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Discussion Starter · #1 · (Edited)
2016 sti
New Iag magnum closed deck short block w/14mm arp headstuds
Oem heads decked/ported/polished gsc s2 cams 272/272, beehive springs and titanium retainers, stock/oem valves not changed but valve lash (head work done at a mashine shop)
Iag Rails
Iag tgv deletes
Iag Comp aos
Iag phenolic spacers
Iag oil pickup/baffle/windage tray
-6an parallel ss braided lines
ID1300cc (not the new x2 injectors)
aeromotive adjustable fpr
Radium hanger w/walbro 450 hard wired 100% duty cycle
Cobb V3 Access port
Cobb flex fuel
Cobb fuel pressure sensor
Cobb Sf intake
Grim Speed 3 port ebc
HKS equal length headers
Grimspeed uppipe w/38mm tial ewg
Invidia catless downpipe
Fp Black turbo w/fp 84mm inlet
Aem wideband and gauge
Glowshift oil pressure gauge
Air pump delete
Cylinder 4 cooling mod
New NGK iridium plugs gap set to .029
Oem stock coil packs
gates timing belt kit w/water pump and new thermostat
12mm Subaru oil pump
removed banjo bolt screens
front mount prl intercooler
perrin motor mounts

Currently on a break in map
I have been monitoring cylinder roughness since i feel/hear misfire on idle and when holding rpm steady anywhere between idle and 3k rpm. Receive P0300 and P0000 (while vehicle is stationary 0 speed) (idle set to 950rpm)
Idle: cyl roughness count is 1-2 on cyl 4 every now and then (smooth idle)
Hold rpm 1200rpm; cyl roughness cyl1-5 cyl2-5 cyl3-1 cyl4-11
Hold rpm 1500rpm; cyl roughness cyl1-61 cyl2-3 cyl3-1 cyl4-18
Hold rpm 1700rpm; cyl roughness cyl1-9 cyl2-7 cyl3-1 cyl4-22

while driving/cruising misfire counts are rare and if i do get any its a count of 1 on cylinder 1 and 4
I have spent 3 weeks diagnosing this issue with my tuner
I have moved coil packs/plugs/injectors around with misfires not changing (even reinstalled my old 1050x injectors that I know were working flawlessly and issue still there)
I have monitored avcs and its +- 1 left to right for both intake/exhaust
no fuel leaks, fuel base pressure set to 43.5, cleaned maf, map, crank sensor, both cam sensors, cleaned/tested oil control valves, I even added 3 grounds, triple checked timing marks, I have spent the last 3 weeks reading and trying what ive seen in forums with no luck.
I ran a compression test even though this build only has 200miles and ring are not seated yet (hot test)
Cylinder 1- 94
Cylinder 2- 94
Cylinder 3- 97
Cylinder 4- 97
I also performed a pressurized boost leak test (10 psi) and smoke test and passed. (told the shop about these compression results and said that's normal.

Today I had a shop in Colorado Springs run Diagnostics and after 3+ hours they didn't find anything. (shop performed smoke test and passed) I will be dyno tuning with this shop, they said they would need a couple day to fully diagnose the issue but at $140/hour that's gonna get expensive real quick.

I am sure there is something I have done that I missed to mention here, I am out of ideas and don't know what else to do please help.
 

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My guess is that you are misfiring because your compression is so low. Mid 90s, I'm i reading that correctly? If the fuel/air mix cannot be compressed properly it will lead to uneven burn/ inadequate combustion. That most certainly would result in a misfire. Just a hunch though. Good luck man.
 

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Discussion Starter · #3 ·
If that were the case then wouldn't every built motor with new short block/internals misfire the entire break in period untill the rings are seated, I would say otherwise if the motor already had a couple thousand miles but not new. Thanks for the help tho.
 

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I have a new IAG stg 4 with new everything in breakin and have almost the EXACT same issue As OP.

My misfires are on bank 2 only, with no load. Driving I see hardly anything of concern on roughness counts. I am running IW1700s and my tuner insists we’re fine, that the injectors are the culprit. My shop thought it was a bad tank of gas because it just started happening with no changes, but not sure about that since I’ve refilled with 93 again.

Like yourself, cant leave this alone. Perhaps we can share info on mutual troubleshooting!
 

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Discussion Starter · #5 ·
Originally I was running my fuel feed parallel and returns T into the fuel pressure regulator, I have split them up and now running a return individually to a port on the FPR, this seems to have helped a bit, idk how your fuel system is set up, I'm currently waiting on a in line radium fuel damper to run in the feed and see if that helps. My tuner thinks it might be harmonic pulses in my fuel system. I also have my own break in map I made to try some things out; decided to force open loop in the tune and commanded 13.5 afr in the trouble rpms and roughness counts are now only on Cyl 1 & 2, I'm thinking my tuner may be right and might have to do with fuel. My previous setup before this build I was using cobbs fpr which has a damper and no issues and should have the radium damper installed soon so I'll post an update.
 

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Originally I was running my fuel feed parallel and returns T into the fuel pressure regulator, I have split them up and now running a return individually to a port on the FPR, this seems to have helped a bit, idk how your fuel system is set up, I'm currently waiting on a in line radium fuel damper to run in the feed and see if that helps. My tuner thinks it might be harmonic pulses in my fuel system. I also have my own break in map I made to try some things out; decided to force open loop in the tune and commanded 13.5 afr in the trouble rpms and roughness counts are now only on Cyl 1 & 2, I'm thinking my tuner may be right and might have to do with fuel. My previous setup before this build I was using cobbs fpr which has a damper and no issues and should have the radium damper installed soon so I'll post an update.
On my setup both bank’s line go direct to an aermotive FRP without a T. Seems similar to your.
 

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2016 sti
New Iag magnum closed deck short block w/14mm arp headstuds
Oem heads decked/ported/polished gsc s2 cams 272/272, beehive springs and titanium retainers, stock/oem valves not changed but valve lash (head work done at a mashine shop)
. . .

I ran a compression test even though this build only has 200miles and ring are not seated yet (hot test)
Cylinder 1- 94
Cylinder 2- 94
Cylinder 3- 97
Cylinder 4- 97
I also performed a pressurized boost leak test (10 psi) and smoke test and passed. (told the shop about these compression results and said that's normal.
This is normal??? I think not . . . unless it has to do with heads and cams. IE your rings should be seated better than that in a few minutes, no?
 

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Im not a built engine expert here but having 94psi on a compression check dosent sound right to me. Was the compression ratio changed during your build process? What i mean by that is, is that what the block should be seeing?

Was that 94 compression result on a cold check? Did it raise after adding oil and rechecing? Just curious.
 

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Discussion Starter · #11 ·
Sorry for the late response, I've been busy
The compression test was done dry and car warm ( my compression tester is pretty old and may need to buy a new one and retest). My car has the oem alarm system as well. I just finished fixing another issue I was running into (low fuel pressure). My walbro 450 went bad ( the pump was new but could have gotten damaged while troubleshooting the misfires since I was ideling for a long time and pump was hard wired for 100% DC. I decided to upgrade to the walbro 525, install a FuelPro pump controller to retain the oem 33/66/100 duty cycle, and ran -6an ss ptfe lines from the tank to the engine bay. While driving I rarely have misfires and only while cruising at low load (cylinder roughness count 1-3) and no cel. I'm going to continue driving to finish break in and for dyno day I'm converting to speed density (its already prewired)

I also removed the screens located inside the exhaust avcs housing (driver/passenger) and did notice driver side was dirty. I have attached pictures for reference.
61208
61209
 

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Sounds like a good fuel system upgrade !

Still concerned about the compression number you posted. Hopefully they are just inaccurate.
 

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Sorry for the late response, I've been busy
The compression test was done dry and car warm ( my compression tester is pretty old and may need to buy a new one and retest). My car has the oem alarm system as well. I just finished fixing another issue I was running into (low fuel pressure). My walbro 450 went bad ( the pump was new but could have gotten damaged while troubleshooting the misfires since I was ideling for a long time and pump was hard wired for 100% DC. I decided to upgrade to the walbro 525, install a FuelPro pump controller to retain the oem 33/66/100 duty cycle, and ran -6an ss ptfe lines from the tank to the engine bay. While driving I rarely have misfires and only while cruising at low load (cylinder roughness count 1-3) and no cel. I'm going to continue driving to finish break in and for dyno day I'm converting to speed density (its already prewired)

I also removed the screens located inside the exhaust avcs housing (driver/passenger) and did notice driver side was dirty. I have attached pictures for reference. View attachment 61208 View attachment 61209
Post updates on if you see any improvements. I never have misfires under boost, just low load situations like you’re seeing.
previous owner of my car put in an obnoxious viper alarm with remote start - my next thing to try is removing this to rule out all types of electrical interference.
 

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Well after installing the damper I just wanted to report back, unfortunately my misfires are still present, hopefully someone else will chime in and help us out.
Sorry for the late response, I've been busy
The compression test was done dry and car warm ( my compression tester is pretty old and may need to buy a new one and retest). My car has the oem alarm system as well. I just finished fixing another issue I was running into (low fuel pressure). My walbro 450 went bad ( the pump was new but could have gotten damaged while troubleshooting the misfires since I was ideling for a long time and pump was hard wired for 100% DC. I decided to upgrade to the walbro 525, install a FuelPro pump controller to retain the oem 33/66/100 duty cycle, and ran -6an ss ptfe lines from the tank to the engine bay. While driving I rarely have misfires and only while cruising at low load (cylinder roughness count 1-3) and no cel. I'm going to continue driving to finish break in and for dyno day I'm converting to speed density (its already prewired)

I also removed the screens located inside the exhaust avcs housing (driver/passenger) and did notice driver side was dirty. I have attached pictures for reference. View attachment 61208 View attachment 61209
Hey man what happened after your tune? Was it a tune issue? I have pretty much the same build as yours except Trojan block and gsc stage 1 cams. Having random misfire codes and after about 400 miles on break in map got cylinder 3 misfire. My builder swears it’s a tune issue relating to the ID1700. Definitely feel hesitation and roughness between 2-2.5k rpm.
 

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Hey man what happened after your tune? Was it a tune issue? I have pretty much the same build as yours except Trojan block and gsc stage 1 cams. Having random misfire codes and after about 400 miles on break in map got cylinder 3 misfire. My builder swears it’s a tune issue relating to the ID1700. Definitely feel hesitation and roughness between 2-2.5k rpm.
I am curious as well... i have an 05 sti i got last year..built block with low miles and having similar issues only misfires and roughness at idle. Currently have the block torn apart because I was thinking issue with the heads... (gsc stage 1 cams / valves etc).. but i have found no discrepancies in the heads or block itself. I am also running id1700s.. and running off strictly speed density. I did not do the fuel system personally but it also is running an lines with an aeromotive fpr and aem pump. Ive been chasing this issue for awhile myself and am not sure the cause still... and havent been able to get the car inspected since I bought it unfortunately
 

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I am curious as well... i have an 05 sti i got last year..built block with low miles and having similar issues only misfires and roughness at idle. Currently have the block torn apart because I was thinking issue with the heads... (gsc stage 1 cams / valves etc).. but i have found no discrepancies in the heads or block itself. I am also running id1700s.. and running off strictly speed density. I did not do the fuel system personally but it also is running an lines with an aeromotive fpr and aem pump. Ive been chasing this issue for awhile myself and am not sure the cause still... and havent been able to get the car inspected since I bought it unfortunately
I had my stock heads done by Headgames in NJ, car is definitely a lot louder in the engine bay than expected. Car drives and runs great as long as I stay out of the 2-2.5k rpm range.
Got protuned by Evans tuning in PA, made ~500 torque 500 hp. After tuning, car definitely runs a lot smoother, I still get some roughness in the cylinders maybe upto 5 on light load/idle but I believe that’s nothing to worry about.
According to tuner, Subaru misfire thresholds are Super sensitive, with all that’s done to the car he believes it’s false readings, so he disabled them.
At first I was like huh, so how the hell would I know if something’s wrong and how to pass inspection.

said You’ll know if something’s wrong, car will start to drive like shit. Honestly car runs beautiful, I’m monitoring DAM, FKL, Feedback knock and all is good. Quite frankly car is scary to drive at this power level, really haven’t gotten over the 50% throttle or even 10psi boost.

In NY we are allowed one system to be “not ready” so hopefully that’s the only one.

good luck with your build, hopefully I was somewhat helpful.
 

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I had my stock heads done by Headgames in NJ, car is definitely a lot louder in the engine bay than expected. Car drives and runs great as long as I stay out of the 2-2.5k rpm range.
Got protuned by Evans tuning in PA, made ~500 torque 500 hp. After tuning, car definitely runs a lot smoother, I still get some roughness in the cylinders maybe upto 5 on light load/idle but I believe that’s nothing to worry about.
According to tuner, Subaru misfire thresholds are Super sensitive, with all that’s done to the car he believes it’s false readings, so he disabled them.
At first I was like huh, so how the hell would I know if something’s wrong and how to pass inspection.

said You’ll know if something’s wrong, car will start to drive like shit. Honestly car runs beautiful, I’m monitoring DAM, FKL, Feedback knock and all is good. Quite frankly car is scary to drive at this power level, really haven’t gotten over the 50% throttle or even 10psi boost.

In NY we are allowed one system to be “not ready” so hopefully that’s the only one.

good luck with your build, hopefully I was somewhat helpful.
Wow, thats funny cause i was told my car was tuned by Jeff Evans as well... lol. And the guy i bought the car from did say it was just a “subaru issue”. I have always went to stimikey and he recently told me that the 1700s can be “tricky” so wondering if that has been my problem all along... now i just have to slap it back together and hopefully get it tuned out also. Mine also pulled hard and had no issues really besides a shit idle making roughly 430 on e85. I have upgraded to an fp green to hopefully get around the 500 range. Definitely helps hearing ideas and people talk about similar issues 👍
 

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Having the same issue as everyone else.
Fully built motor with an IAG stage 2 block, built heads with gsc cams/valves/springs, custom fuel system, ID1050X injectors, etc. At idle, it's misfiring with small counts on all four cylinders. When I hold revs to about 3k, they go away with a sporadic 1 count on cyl 4 usually.
I did drain my fuel and refill, which made the problem less. Problem is, this is a break in map that needs adjustment before really driving. Should I take the care down my road and monitor cyl roughness? Since it will be under a load. The misfiring was way worse prior to draining the fuel (well over a year old). I am leaning towards still not the best fuel since i left probably 3 gallons in it and mixed in fresh.
 

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Having the same issue as everyone else.
Fully built motor with an IAG stage 2 block, built heads with gsc cams/valves/springs, custom fuel system, ID1050X injectors, etc. At idle, it's misfiring with small counts on all four cylinders. When I hold revs to about 3k, they go away with a sporadic 1 count on cyl 4 usually.
I did drain my fuel and refill, which made the problem less. Problem is, this is a break in map that needs adjustment before really driving. Should I take the care down my road and monitor cyl roughness? Since it will be under a load. The misfiring was way worse prior to draining the fuel (well over a year old). I am leaning towards still not the best fuel since i left probably 3 gallons in it and mixed in fresh.
Mine had me worried as well, got the car tuned, makes great power, tuner believe it was related to how much modifications has been done, that’s what trying the misfire codes. Eventually turned off the monitors and have been monitoring the feedback knock and fine learn on AP. Car runs and pulls hard. No issues
 

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I have two theories.
1. there is an intake manifold leak i am unaware of. i have the small phenolic spacers and maybe something is off. i needed them because my aeromotive fuel rails were hitting the avcs oil valve's on the heads.
2. my fuel is still not 100%

i plan on checking for the leak first, then want to drive it under a load and monitor some parameters.

Also, what is everyone's base fuel pressure set to at idle?
 
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