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Old 11-19-2001, 09:06 AM   #1
fab9joe
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Default flat spot

hello all! i need some help. i am experiencing a 'flat spot' in power from 3400rpm - 4000rpm. it seemed to start when i removed the intake silencer but when i put it back on it appeared again. the only mods i have done have been removing the second air filter in favor of a gasket and an amsoil air filter in the small air box. i always reset the ecu after any change i make. any thoughts before i put the factory filters back and take it to the dealer??? thx- adrian
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Old 11-19-2001, 12:40 PM   #2
wcbjr
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It is common to the 2.5's. We all have it.
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Old 11-19-2001, 01:03 PM   #3
Richard L.
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Isn't that the infamous hesitation problem? I don't have it after the front O2 sensor recall and cold air intake. I don't remember having it when my car was bone stock either.

--
Richard
http://www.whitetyphoon.com/impreza/
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Old 11-19-2001, 03:48 PM   #4
fab9joe
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but how bad a flat spot?
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Old 11-19-2001, 04:03 PM   #5
fab9joe
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why does the flat spot disappear after an ecu reset only to resurface after 50 miles?
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Old 11-19-2001, 07:02 PM   #6
Ripper
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I wouldn't know but nothing a little modding doesn't help.
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Old 11-19-2001, 09:15 PM   #7
ColinL
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I've got this too and have the exact same intake setup. Mine does not come back nearly that fast though. Two questions:

How's your oil level?

Ever gotten a check engine light?
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Old 11-19-2001, 09:33 PM   #8
Keith99RS
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To the guys with the flat spots - How many miles on your cars?? If it is near or over 30K or so and you still are running the original plugs, it could be them. My car developed a flat spot at about the same RPM, then when it went over 4000, it would take off. I put new plugs (NGK's) in this weekend and the car is smooth like new again with no hesitation. Just a guess
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Old 11-19-2001, 10:18 PM   #9
fab9joe
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my car only has 16000 miles on it and i just put the ngk iridium plugs in. i think iwill take it to the dealer( after putting back the stock filters). i run amsoil synthetic every 4000 miles.
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Old 11-19-2001, 10:20 PM   #10
fab9joe
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i havnt had a check engine light recently
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Old 11-19-2001, 10:21 PM   #11
ColinL
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38k miles, second set of replacement NGK copper plugs. about 5,000 trouble-free miles on them without incident, then I start getting lag. it's worst around 4,000 and at 4,800 it's like I hit the VTEC cutover as all the power comes back.

it is 100% gone if I reset the ECU and has been happening for about 1,500 miles. since a second reset and oil change 500 miles ago, no problems. of course I don't really care right now because I have a little coolant issue. not that anyone has read that thread.
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Old 11-19-2001, 10:57 PM   #12
Aaron'z 2.5RS
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I to have had the hesitation problem.

I found it to be the "over sensitive" knock sensor.

This was al over the board about a year ago.

After i modded my knock sensor, the problem is gone, it only comes back when i get a bad tank of gas.Even then it rairly happens.

The theroy is, the sensor is to sensitive and picks up things that the ECU thinks are knocks/pings and decides to retard the timing.

1-2 rubber washer made from a bike tire patch, worked for me. The rubber is about 1/16" thick or less so when you tighten down the sensor it skwishes and is almost NOT there. It dampens the knock sensor kinda like dynamat.

(insert non-responisibiliy discloser here)

Seem to work great for me....
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Old 11-20-2001, 04:42 PM   #13
Fmdeadrick
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I had the same problem after fitting a prototype intake about 2 years ago. I got an AFC and that solved the problem for about 18 months. About 3 months ago it came back with or without the AFCs help, and ECU resets only lasted for 50 miles or so. I found that adding FI cleaner seemed to help briefly, but not for long. I have platinum plugs with less than 5000 I think, so I don't suspect them. I may try the Knock Sensor mod. This car certainly is a piece of work, just about as tuned as an old 60s muscle car. Hopefully with the turbo and TECII there will be better control over it, though I doubt much less work!

Mark
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Old 11-20-2001, 06:04 PM   #14
fab9joe
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mark, let me know if that knock sensor mod works- adrian
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Old 11-20-2001, 11:53 PM   #15
Fmdeadrick
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Ok, I guess I better figure out where it is. Anybody want to help me find Waldo?


Mark
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Old 11-20-2001, 11:57 PM   #16
Aaron'z 2.5RS
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WALDO



Peas,
AaRoN

A 12mm socket and a few extension with an elbow will get you there.....
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Old 11-20-2001, 11:58 PM   #17
jeffg
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Almost all of the cars here have it. Differnt intakes, no intakes, some with exhaust, some with headers, some with AFC. It does not seem to matter what parts you put on the car. Some mods definitly do make the problem worse though. Intakes for one.

I noticed on a dyno chat posted about AFC's that there was a pretty serious dip in AFR at ~4000 rpm. From looking at the dyno charts posted you could see a drop in HP and torque inline with this change in AFR. Could this be a ECU "defect?" Does anyone know if there are any high versus low rpm vacumn switches? Some sort of Emissions thing that helps kill high rpm polution? or vice versa. If the knock sensor thing works for you Mark, I will have to have a knock sensor day here.

Jeff
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Old 11-21-2001, 11:46 AM   #18
ChaosThom
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After installing a free-flow intake (Hyper Weapon-R) the flat spot was reduced. But after installing a 2.25" muffler (glasspack) , the flat spot is completely gone.
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Old 11-21-2001, 12:12 PM   #19
ColinL
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the '99 ECU is waaaaay different from the 00-01, so don't try to draw too many conclusions about what you might have seen recently relating to an AFC on a 00-01.

notice how everyone else posting has a 99? there's something very specific here, but I know that other model years had some hesitation too.
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Old 11-21-2001, 12:54 PM   #20
Fmdeadrick
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Thanks, the problem is certainly different on the '99s. I'll try to take a look at it within the next few days.


Mark
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Old 11-21-2001, 09:15 PM   #21
Peaty
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Old 11-21-2001, 11:18 PM   #22
fab9joe
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just got back from the dealer and #!*%#!*& they said they couldnt find a problem, the computer couldnt find a problem. i took the service advisor for a test drive and he said he could feel a slight ' hesitation' and scheduled an in depth appointment for 3 weeks from now. does anyone know of a good subaru tech in the southern arizona area? or i could just reset the ecu every 3 days!?!?
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Old 11-22-2001, 04:16 PM   #23
fab9joe
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i just conducted an interesting test: i unplugged the knock sensor and low and behold flat spot gone! the car did however feel a little slugish. i then plugged it back in and the flat spot was back. of coarse i did get a CEL. does anyone think i should reset the ecu with knock sensor unplugged? that does seem to be the problem. unbelievable that the dealer didnt try that! thx -adrian
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Old 11-22-2001, 07:07 PM   #24
Aaron'z 2.5RS
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I've been told you can get a MY00 knock sensor, whom ever told me that said they wern't as sensitive.....

I haven't tested that theroy so I donno...
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Old 11-22-2001, 07:24 PM   #25
imprezawd
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I have the same problem. My check Engine light came on and on and on and on. every time it says it is the Fuel control selinoid valve(locate under the rear right bumper.. I changed it and check engine came on and same thing flat between 3-4k. Only for a while then it will go away and then it will happen again..
I dont even botherany more. when it runs good it runs good. I hate it but this isout of my control and my techs at work dont know for sure what it is either???

Richie
99rs
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