Forum » General Tuning Discussion » Ej257 Stumble issue (Not the common one)

Ej257 Stumble issue (Not the common one)

General Tuning Discussion

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EJ257... Cobb Intake - Parrin Inlet hose - TGV Deletes - IAG Rails - Parallel lines with an Aeromotive AFPR - ID 1050X injectors. Bigger turbo, forged block and so on.

Having a bit of a hard time nailing down an stumble issue. It occurs with light tip-in between 2000 and 3000RPM. Additionally while lightly accelerating between the aforementioned range. Also, when cursing if there is a light downward grade its quite sluggish when the load drops due to the grade change.

I wanted to make sure my injectors were scaled correctly so I install the factory air box and ran on the factory MAF calibration. The scaling and latency did need a bit of work, but everything is really nice as far as that goes now. Tip-In also seems to be quite happy. The GR STi's had a stumble issue from fuel reversion, but this really isn't the same, there is no notable STFT that accompanies the stumble. Thoughts?


Hi Jordie. You mentioned GR STI, but it was t clear if that’s what you have. if it is, the length, construction of your fuel lines are critical factors as are a few other things.

For now regarding tip in, people often forget to adjust the minimum ipw limit and that can prevent any tip in enrichment from occurring at low deltas if you’ve compensated for increased injector flow vs. stock.

if you’d like to post a log and cal I can take a peak.

Hi Mike,

Thanks so much for the reply. The vehicle in an 09 STi. The lines are (IAG Kit) -6 and are as equal as I could get them on both the supply and return. I'm running 45PSI of base pressure with the manifold reference "T"ed off the Recirc valve . My Tip-In Activation (Min P/W) is currently set to .108.

I literally just reinstalled my Cobb SF intake after having the factory air box in for checking the injector scaling and latency. Still need to flash the MAF calibration and touch it up if necessary. I'll get a log ASAP.

Thanks again!

Sorry for the late reply, I was recovering from a blown back... again. I've attached a log of the AVCS system. One thing I noticed is that the left hand exhaust cam AVCS seems to be having trouble controlling itself. As you can see, its all over the place, reacting slow, then over phasing. Perhaps malfunctioning OCV? I had an issues with the left intake cam a few months ago, it was stuck at 50 degrees, I pulled the OCV, tested and cleaned it which resolved the issues. I guess I'll do the same with the left exhaust OCV. Any other thought would be appreciated.

Attached Files

Since the log is only a few seconds long and I coolant temp wasn't included, I couldn't make a comment on whether the AVCS behavior is concerning or not. Until the engine warms up you'll often find cams aren't tracking in parallel on cars in good working order, but if this is occurring when the engine is warmed up, yes that is very concerning and could certainly cause stumbling among other things. This is one of the reasons I stress to people not to drive these cars aggressively until they're fully warmed up. There's also a period of time after each restart for the cam phasing system to come online and sync everything up.

The stumbling in this case could be due to the two halves of the engine operating with significantly different VE based on cam phasing differences, causing one side to run leaner, the other richer, with the average of the 4 seeming in the ballpark of normal to the oxygen sensor. Oil and filter change, removing the AVCS filters, cleaning the solenoids is a good next step, but cleaning rarely resolves it even if you have access to an ultrasonic cleaner. The solenoids usually end up needing replaced. Hopefully you'll get lucky. If this is an actual AVCS issue and those procedures don't resolve it, after replacing solenoids, gears come next, and last cams (unless there's an electrical issue). I would check the ground straps near the exhaust solenoids as well.

In case this car has ever had engine work, as a rule I always replaced the solenoids and remove the filters during engine repairs/upgrades because this is so likely to occur. Gears get replaced automatically if the turbo or an engine bearing had an issue.

Best of luck with the car!


Thanks for the detailed reply. Sorry for the log length, it was the best one I had from that day to show the general trend of the issue, it's intermittent and can have varying levels of severity. The engine was most certainly warmed up and had not been restarted for 20 min or so. The screens were removed during the engine rebuild. Funny enough the cam phases as it should under high loads, something to do with the higher oil pressure? I'll start by checking the OCV's and go from there.




If the engine had to be rebuilt, I would definitely replace all solenoids with new OEM units.


It was just a ringland, the oil was in good shape. But who knows. If a OCV replacement fixes it, I'll probably do both exhaust side OCV's.


Yup I know what you're thinking, because I thought the same. While you wouldn't think that would have an impact on the oiling system, solenoid issues on ringland failure cars were so common we made replacing the solenoids standard procedure.

Good to know.

This afternoon I removed both the exhaust OCV's and the holders so I could clean and test them. The resistance at 8 Ohms was within the spec of 6-12 Ohms. I also applied 9 volts to them to observe their operation and both appeared quite responsive and strong. Compared to the Intake OCV's I had previously removed they actually looked cleaner. One thing I did notice is the filters were still in place, I could have sworn I removed them when I reassembled the engine last September. Naturally I removed them however, they were spotless. I reassembled everything, swapping the 2 OCV's and went for a drive. I got what I would consider a positive result. The exhaust cam phasing is much more stable and consistent, The left was damn near perfect. There were a few instances where the right cam would hunt a bit, but nowhere near the 0 - 45 Degrees of before, closer to 13 - 23 with the target being 17. I definitely noticed a difference in engine smoothness. I would like to put it through a few heat cycles before I celebrate too much but I like the direction we're going here. If the right cam starts exhibiting worse hunting I'll swap the OCV with one of the intake OCV's which I know to be rock solid, if the issue follows I would be confident to replace that OCV.

Sounds good. For what it's worth, I've never had one that wasn't functioning properly be out of range on an electrical test. Also, I recall the exhaust solenoid being a different part number from the intake solenoids, but it's been a few years since I've looked at them.

Hey Mike, I ended up replacing that OCV and now all cams are phasing damn near perfect however, I'm still having some stumbling issues. Do you mind having a look at this log? Its kind of short but it encompasses 2 situations that I would like resolved. The first part of this log is cruising with the cruise control on and experiencing stumbling. The second (around row 107) is when starting down hill and the load starts shedding, this particular point is quite rough. Just to clarify, the stumble is present cruise control or not.

Notice first section, while the stumble is present, there doesn't seem to be any fueling anomalies. In the second, once gravity starts lending a hand load drops, but the fuel trims report a lean correction. I have spent a lot of time on my MAF especially down low where it can sometimes be a bit harder achieve a steady state in which it gather MAF calibration date, so needless to say, I'm quite comfortable with my MAF calibration.

P.S. Looking forward to seeing what you can do with Speed Academy's G25 660 setup. That's the turbo I'm leaning towards.



Attached Files

Just a thought your primary ignition is approaching 50 deg. which sounds a little high. if you are running dynamic advance that just adds to it. maybe dial the total advance in that area back to around 40 and see if there's any difference. ( I realize theres no feedback knock in the log)

Have you checked out any non-tune related issues like a melted/clogged catalyst(if it exists), or a misbehaving ignition system?


Thanks for having a look. I don't have any extra Dynamic Advance in those areas. I have tried bringing the Primary Advance down to 45 but it didn't have any effect. Perhaps I can bring it down a bit further.

I don't think the cat is an issue, as it revs strong to 7200 @ 22 PSI. I did check all the grounds a while back and everything looked good. It's just strange it only happens between 2 and 3 Grand under 1.0 load, it's smooth as can be everywhere else.



Hmm the only other thing that comes to mind is that those years had trouble with the engine harness at the main bulkhead near the right side firewall and where the grounds and powers split up in the harness between there and the engine. Usually if that's the case with the engine running you can grab the harness ,shake it around and make all sorts of weird things happen. may be worth a try. I've re-soldered connections in the harness and zip tied the bulkhead together to fix those in the past.

Thanks for help, I'll have a look at that.



The harness issue jb mentioned relates to 11-14 WRX (mostly 13 WRX), rather than your 09 STI. That said, the main harness connector on all the GR cars coming unlatched and pins backing out of the connectors does happen. I zip tie the latch so it can't come undone.

Air/fuel ratio in the 16s will certainly stumble on a Subaru. They aren't as forgiving as some.

If you're confident the lean conditions are not caused by the calibration, I would investigate the car mechanically and electrically further.


I'm pretty confident at this point that its not. I'm going to check all the grounds, coils and plugs first, then go from there.

Have you run into any issues with the factory grounding setup, other than the grounds on the Exhaust OCV? Would I benefit from upgrading to thicker grounding cables?