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Old 11-08-2006, 09:05 PM   #1
linsavy
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Impreza GC8 Severe stuttering under high boost

I want to share my story so that others may learn.

C/N: Stock EJ20K stuttering badly at high rpm and boost over 10psi. Fixed by swapping coil pack.
Edit: It's bad again, wasn't the coilpack.
Edit2: It's fixed!, the induction pipe to the turbo had a hole in it.

For the past month I have been experiencing very bad misfire/stutter wherever I was at WOT and boost had built up. No CEL of any kind was present. The mileage was normal and the car ran just fine up to 10 psi right up to redline. Didn't burn any oil.

I had just finished getting a used larger fuel tank and pickup installed.

Thought the problem was fuel related with possible fuel starvation due to the new tank set up. So I replaced the fuel filter in case it was gunked up with crud from the new/old tank. Nope.
Next the fuel pick up was inspected. It was not perfect, but was cleaned up and put back it. We hooked up a fuel pressure gauge at the fuel rail and drove around. The pressure was A-OK even at WOT with the stuttering going on.
Next we figured it was the MAF because I foolishly cleaned it with some electronic cleaner a few weeks earlier. These MAFs apparently do not like to be cleaned so this is NOT a good idea. They are also hard to find as they are larger and only exist on Version 3/4 and JDM Legacy turbos and cost $$$. So the MAF was swapped with a known good one. No dice.

Spark plugs were recently changed and the plug wires are new. No boost or vacuum leaks. Compression test a little low, but expected for the mileage on the motor. Leakdown normal. Headgasket recently replaced.

So I once again turned to the wisdom of Nabisco and found a few references to similar symptoms. A new coil or coil pack often cured the problem. I checked the coil for resistance using a multimeter. It measured a little low on the secondaries but still in spec. So tonight I installed a used but known good coil. SUCCESS!!

Car now runs like a champ.

Enjoy.
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Last edited by linsavy; 12-06-2006 at 11:17 AM.
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Old 11-09-2006, 12:48 AM   #2
snowman4us
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Thats roughf. See if anyone local has a ECU you can swap for 5min to see if anything changes.
Also what mods do you have?
and how do you know that your maf and map are hooked up properly(not trying to be rude). See if you can get an osiloscope(sp?) and look at the patterns for maf, map, crank, cam, IAC,knock count, all that stuff. also look for small stuff, like loose hose clamps, uppipe leaks, just basic things. Also what if you go partial throttle up to redline? does anything change.

Kirill
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Old 11-09-2006, 01:17 AM   #3
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Glad ol misty blue is running well Bruce!
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Old 11-09-2006, 10:26 AM   #4
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what was the coil pack from. USDM what? i have a similar stutter/hesitation with an otherwise perfect running car. it won't do it all the time, but when it does it's annoying as hell! thanks for this thread too. i was just too lazy to post about it. me and my other tech did all the same things to find the problem(minus HG) and found zip as well. glad to know what it was and you're running good again!

--keith
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Old 11-09-2006, 11:53 AM   #5
linsavy
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Quote:
Originally Posted by snowman4us View Post
Thats roughf. See if anyone local has a ECU you can swap for 5min to see if anything changes.
Also what mods do you have?
and how do you know that your maf and map are hooked up properly(not trying to be rude). See if you can get an osiloscope(sp?) and look at the patterns for maf, map, crank, cam, IAC,knock count, all that stuff. also look for small stuff, like loose hose clamps, uppipe leaks, just basic things. Also what if you go partial throttle up to redline? does anything change.

Kirill
No problem, now. The car runs great. It was the coil all along.
Partial throttle was OK up to redline.
By the way, the swap has been in all good for 2 years now.
Thanks anyway.

Thanks James. How's is Vegas?

Kheff. I borrowed a V4 coil pack. It has a different number on it than my orginal, but works well. I am trying to find a USDM equivalent still. I am thinking that a 1995 to 1997 DOHC 2.5l might me the same. I have a 99 Legacy but the coil pack is quite different.
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Old 11-09-2006, 01:28 PM   #6
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so i should find a junkyard 95-97 2.5 dohc car to see if they're the same? does anyone you know have a dohc 2.5 so we can get a pic, if none are in my local junkyard? i'll try matt monson to see if he has anything layin around too. thanks!

--keith
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Old 11-09-2006, 03:40 PM   #7
linsavy
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My mechanic guru tells me that a 94-95 2.2l has the same or similar coil pack. I will try to compare it to one before I order it. Also Dave at rallispec is very knowlegeable about JDM weird parts.
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Old 11-09-2006, 03:41 PM   #8
johnkaldar
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Hope there is help with this. I thought by reading the search that this was blamed on a bad BCS. I have the same issue and I was ready to try a different BCS. However, I had to install a plug in the end of my muffler to quiet it down and today I get a little overboost, but then it settles down to my setting (180 Kpa abs) and sticks even with WOT. I am not sure if it still is the BCS or maybe now a coil pack based on your experience. I am still leaning towards the BCS, since I now have more backpressure. Hopefully, someone can dig up the USDM part #.
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Old 11-09-2006, 04:02 PM   #9
linsavy
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My car was not overboosting. The boost gauge would jump all around with the stutter, but did not exceed 17psi. We too thought it might be overboost but the gauge said otherwise. Also I am running the JDM stock ECU and they will not get too fussed by 18 psi.
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Old 11-09-2006, 09:21 PM   #10
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I am using an Autronic and can set the boost. I had it set at 20 psi, which I found is too high for the turbo, so I turned it down to 12 to see if the wastegate would cycle. With 2.5" straight to a flowmaster exhaust, I still would spike to 17 psi and then stutter with a huge loss of power. Tonight with a plug in the end of the muffler, I was able to get the gauge to bounce between 12 and 14 psi, again with loss of power under higher throttle position.
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Old 11-12-2006, 01:03 AM   #11
linsavy
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Aak!
It's back again! Stuttering away worse than ever. Now I am really befuddled. I swapped plug wires to see if that could be it, but no.
I am going to pull the spark plugs tomorrow, but the ones in there are only 1 month old.
It ran well for one day and about 5 hard pulls. Tonight, I went to gas up and it started going wacked again on the way home.
Grrrr!
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Old 11-13-2006, 02:05 PM   #12
kingsubi
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What plugs are you running Bruce? If coppers, what are they gapped to?

What kind of stutter are we talking about? What kind of sound does it make when it happens?

Can you accelerate through 3-6K at half-throttle (~10 psi)?? i.e., does it only happen at full boost.

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Old 11-13-2006, 03:27 PM   #13
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What is the spring pressure on the bov supposed to be? What kingsubi is asking is just what I have. I can accelerate up to 50% throttle around 10psi. Could a leaky bov be the issue?
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Old 11-14-2006, 09:50 AM   #14
linsavy
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Yes, I can accelerate up to redline at 1/2-3/4 throttle as long as I don't go over 10psi. The motor will cut-out or stumble severely if I do and keep doing it until I ease off the throttle. I don't notice any sounds other than the motor going from smooth to on/off, on/off. There is no power when it stutters and the boost gauge swings irratically.
John is sounds like we have the same thing. My BOV was replaced 2 years ago with the swap for new stock one. I am running a VF23.
I currently have copper NGKs gapped at .28. I took out the NGK platinums that were in for a month. They were gapped at .30. They looked good except the outer ring was black with soot. The inner parts were clean.
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Old 11-14-2006, 10:14 AM   #15
kingsubi
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Any chance it could be the clutch? (for both of you...)
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Old 11-14-2006, 10:29 AM   #16
linsavy
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Not the clutch.
Iain thinks it may still be fuel related because it all started after the fuel tank swap. The fuel pressure was good (50 psi) though so maybe it is the injectors? I don't know. It wouldn't hurt to put in a new sock at the pickup and send out the injectors for cleaning and flow testing.
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Old 11-14-2006, 11:46 AM   #17
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my clutch is bad, but it's a waay different feeling. just as they said its a no noise just a severe off power feeling like a fuel/RPM cut out. i have a brand new fuel system. fuel cell, pump, lines, filter, cleaned injectors, etc. so it may be fuel pressure, but definately not contaminated/dirty/plugged fuel system. i have a very high pressure pump and stock FPR. i thought mine was too high psi at idle causing a fluctuation, and possibly the cut out.

i attributed the idle surge to recovery line being fed all the pressure besides the minimal it takes to idle, and then trying to send remaining psi into fuel feedline when it begins to starve a lil. but that was just a guess. im not sure how this would affect the actual cut out up high. bad OEM FPR or going bad? not sure.

linsavy said 95 legacy dohc coil is same visually minus part number on back. i tried to locate OEM BCS thru parts store, but nothing even listed for any model. imgoing to try suby dealer right now!

--keith
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Old 11-14-2006, 02:12 PM   #18
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Seeming both of you have changed out the fuel tanks and pick ups - I sense a theme...although Bruce's psi seems to be fine before the rails (not sure where you measured it??)

This could be a series vs. parallel fuel delivery problem that the OEM fuel system dealt with somehow and the new system isn't??
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Old 11-14-2006, 03:03 PM   #19
linsavy
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Maybe, I think I will swap the whole pickup, pump and all out of the other car and see if that helps. The fuel pressure gauge showed 40 psi at idle and it kicked up to 50psi and was steady with the misfire/stutter. I would expect it to be jumping all about if there was an issue.
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Old 11-14-2006, 03:23 PM   #20
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I swapped out the stock rs pump for a 255 wahlboro that I got from kheff. I haven't checked to pressure yet though. Wouldn't increasing the duty cycle on the injectors work if the pressure is down (just as a test)? I know that I kept raising my afr to 9.7:1 with no change.
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Old 11-14-2006, 04:29 PM   #21
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Quote:
Originally Posted by linsavy View Post
Maybe, I think I will swap the whole pickup, pump and all out of the other car and see if that helps. The fuel pressure gauge showed 40 psi at idle and it kicked up to 50psi and was steady with the misfire/stutter. I would expect it to be jumping all about if there was an issue.
If your FP goes from 40-50psi, idle to full boost, then you have a problem.

It means differential pressure across the injectors is going from 50 psi dropping down to 35psi (assuming 20in/HG vacuum (-10psi) at idle and 15psi max boost).

-Rob

Last edited by rob; 11-15-2006 at 12:41 AM.
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Old 11-14-2006, 04:33 PM   #22
kingsubi
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Quote:
Originally Posted by rob View Post
If your FP goes from 40-50psi, idle to full boost, then you have a problem.

It means differential pressure across the injectors is going from 30 psi dropping down to 25psi (assuming 20in/HG vacuum (-10psi) at idle and 15psi max boost).

-Rob
I would assume the FPR or the pump is not keeping up - not sure increasing the duty cycle is really solving anything.
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Old 11-14-2006, 05:08 PM   #23
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Ooops, simple math error. If you are hitting 15psi, then you dropping your injector pressure by almost half! Sounds like you hooked up your FPR pressure reference line to a port PRE-turbo! Make sure the FPR is seeing the boost reference.

Otherwise, you might have a fuel leak at your fuel pump.

-Rob

Last edited by rob; 11-15-2006 at 12:42 AM.
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Old 11-14-2006, 10:54 PM   #24
linsavy
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You guys are losing me with all your big words. What fuel pressures should I be seeing and how much should there be at WOT with 15psi of boost?
There was no change in the vacuum lines only the pickup was changed.
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Old 11-14-2006, 11:54 PM   #25
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43+15=58
idle shoule be 43 -what ever vaccume you are pulling

now if you had a crack/leak in your FPR reference hose that could cause proublems
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