:

Thanks man, yeah, it seems like a bit of mahi, but hopefully I crack it.

 

Got cylinder 2&3 done (compression 150psi, no change when swapping out coils and plugs), then got bored and tried the fuel pump, haha.

 

Will get onto 1&4 tonight.

 

Tried removing alt belt, but it still ran bad without the alt. Thanks for the idea though.

 

Thinking about it this morn, it seems like something not right with closed loop system. It gets going well on boost and above 2000rpm. So I've got to understand whats involved in closed loop running... O2 + TPS + MAF I guess. But I've swapped all those... scratching head a bit.

 

How does it run with no 02 sensor?

 

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This is promising... last fkn cylinder (#4), swapped coil and plug, idling mostly normal... no shudders and dropping rpm. Will take for a test drive tomorrow. All cylinders 150psi compression, which is good.

 

Is there any harm not having the fuel filter core inserted for a short blat?

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59 minutes ago, keeweechris said:

Is there any harm not having the fuel filter core inserted for a short blat?

 

I wouldn't do it personally... the sock on the pump is all you have between the crap floating around in your tank... and the injectors.

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