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2008 SpecB - Hunting a hesitation problem

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Hey guys,


Here I am asking for some pro insight.


The car is a 2008 Spec-B with 180000km, dont know much about its past. The main issue i'm hunting is a clear hesitation at WOT, between 3000 to 5000 rpm (like when the turbo kicks in). It happen almost every time I push it, but it is quite unnoticeable in mode. Another clue is, after I fill gas, the car is hard to start. This could be related or not (but it is not my priority right now). Other than that, everything runs fine, no strange sound or anything...



What has been done already (Not all related):

Timing Belt

Both OCVS (Had a P0021, changed both sensors)

New banjo bolt with filter on turbo side

New Battery

New grounds (Battery and engine)

New Spark Plugs



I have 2 suspects right now : O2 sensor or Vaccum/Boost Leak.



I've read that a vaccum leak can be easily found by looking at the A/F Learning #1 at idle. In my case, this value goes up at 21.88% as soon as the car finish heating up and idle go down. So, is it right, I have a vaccum leak? My idle is'nt that bad and I quickly checked all the vaccum line I can find and no luck yet.... It does go down when I give throttle.


Then, if I have a vaccum leak, I most likely have boost leak as well, but can this be enough to make those hard hesitation at wot?


O2 sensor

I've been told this is a common issue on the car (also had to replace it on my previous WRX). I wanted to clean it in case it change something but it is really stuck there, even with the right tools I'm about to damage the bolt so I will wait for a "real" mechanic... But, is there any log I can make that could help prove if my O2 sensor is faulty or not?




If you have any other suggestions, you have all my attention!

Thanks folks!



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Yes its in Open Loop.

The Spark plugs are new and I checked the gap before installing them (.030)

Cleaned the MAF with brake cleaner. Did not rub the filaments, did not feel safe about it..


Coils packs... they looked good... Is there anyway I can rule them out?

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Special Ben,


I've experienced an unusual hesitation under WOT conditions before. It came about soon after changing spark plugs. The cause of the hesitation was that a coil pack was not properly seated and had poor contact with the spark plug.


I would check to ensure all the coil packs are properly seated onto the spark plugs.

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Thanks for the tips Peter. But since I have this issue, spark plug has been removed/installed twice without any change (I first did a re-gap before changing them). I did look inside the coil pack and it seemed good.


Is there a way to log that kind of miss fire in RomRaider?

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I would try a boost leak. I was chasing a stumble at idle/low rpms and found a small tear in the hose that goes from the manifold to fuel pressure regulator. I replaced it yesterday and so far so good.


Courtesy Mike from Tuning Alliance for instructions on how to do an easy boost test.


1) unhook air inlet from air box

2) plug it with something. I used an unused oil filter from my Mazda Mx-5. wedged it in there and tightened the worm clamp. Perfect

3) Pull vacuum line off of your by pass valve and put air through it. Mike recommended just blowing in it, but that didn't produce enough psi to find the leak. I ended using my air compressor, with just a few quick shots of air

4) listen and see if you can hear air escaping. If you do, you can spray very soapy water in the general area and the leak should be obvious


Of course it could be other things as well but a a boost leak test is a good place to start. For comparison my A/F learning was around 5.5% prior to this. 0-.78 now.

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Sure, watch your fuel trims, A/F learning, etc. If there are big swings, or a major correction factor (I have heard that more than 7% is a flag), you likely have a leak. But you would need to start with the knowledge that these measurements were correct. A basic service, along with MAF and oxygen sensors would be my first moves (after having visually inspected everything).


Maybe take an hour or two to wipe down your motor, paying special attention to the intake tract. If you get these surfaces clean, especially where each part joins/seals, you'll see any new blow-by accumulating.

LW's spec. B / YT / IG
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I think brake cleaner and the MAF are not a good mix.


I use a can of MAF cleaner from the auto parts store.


The wires on the MAF are the IAT the MAF is the Tube

305,600miles 5/2012 ej257 short block, 8/2011 installed VF52 turbo, @20.8psi, 280whp, 300ftlbs. (SOLD).  CHECK your oil, these cars use it.


Engine Build - Click Here

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