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2003 Forester 2.5XS AT
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12 Posts
Discussion Starter · #1 ·
Hi everyone.

Current Issue: during WOT, the car will stutter/jerk starting around 3500 rpm. no engine lights either. during light to mid throttle, nothing, car drives normally but as soon as i step on it, jerk/stutter galore. it feels like a misfire but theres no smell or engine lights.

Preface: my 04 XT 5MT had accessport with stage 2 map, oem TD04, cold air intake, catted downpipe, and catted exhaust and drove with zero problems.

New Setup: saturday i swapped in a VF48 w/40k miles, AEM fuel pump, STI uppipe, STI TMIC, and custom basemap provided by my tuner until i can get it protuned sunday with him. same tuner provided the stage 2 map that ran fine pre-swap.

Checked: i have looked at every hose/line i could think of and nothing is loose, leaking, or kinked.
 

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2018 X3 M40i / 2016 X3 xDrive35i
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I have an issue just like this that I attributed to ID1000cc injectors. But I also had a fuel pump installed at the same time. It was a problem with harmonic resonance within the fuel lines and caused surging/hesitation in that RPM range. Fixed with fuel dampers installed on the inlets of the fuel rails.
 

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2003 Forester 2.5XS AT
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12 Posts
Discussion Starter · #4 ·
I have an issue just like this that I attributed to ID1000cc injectors. But I also had a fuel pump installed at the same time. It was a problem with harmonic resonance within the fuel lines and caused surging/hesitation in that RPM range. Fixed with fuel dampers installed on the inlets of the fuel rails.
Do you have any links to install and where to get? never heard of this before.

Solution: stop going WOT on an untuned car running a base map.

You probably need larger injectors and are running out of fuel.
understandable. maybe i should wait and see if the tune on sunday will solve this issue. various sources online indicated injectors weren't necessary upgrade with this swap but problem could be attributed to fuel, although i figured installing the AEM 50-1200 pump would prevent that
 

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2018 X3 M40i / 2016 X3 xDrive35i
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Do you have any links to install and where to get? never heard of this before.
Believe me, I hadn't either and it took a lot of troubleshooting. The parts I purchased will not be relevant since I had Injector Dynamics rails which utilized AN fittings I was able to utilize.

if I remember right, Cobb shortly after started making a stumble mod fix for 08+ STI's that wasn't able to be utilized on ours, but the concept was the same.
 

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2018 X3 M40i / 2016 X3 xDrive35i
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Sounds like it likely is more fuel pump related than injector related as I had theorized. Some others have resolved by going to the older style FPR (which is the same as you already have) but I believe the newer fuel rails have more in line dampers already so the additional dampening of the FPR might have been the resolution.

I know on my 04 I had something like 3 OEM dampers added to the rail and those didn't fix it until I added the additional dampers right on the inlets to the fuel rails.

Here's my old thread:

https://www.subaruforester.org/vbulletin/f115/my-2004-foresteration-opportunity-317034/#post3547178
 

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2003 Forester 2.5XS AT
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12 Posts
Discussion Starter · #8 ·
hmm wonder if it could be clutch related as well. i replaced my fuel pump with the AEM 50-1200 and ran it a few days before swapping vf48, tmic, uppipe, and the car ran fine no stumbling through rpm during WOT, that tune was around 15.5 psi. this new basemap i seen puts me around 18.5 and it'll stumble only during WOT. asked my tuner and he indicated hes never had anyone tell him problems like this with basemap.

im wondering if possible inlet leak issues? accessport log doesnt show anything out of ordinary that i can tell. a/f is about 11.2 during WOT and a/f leaning stays 0. i'm actually not even sure what the symptoms are for inlet leak. i cleaned MAF lastnight and that didnt change anything. even did the traditional pull MAF during idle and engine died to check if it failed or not.
 

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2003 Forester 2.5XS AT
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Discussion Starter · #9 ·
Problem solved!! Ending up being bad MAF. Switched mine out with a friend's 06 STI and my RPM stumble/hesitation/misfire thing is solved! WOT is wicked smooth now all the way to red line.

Really weird though...when I was stage 2 I didn't have this issue...guess I must've bumped into the MAF the wrong way during swap.

Thanks so much for your help! :grin2:
 

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2018 X3 M40i / 2016 X3 xDrive35i
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Great to hear! Makes sense it would be MAF going WOT, there you're pretty much entirely dependent of that being functional/accurate in open loop fueling.

Before you spend the money to replace it though, try CRC MAF Cleaner on it first.
 

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2003 Forester 2.5XS AT
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Discussion Starter · #12 ·
I used MAF cleaner on tuesday and let it sit and dry for 1 hour before installing and it still did the hesitation...weird.
 

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2005 Forester XT Automatic
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18 Posts
Not sure if you have purchased a new MAF or not yet. I was having false knock issues with my accessport for several months. Car would run fine until I would go to pass or full throttle. The RPM’s would climb and I would start randomly getting knock feedback and fine knock learn. Tried cleaning the MAF a couple months ago and no change. I ordered a new one Denso MAF on eBay for $32.50, installed it and reset ecu. Fixed the last of my issues I have been having. Couldn’t be happier.

https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/312500970349


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