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Old 03-04-2007, 12:33 AM   #1
mccorry
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Join Date: Jul 2004
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2005 Legacy GT Lmtd.
Regal Blue Pearl

Default Off Idle (1500 RPM and up) RPM surging / hunting

Hey everyone..... I've done some searching but have only found idle RPM hunting issues....

First some specifics....

05 LGT 2.5L 5MT
Stock turbo (VF40)
Perrin TMIC
AVO Wastegate actuator
Helix catless UP
Cobb Catted DP
Cobb Stage 2 91 Oct. 1.16 map

Two days ago... I started noticing that my LGT started to surge or "cycle" during slight engine braking. By "cycle", I mean that I could feel the car slow down briefly.... then return to normal....repeatedly... about once per second. It was like the engine braking was inconsistant. I even get a little gurgle out the tailpipe that matches the frequency of the surging.

This happens when in 2nd or 3rd while rolling at a constant speed. It's like the rolling resistance repeatedly increases and then returns to normal. The RPM's don't change and the engine vacuum doesn't change. I can just feel the car slow down a titch... like the brakes are getting slightly applied at a frequency of every second or so. On... off.... on.... off.... on .... off. But it isn't the brakes.... it's the drag off of the motor. I can't see anything wrong in the gauges... but I feel it in the seat.....

At idle its absolutely fine. Under acceleration and highway cruising, its fine.
If I lift all the way off the accelerator... it goes away.

I messed with it some today and found out that I can get it to "hunt" while parked in neutral. Car started... clutch in (or out... doesn't matter).... tranny in Neutral. The car idles at a solid 600 RPM.

If I give it a little gas... it will idle solid at 1000 RPM.
A llittle more gas and it will run at 1500 RPM for a few seconds... then 1300 RPM.... 1500 RPM... 1300 RPM.... etc.
Same thing happens at 2000 RPM, 2500 RPM, etc....

This "hunting" happens at any RPM over 1500 and the cycling is always around 300-500 RPM. Really weird.

Anyone have any ideas on what to check? My air filter is VERY dirty, so I am going to change it in the morning. All of the hose clamps on the intake tubing appear to be tight. Still haven't recieved any CEL's at all. I am a little confused right now.....

At first I thought it was drivertrain issues, but I ruled them out. Then I thought that it acted like I might have a bad spot on the throttle position sender or something..... but the hunting at various RPM's kinda disprove that.
If the air cleaner doesn't help, I plan on pulling some data longs to see if the throttle plate is moving.....

Has anyone else experienced this?
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Last edited by mccorry; 03-04-2007 at 12:46 AM.
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Old 03-04-2007, 02:04 AM   #2
Mezmerik
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Default

Interesting... my car does the same thing but I have an 07 WRX completely stock. It's done it since I drove it off the lot new, so I just figured it was supposed to do it; maybe not? My old 86 mazda never did anything like that. Same stuff, I engine brake then roll up to a stoplight at a constant speed in 2nd at about 1000 rpm and like you said, I feel the engine resistance, and a second later, it feels completely neutral even though speed/road slope/throttle position are still constant, then I feel resistance again, repeat. It also does this if I'm driving really slow in traffic at a constant speed on flat ground. Also, when I start it and let it warm up a bit, after about 30 seconds, the rpms start to drop, but they jump around a bit, 1300 then 1700, then 1400, etc then level off.
I like to pretend my car has a personality, so I imagine it's like "oh you want to idle in gear do you? well then you're gonna need a little more gas so I don't stall... so here you go... oh wait, you don't want it... or do you?". Yeah, I'm a dork, I know.
But seriously, I read some time ago that this activity was related to emissions controls... but I don't know if that was legit or just a junk thread.
Bottom line is that I've experienced similar stuff with a stock setup on my 07 but don't actually know what's up.
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Old 03-04-2007, 10:07 AM   #3
mccorry
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Regal Blue Pearl

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Are you going to take it in for the dealer to look at it? Mine is at 50K, so I'm out of warranty.
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Old 03-04-2007, 12:47 PM   #4
DiscoGsus
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You aren't out of warranty at 60K. Just bumper to bumper. Powertrain is 60K. Though your mods make that harder to argue.

The jumping around off idle, under no power, like if I coast with the car still cold, the revs will climb a little. Those are all emissions related cold start things in the ECU. Its totally normal. (Well, at least in an impreza.) My sisters 06 RS, and my 06 STi both do it. I haven't heard about it in a legacy but it wouldn't surprise me.
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Old 03-04-2007, 01:36 PM   #5
mccorry
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Well... I changed the air filter and it the car runs better... but it didn't fix the surging issue. This isn't minor.... it's fairly pronounced....

I don't really want to deal with the dealership since mine really SUCKS and I don't want the hassle of arguing with them.

Here is a datalog I just pulled....

Starts at idle... nice and steady....
I bring it up to ~1700 RPM and you can see the A/F sensor, A/F correction, and RPM bounce. The RPM bounces between 1700 and 1500 RPM about once per second...
After a while I rev it up slowly to 2800 and then settle back down around 2200 RPM.
It starts to bounce (hunt) there again for a while. Between 2250 RPM and 2000 RPM.
Then I let it come back to idle.

http://legacygt.com/forums/attachmen...2&d=1173032585


My gut says front O2 sensor?!?!?!

It looks to me like the A/F ratio is bouning all over the place...a nd the car is chasing it with Injector duty cycle. But I'm a newb to this stuff...
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Old 03-06-2007, 08:28 AM   #6
wrxxed
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try cleaning(carefully)the maf sensor
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Old 03-06-2007, 09:53 AM   #7
jradams38
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Interesting. I'm seeing the same symptoms on my 20g'd '04 STi. It seems to have gradually become worse with the exception of a recent tune where the MAF scaling had to "consume an unusually large amount of ECU space (I have to use a real-time map at all times)".

What you and I seem to have in common is an extremely dirty air filter and possibly a gunked MAF. I have the Perrin big MAF intake with the wet filter. Perhaps that catalysed the buildup of grime on the MAF.

Tell you what: I'll clean my MAF and my filter, see what happens to the problem, and report back. Anyone have any quick links on how to clean the MAF?

J
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Old 03-06-2007, 05:02 PM   #8
TurboRex
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I was getting ready to post a similar problem and found this thread. Like you say it is off throttle and trying to hold a steady rpm whether driving down the highway or with the car in neutral and rpms are above 1500 or so.

My problem started with a p31 code which I thought denoted a bad front o2 sensor. I replaced it and the cel went away but now I have the surging when it did not do it before. I tried resetting the ecu and it is not as bad but feel it will be after the ecu learns some more. The problem is not consistent, ie some days it seems worse than others.

By the way, I have an 04sti with an fpred, ecutek, utec, big maf, etc. My filter is also really dirty. What are ya'll cleaning these filters with....soap and water? I don't have the oil to put back on the filter though.

Greg
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Old 03-06-2007, 06:43 PM   #9
bboy
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Anybody have a boost gauge that is having the surging. What's it read during he surge?
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Old 03-06-2007, 09:22 PM   #10
modaddict
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Quote:
Originally Posted by jradams38 View Post

Tell you what: I'll clean my MAF and my filter, see what happens to the problem, and report back. Anyone have any quick links on how to clean the MAF?

J
I think I wrote a thread in this forum "how often do you clean your maf" or something like that.

look at it, it has pics.
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Old 05-18-2008, 10:08 PM   #11
mccorry
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Just a follow up... forgot to cem back in there to report back.

Turned out to be a fouled front O2 sensor. Swapped out the sensor and the problem went away.
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Old 05-18-2008, 10:21 PM   #12
k mier
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Just found this post. The front o2 was going to be my guess. Had one fail on me w/less than 1500 miles. Under light loads it was very noticeable, not so much under moderate load. Logs would show the a/f's bouncing from full lean to full rich. Cheapest I found one for was, IIRC, $260.
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