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Old 07-17-2012, 05:55 PM   #4
Scooby Newbie
Member#: 34084
Join Date: Mar 2003
Location: cleveland,oh
2002 wrx


It's a fueling issue. had it on my 2011
Changing the FPR from an earlier year may or may not help
a tuner may or may not be able to tune it out
I bet you didn't notice it until you changed the turbo and injectors
I had the same problem and my tuner tried to tune it out for many weeks, changed out the fpr, tried different injectors, different fuel pumps. nothing fixed mine so I eventually got rid of the car.
If you want to read more about it(you aren't the first to have this problem) do a search for "08+ hesitation" or "low rpm stumble" lots to read, I've read every page.
I came across this post recently and you should read it because it may be an option for your to check out. looks like the guys at top speed may know how to get rid of it. maybe, maby not.
good luck to you.
Originally Posted by threelete View Post
For starters I logged this AM on the way to work
@ WOT:
.80 Lambda - Max duty cycle was 92%

History of my car.

1000k- flashed to Cobb Stage 1- 93
1500- added Cobb SF and Flashed Stage 1 SF 93 octane
2200- Added Invidia DP mated to stock catback, Flashed Stg.2
8500K- drove to Cobb Plano. 20gxt7cm, Walbro 255, ID 1000cc, Grimmspeed EBCS, E85 and 93 Tunes. 373/372 on Mustang dyno
13,500k- #4 cracked. Sent car on truck to Topspeed
Got back: Car had 16,000k on it last fall, Topspeed broke it in.
I have 33k now. I'm high mileage driver

SS1 forged CP block installed (new block), Aeromotive FPR, Aeromotive 340 fuel pump (I'll explain), Crawford AOS.
Got the car back with E85 and 93 tunes. there was E85 in the tank when I first drove it and it felt basically the same(386/412) dyno jet.
While my car was getting broken in Topspeed called me and asked it I had noticed a big hesitation between 2500-2900 rpms before the car failed and I told them I had!

Well, the car was still doing it with the new engine.

BEFORE THE PISTON BLEW HERE ARE SOME SYSMPTONS I WAS SEEING: My Walbro had started getting very loud before I had a piston issue on stock block. I sent logs to Cobb and they said it was fine. the walbro was making the start up hum noise longer and sometimes for an entire trip it would hum .Then one day I'm driving on a 98 degree day and not even driving hard and when I stopped the car threw a code? and was shaking. I sent logs to Cobb again and this time they confirmed bad misfire. Local compression check verfied. When I would start the car it was skipping #4, it was obvious at idle. My oil levels were checked weekly and they were OK, very small losses over 1500 miles.

SO, when Topspeed was still seeing the hesitation on the new engine I got worried. they replaced the Walbro with the Aeromotive 340 and also installed an Aeromotive FPR. The problem went away. So, my best guess was that I had a fueling issue. Originally when I left CObb i was told to continue using stock Subaru or Equal Oil. Oil change intervals were not stressed to me so I went with 2000 mile intervals, which Topspeed recommended was borderline too high. I'm going with 1500 mile intervals now and using eneos 0w50 or motul 10w40 when running E85 only. I do 3000 on 93 through the winter and I stick with the good oil. the Crawford AOS is good insurance also. I put the EL headers on also and the increased engine Vol. Eff. probably helps too. Bottom line after reading upon end is that our cars seem to fail selectively. Letting the car warm up in the AM, etc. all helps. When I start it in the AM and it's initially very loud and settles down after about :30 seconds then I sit for a minute before I go. I treat her like one of my kids now.

Another key thing Doug Wilks noticed on the dyno last week was that Fuel Pressure was low at start up. One of my lines on the fuel pump was loose. The toggle on the plastic body of the was slightly bent so the line was slipping. he wired tied it so it can't slip now.

I hope this helps.

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Last edited by cdoliver; 07-17-2012 at 06:01 PM.
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