Help, someone, anyone!!!! hesitation returned.

[jc54bv]

Hey.... so Ive been battling a intermittent hesitation/bucking when accelerating, right around 2000-3000 rpm or pulling from low revs at high speed. I finally thought I had it licked with a ground wiring mod but no luck, after about three weeks it has returned. I have tried everything, o2 & knock sensor replaced, no vacumn leaks, new fuel filter, cleaned or replaced EGR and pcv, new plugs and wires( NGK blue wires, and NGK laser platinum plugs), new airfilter and all the usual tune up stuff. I tested the TPS and it is was within the specs reccomended. Each thing has made a very slight improvement but not eliminated the problem completely.

I think it must be sensor related, the reason I say that is that after resetting the ECU (unplugging negative on the battery for 8 hours +, the hesitation completely dissapears for about 10 cycles. It pulls well and has lots of power. When the hesitation returns it is bucking and has what I can only describe as a step by step acceleration- flat spot-accelerate-flat spot-accelerate-flat spot-accelerate etc,etc . At wide open throttle no hesitation?????

This is driving me nuts, It is still driveable, but it is just so unpredictable

Any ideas???

[PaulO]

Oh dear, I finally sold my 2003 because we could not get rid of that damned hesitation. I have now also noticed it in a Forester that I had for a month, but it was only noticeable on the Forester when the CEL was on, due to a leaky front exhaust.

This must be the most frustrating thing about these beautiful cars, sorry I cannot be of any help.

[ETC]

Bummer,

No codes ?

Does it do it when it's cold ?

Does it idle ok when in this "mode" ?

When it's dead cold, start it, take it out and drive it normally and let us know.

The ECU doesn't reference sensors when it's cold and I don't believe it uses most sensors at WOT [other than TPS].

The disconnecting the battery thing would lead me to think it's more of an ECU issue than a sensor issue. The ECU essentially "resets" when power is cut, sensors don't reset as there is no memory or processor to reset.

Definitely check all the plugs on all the sensors and the ECU and all the wiring for any damage.

Without logging software, being able to compare sensor readings when it's not doing it and when it is or, to see what the ECU is reporting ... I'm not sure how to go about trouble shooting this.

Can you go over again when this started and what if anything had changed prior to this condition ?

[cutback]

I have a similar problem with mine. It only happens when trying to accelerate quickly. Just cruising around or reving the engine it's fine. I haven't had it long and haven't had a chance to try anything like a tune yet, but when I first got it, it was going well.

I hardly go anywhere these days so it's not bugging me too much yet.

The only thing I can add is that mine came good for a while after disconnecting the battery as well.

[jc54bv]

Hey ETC..... Yep no codes, no CEL, It idles smooth as anything with no jumps or drops in RPM. It does Idle low at around 200 rpm, but it has always been like that.
From a dead cold start the bucking seems marginally worse, but then continues into the normal operating temperature.
Thats interesting that you mention the ECU itself, I had begun to do some research about people having them re-flashed, but I will defintly have a poke around under the dash later see if anythings loose or unplugged. Im wondering if it is maybe a combination of things, I hvent rechecked for vacumn leaks recently, maybe thats a contributing issue????

I bought the car around 4 months ago, but didnt notice the bucking at the time, when I did finally notice it I put it down to a clogged air filter or something?? but it wasnt, then I tried the fuel filter and it wasnt that either. Those where what I thought would be the most likely to cause this condition.

Anyway Im not going to give up on this car, cause I still love that boxer rumble (just wish it would rumble properly)

[jc54bv]

Ok so I took it for a run this morning from dead cold, and actually it performed quite well, the hesitation was very faint, and it seemed to be marginally better throughout the whole day. But this only emphasises the unpredictability of this problem

[ETC]

Normally I would be guessing fuel issue [pump, relay, pressure regulator] or weird vacuum leak [PCV, purge valve]but, you say it goes away when you reset power so ... I'm back to ECU.

Even electrical things like the TPS or other sensors shouldn't be temporarily "fixed" by a reset. Most of the sensors should code pretty quick after the car gets up to temp after a "reset".

And, what was the deal with the 3 week "fix" by extra grounding ?

The only other thing that comes to mind would be a failing battery or charging circuit.

This may be a "leap" but, maybe engine coolant temp sensor ? starting to fail and giving intermittent "cold" signal to the ECU causing it to go in to open loop. Again, this shouldn't be affected by the "reset".

[dpregs]

Originally Posted by jc54bv

Hey.... so Ive been battling a intermittent hesitation/bucking when accelerating, right around 2000-3000 rpm or pulling from low revs at high speed. I finally thought I had it licked with a ground wiring mod but no luck, after about three weeks it has returned. I have tried everything, o2 & knock sensor replaced, no vacumn leaks, new fuel filter, cleaned or replaced EGR and pcv, new plugs and wires( NGK blue wires, and NGK laser platinum plugs), new airfilter and all the usual tune up stuff. I tested the TPS and it is was within the specs reccomended. Each thing has made a very slight improvement but not eliminated the problem completely.

I think it must be sensor related, the reason I say that is that after resetting the ECU (unplugging negative on the battery for 8 hours +, the hesitation completely dissapears for about 10 cycles. It pulls well and has lots of power. When the hesitation returns it is bucking and has what I can only describe as a step by step acceleration- flat spot-accelerate-flat spot-accelerate-flat spot-accelerate etc,etc . At wide open throttle no hesitation?????

This is driving me nuts, It is still driveable, but it is just so unpredictable

Any ideas???

Hey JC,
I registered here just to type this for you! We own a 2004 outback sedan H6 with 139k miles on it. We have had that bucking since about 40k miles. I took it to the dealership 5 times between 40k and 100k miles when it would act up and they would always tell me that they could not reproduce it or get a code of the problem. Well, last year coming back from Boston in August the car just would not run without bucking. The entire drive from MA to CT the car bucked when you would hit the gas, it was like the transmission was confused about what gear it should be in! IT was a SCARY ride home! I finally gave up and brought the car to Firestone near us, and within one hour the had it diagnosed and in the repair bay! It was the "TPS4165 Throttle Posn sensor". Since Firestone fixed it, we have not had one incident of bucking!

Dan Perkins Subaru is a joke and should not be allowed to have a repair license! So many times there, so little help! Maybe it was because we had the 100k mile warranty that they didn't want to work on it, maybe they just don't know what they are doing, regardless, stay away!

[jc54bv]

Well thanks for making the effort....... did you ever get a check engine light? I have no codes or CEL, and I have also done the voltage/resistance testing on the TPS too. Maybe, just maybe it could be faulting, but not enough to throw a code? I am also suspecting the knock sensor is playing a role in this, there has been many reports from subaru about hypersensitivity of the knock sensor in the 5 speed models 1995-99. I may re test the TPS tonight to make sure I didnt miss any warning signs.

[dpregs]

Hey,
Nope, never got a light to come on. Maybe the dealer never picked it up because it wasn't enough to throw a code, and when it had a complete meltdown it finally threw a code.