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My AT 2007 OBXT has been having a strange thing happen lately...


when I am coming off the gas (completely) the engine has a slight rough idle for like 1 second.


I have replaced the plug and packs and had a valve job done 4k ago.\


I am running stage 1 Cobb with no other mods.


Any ideas?


Thanks!
 

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2005 OBXT Ltd Obsidian Black 5MT
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1,079 Posts
Check for air leaks. Both intake and exhaust.
 

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2005 OBXT Ltd Obsidian Black 5MT
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1,079 Posts
my mechanic did a leak down and said it was solid. :|
Sorry, not a leak down test. That's internal to the cylinders. What I'm referring to is the air path from the airbox where the air filter is, the inlet tube through the turbo, intercooler and then in the actual intake manifold.


What the testing of the intake side usually entails is hooking up an air compressor to a gauge and valve and putting air pressure through the entire inake, listening for air leaks.

I made this tester for myself, but you can buy or cobble together one.




Even after replacing the entire engine, I found tiny leaks and problems with the bits that I reused or broke.

Torn TMIC to intake hose:



Ripped blowoff tube:



Crushed PCV crossover pipe:



But after lots of digging, I've been able to pressureize the intake size and entire engine to 20psi:



Now I've got a leak on my exhaust side somewhere that's causing it to pick up extra air and messing with the AFR sensor and it's making it do funny things when it's cold.

Check for bad vacuum hoses too. I've got good money that the rubber hoses are all bad that are more than 6 or 7 years old on there.
 

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2009 Subaru Outback 2.5i
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276 Posts
Check for air leaks. Both intake and exhaust.
Great advice and great execution. Another option would be to use a smoke machine. Uses mildly pressurized air and a smoke source to identify leak sources. Aside from just the sound of a leak, you also get visual confirmation as you can see the smoke leaking out. The commercial/professional smoke machines are prohibitively expensive ($800-$1K), but for moderate home use there are some cheap DIY options. Since I have an air compressor, I rigged a DIY smoke machine out of about $15 worth of materials from Harbor Freight and Home Depot using incense sticks as a smoke source. Works great. Used it to find a TINY fuel tank leak on a customer's Forester. Would just as well work for intake leaks.
 

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2005 Legacy GT wagon 5MT Limited
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320 Posts
When you say Stage 1 Cobb, what does that include (as "stage 1" is by no means a 'standard')? Intake? Catback? Tune?
Cobb Stage 1 is a very specific standard. It means a Cobb Stage 1 OTS tune with no significant hardware mods. Add a Cobb intake, you're at Stage 1+. And of course the catback is irrelevant.
 

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2005 OBXT Limited, VF37, STI intake, 5MT
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Cobb Stage 1 is a very specific standard. It means a Cobb Stage 1 OTS tune with no significant hardware mods. Add a Cobb intake, you're at Stage 1+. And of course the catback is irrelevant.
Yes, Cobb does package and sell a "Stage 1" kit, but that changes from model-to-model. ****, Cobb doesn't sell a Stage 1 Power for the 2007 OBXT. An LGT map will probably work, but it's also an OTS map. So saying that car has "Cobb Stage 1" is a crap shoot.

My point was that some people call an intake/exhaust/OTS tune "Stage 1", others say intake and tune, some same just a tune, etc.

Assuming the OP has an AccessPort, what does the LTFT, STFT, and IAM look like as of recently?
 

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Master Caster
2005 XT, Mildly Modified...2006 XT Limited, Highly Modifed
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16,120 Posts
Assuming the OP has an AccessPort, what does the LTFT, STFT, and IAM look like as of recently?
Yeah, would like to get some data....

My AT 2007 OBXT has been having a strange thing happen lately...


when I am coming off the gas (completely) the engine has a slight rough idle for like 1 second.
Does this happen when coming off the gas? Or when you are approaching a stop. What are you noticing...is it a low RPM drop or are you seeing a blip or misfire?
 
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