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08 Sti Asphen white
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96 Impreza L with almost 170k Miles on it ... Just started getting a slight hesitation on initial tip in .. every other gear cruising is fine .. any ideas? .. yes I've read through most of this but haven't found anything specifically towards this .. completely stock ..
 

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I've read this entire thread and several others but haven't seen any concrete answers. I'd really like to avoid just throwing parts at it if possible.

2002 Impreza 2.5RS, 5spd, 81k miles, only mods are CAI and an aftermarket muffler welded to the full stock exhaust. About 2k miles ago I had the engine out and did head gaskets, timing setup, plugs and wires, and new clutch. None of the issues seem to be related to the timing of that work.

The problem is entirely intermittent, sometimes it'll bog some off the line, and cruising or accelerating it'll cut out and lose power under load. I had an intermittent P0328 knock sensor code and it'd occasionally cut out like described. One morning it got extremely bad, ran like crap, and had no power. I replaced the knock sensor and that fixed the CEL and poor running. It's still had the intermittent hesitation and gutlessness and has gotten worse the past few days.
I haven't removed or adjusted the TPS but checked it anyway. .46-.47V at idle. On the low end of the spec, but still within spec so I left it alone. I doubt that's the cause anyway since it's an intermittent problem.
I cleaned and inspected the PCV, IAC, and MAP and all seem fine.

Here are what I think the 3 more likely possibilities are:
-Bad front O2 sensor. Could be bad being 11 years old with 81k miles, and the bad knock sensor could've sped up the failure. Here's the catch: several posts said with a bad O2 sensor it should run fine above 4k when it goes in open loop. I did a couple 4th gear runs today above 4k and had it cut out momentarily around 4500rpm or so. Could the O2 still be the culprit? Shortly after I did the high RPM runs it seemed to start cutting out much more, sort of bucking and shuddering even just cruising down the road. It also started raining around this time, but I doubt it's related.
Wouldn't the O2 just change the A/F ratio somewhat if it detects a lean or rich condition? Seems like the rapid changes in power would more likely be changes to ignition timing from a different sensor.
-Faulty knock sensor. Even though I just replaced it, could the new one be sending false knock signals and causing retarded timing occasionally? I did position and torque it properly and made sure the bolt and mating surfaces were clean and smooth.
-Bad ignition coil. As far as I know, this is still original. How often do they just go bad? I cleaned the connections(1 plug did have some corrosion when I replaced plug wires) and made sure everything is fully connected. The past few days when the problem has been worst have been pretty humid. If the coil is cracked I can see how this would be a contributor but it seems random.

Any ideas? Anything else I should check? Has anyone had the same issue and had a specific part fix it entirely?
 

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Update:
I disconnected the upstream O2 and drove it for a few days. It obviously got the CEL, and seemed a little lower on power(probably running rich to be safe), but never seemed to have the shuddering or cutting out like before.
I plugged the sensor back in and the issue came back almost immediately, so I figured that was the problem and ordered the Denso sensor last night.

Now here's the catch: I unplugged the sensor again last night so it would stop cutting out when I drive, but it still cut out intermittently this morning for my entire 40mile drive. So is it just a coincidence that the issue came and went when I unplugged the sensor, or do I need to plug my scanner in and reset things? When I first unplugged the sensor last week I had scanned it to make sure nothing was up, so maybe that changed things.

I'm a little concerned here, when I think I got somewhere another detail pops up that makes me wonder. I hate diagnosing electrical gremlins.
 

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2000 2.5RS/C BRP
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Can your scanner read freeze-frame data? There may not be any without a CEL, but it's worth a shot.
 

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2000 2.5 RS, Rallye Blue
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replaced the IACV with another and drove it for 2 days without problems and now the idling issue is back. and hesitation still there misfires always been there :/
 

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2001 RSTi, 2011 WRX Hatch
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I'm still hesitation free, but I've replaced nearly everything except the crank sensor at this point, so I should expect it to idle well =)

Again, my issue was just spark plug gap.

-- Dave
 

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02 WRB wrx, 99 AW wagon
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I was lucky that it was just my plugs

Normal commute, up a big hill car starts losing power....and I can feel the car gain and lose power from inside the car. (car idles fine)
On the way home, another hill, I drop it in 3rd (4eAT) and had to floor it to go up the hill. It wouldn't pass 60.

Car has 180k, plugs were all black and way out of gap...New NGK copper, all gapped >.40 and the car not only has more power than before, but rpms at 65 is lower.

That feeling when you fixed it: Priceless
 

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MY04 WRX STi Silver
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For people that are having hesitation issues that can't seem to be rectified on modified MAP sensor based cars please note:

Your vehicle is tuned on a speed density system where Volumetric efficiency is told to the ECU, which is used to calculate how much air is entering the motor.
If you change say your exhaust.. This changes how much air will flow through the motor.

THE ECU does not know this has been done and will still only inject the amount of fuel for the air it's expecting!
If your mods created an substantial increase in flow at a given manifold pressure or rpm, it will run lean at this point of increased efficiency.
This leaning out may result in knock as the ignition timing is still on a factory map, possibly causing your hesitation.


I'm in the midst of setting up my Carberry rom for my 04 ADM STI and if I change AVCS in the slightest, I can immediately see the AFR's change in that region.

This doesn't apply to MAF equipped cars as they measure air mass :)
Just be aware it may not be fixable :)
 

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'00 BRP RS Sedan, '04 FXT PSM
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Thought I would add a bit as I was experiencing some stumbling/hesitation issues. No check engine codes were thrown which made diagnosis a bit tricky.

The car has 176K miles on it (we've owned it since new) and I just finished installing the following:

new fuel filter (what a BITCH to get off)
new OEM coil pack
new NGK copper spark plugs
new NGK spark plug wires
new 2005 Impreza WRX STi Alternator (Ultima brand from O'Reilly Auto Parts)
new PCV valve

All of the parts that were replaced, aside from the spark plugs and possibly the PCV valve, were original parts that came on the car. As well, we Seafoamed the engine roughly six months ago.

When I removed the plug wires I noticed heavy oxidation on one of the coil pack posts and another post that looked like it had been burnt. Rather than try to clean up the original coil pack I decided to pull the trigger on a new OEM coil pack since the original went 176K hard hard miles. Luckily the local stealership had one in stock and was willing to sell it to my for $28 more than I could find it online, shipped. Thank goodness for those wonderful parts departments ;)

The 2005 STi alternator might have been overkill but with the original still in the car I figured it was a good idea anyways. Plus, the 2005 STi alternator (90 amp) was $55 cheaper from O'Reilly auto parts than a 2000 Impreza RS (75 amp) would have been ($124.99 vs. $179.99) and it is a direct drop-in/plug-and-play upgrade. Crazy but true!

I also cleaned up all of the factory grounding points that I could locate in the engine compartment. I will be installing a DIY grounding kit in the near future as I had purchased all of the components prior to having these issues.

I hope this helps someone down the line as I was having a hell of a time.

-Brian
 

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So I have a 98 2 Door Impreza that recently started having really bad hesitation and misfire issue. About 3 weeks ago it first started having some hesitation and idle issues so I replaced the spark plugs and put in some fuel injector cleaner and that seemed to help the issue out for the most part but then a couple of days ago I started the car and it was having horrendous misfire issues and having trouble idling at all, either the idle will sit at 1k+ and will bounce around from <400 to 1.5k or just sit at around 200-300 before stalling out while having very terrible misfire that is easily heard from the exhaust.

In first and second and in general at low RPMS the car bogs down very badly and will kind of limp with the gas pushed all the way down but as soon as the car gets to 3k+ RPMs it just gets a huge jump of power and seems to work fine though you can still feel/hear some misfiring (it finally through a misfire cylinder 1 and 2 code while I was doing some testing). The car also seems to work all right in 3rd gear + if I can get it started and rolling at all and has a little bit of bucking around 2k RPMs but then again feels like it gets a surge of power and will accelerate just fine and have no problems at highway speed. But as soon as I come to a stop or just coast in neutral for a while the car will idle at <300 RPMs and eventually stall. The car also has issues when in neutral when it's below 3k RPMs of not responding well to the throttle but when I can sit there with the pedal down and hit higher RPMs it will sit there and act fine and let me rev to 5k+ with no issue.

After talking with some friends and family they said it's most likely the fuel pump or fuel injectors, and after doing some reading and research I'm inclined to believe that could be the case but wanted some other opinions on the matter on what I should replace first. I'm obviously going to replace the spark plug wires when I get the chance as well just for general principle, but I'm not sure what I should go for first to try and mitigate the problem between the MAF sensor, the fuel pump, the coil pack, and the injectors. As far as price and easy of work goes, coil pack and wires seem like the obvious choice, but the way the car is acting makes me feel like it's something to do with fuel getting to the engine. It could also be the MAF sensor but I'm not sure if it would affect the car in only low RPMs and Idle.

On another note the car used to have really bad idle issues in general and would throw random CEL codes that I was able to get cleared up simply by cleaning out the IACV so that might be an issue as well.

Any thoughts or opinions would be great, I need to buckle down and do some work on her but I only have so much money I can throw at the problem every week.
 

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I had issues like many describe in this thread..

'02 ReallySlow 2.5 N/A slushbox.

: Revs freely in P or N. In cold temps, 40 degrees or lower, I could drive for 3 to 4 hours before issues started.. in 50+ temps, it would take a little as 15 minutes around town normal driving, but if I had to romp on it - say to enter the highway, it would just lay right down and die.. Really bad from a start once it started happening, but fine once moving, and only really bad again if for instance it dropped down a gear - as you were climbing back up the RPM range. Basically, any real load made it crap its pants. (and nearly made me crap mine on a few occasions)

After literally plowing down every list of potential fixes to no avail, scratching my head and standing at a virtual dead end...

I decided to replace the fuel pump...

My buddy and I didn't figure that would be it either and he actually suggested we check it - but, given it's location, I decided, if we were going in, it was getting replaced... so we did... nothing even seemed obviously wrong with the old one, other than the filter media looked like it had been soaking in 10 years worth of gas.

My mechanic buddy figures, since it seemed heat related, both ambient and operational, that the heat was simply building up in the pump motor, and lessening its efficiency to deliver steady fuel under loads, especially if it was hot out, or the car had been running steadily for long periods of time.

The car has never hesitated again since the pump was replaced.

Hopefully this helps some other lost souls on their journey to end the hesitation...

Good Luck SubyFriend
 

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2000, OBS, RustRed
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For people that are having hesitation issues that can't seem to be rectified on modified MAP sensor based cars please note:

Your vehicle is tuned on a speed density system where Volumetric efficiency is told to the ECU, which is used to calculate how much air is entering the motor.
If you change say your exhaust.. This changes how much air will flow through the motor.

THE ECU does not know this has been done and will still only inject the amount of fuel for the air it's expecting!
If your mods created an substantial increase in flow at a given manifold pressure or rpm, it will run lean at this point of increased efficiency.
This leaning out may result in knock as the ignition timing is still on a factory map, possibly causing your hesitation.


I'm in the midst of setting up my Carberry rom for my 04 ADM STI and if I change AVCS in the slightest, I can immediately see the AFR's change in that region.

This doesn't apply to MAF equipped cars as they measure air mass :)
Just be aware it may not be fixable :)
Do you think that swapping in a catback STI exhaust would cause hesitation?
When I go WOT the car really hesitates and it feels like there are misfires, pedal feels kind of like a machine gun and the engine sound gets very soft.

Not sure if a grounding issue could cause this but it seems like the problem is more apparent in the rain.
 

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Ex-Mooderator
2001 RSTi, 2011 WRX Hatch
Joined
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3,275 Posts
For people that are having hesitation issues that can't seem to be rectified on modified MAP sensor based cars please note:

Your vehicle is tuned on a speed density system where Volumetric efficiency is told to the ECU, which is used to calculate how much air is entering the motor.
If you change say your exhaust.. This changes how much air will flow through the motor.

THE ECU does not know this has been done and will still only inject the amount of fuel for the air it's expecting!
If your mods created an substantial increase in flow at a given manifold pressure or rpm, it will run lean at this point of increased efficiency.
This leaning out may result in knock as the ignition timing is still on a factory map, possibly causing your hesitation.


I'm in the midst of setting up my Carberry rom for my 04 ADM STI and if I change AVCS in the slightest, I can immediately see the AFR's change in that region.

This doesn't apply to MAF equipped cars as they measure air mass :)
Just be aware it may not be fixable :)
I don't believe this to be true, since the MAP sensor is always in operation and still measures the volume of air coming through at a given RPM, fueling is adjusted accordingly. The car won't automagically run rich/lean if you change something, it adapts pretty quickly.

Folks spent several years running apexi piggybacks/etc trying to tune air/fuel and the ECU just tunes around it in short order.

-- Dave
 

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'00 BRP RS Sedan, '04 FXT PSM
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I had issues like many describe in this thread..

'02 ReallySlow 2.5 N/A slushbox.

: Revs freely in P or N. In cold temps, 40 degrees or lower, I could drive for 3 to 4 hours before issues started.. in 50+ temps, it would take a little as 15 minutes around town normal driving, but if I had to romp on it - say to enter the highway, it would just lay right down and die.. Really bad from a start once it started happening, but fine once moving, and only really bad again if for instance it dropped down a gear - as you were climbing back up the RPM range. Basically, any real load made it crap its pants. (and nearly made me crap mine on a few occasions)

After literally plowing down every list of potential fixes to no avail, scratching my head and standing at a virtual dead end...

I decided to replace the fuel pump...

My buddy and I didn't figure that would be it either and he actually suggested we check it - but, given it's location, I decided, if we were going in, it was getting replaced... so we did... nothing even seemed obviously wrong with the old one, other than the filter media looked like it had been soaking in 10 years worth of gas.

My mechanic buddy figures, since it seemed heat related, both ambient and operational, that the heat was simply building up in the pump motor, and lessening its efficiency to deliver steady fuel under loads, especially if it was hot out, or the car had been running steadily for long periods of time.

The car has never hesitated again since the pump was replaced.

Hopefully this helps some other lost souls on their journey to end the hesitation...

Good Luck SubyFriend
That was going to be the next item that I replaced. Thankfully I didn't have to. Glad you got yours sorted out.

-Brian
 

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'00 BRP RS Sedan, '04 FXT PSM
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Do you think that swapping in a catback STI exhaust would cause hesitation?
When I go WOT the car really hesitates and it feels like there are misfires, pedal feels kind of like a machine gun and the engine sound gets very soft.

Not sure if a grounding issue could cause this but it seems like the problem is more apparent in the rain.
I seriously doubt that an STi catback would cause those issues.

-Brian
 

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2000 2.5RS/C BRP
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Do you think that swapping in a catback STI exhaust would cause hesitation?
When I go WOT the car really hesitates and it feels like there are misfires, pedal feels kind of like a machine gun and the engine sound gets very soft.

Not sure if a grounding issue could cause this but it seems like the problem is more apparent in the rain.
I can say from personal experience that No, an STi catback will not cause hesitation on its own. As previously pointed out, the MAP-based cars do still have a range of adjustment (though not perhaps as expansive as the MAF cars), and as the most restrictive part of the exhaust is the catalytics, the catback will do very little. (FWIW, I ran my car with an untold number of different exhausts on the stock ECU, even just running headers and catpipe for a week or so, never had hesitation.)

Your hesitation issues sound like bad plug wires- when wet, they arc to the block rather than actually getting the power to the plugs. When did you last change them?
 
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