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Old 08-13-2013, 08:21 AM   #1
sgoldste01
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Default Engine stumble at cold start

I suspect this is normal behavior, but since I have an appointment at the dealership tonight for a fog light repair, I thought I'd ask to see if I should mention this too.

For the past two mornings when I first start the car, the RPMs will drop to the point where the car shakes a little and I wonder if it's going to stall. It then recovers itself by revving the RPMs high (2500 to 3000 RPM, I think), after which it settles down to a normal idle.

I've seen this on rare occasions since I bought the car 18k miles ago, but this is the first time I've seen it two days in a row. It's possible that this is happening because when I filled the car last weekend, I used premium gas with 0% ethanol instead of the regular unleaded with 10% ethanol that I normally use. So my theory is that the computer is re-calibrating itself to the new fuel type, but that's just a guess.

Has anyone else experienced this? Is it normal, or should I mention it to the dealer tonight?
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Old 08-13-2013, 09:14 AM   #2
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Originally Posted by sgoldste01 View Post
I suspect this is normal behavior, but since I have an appointment at the dealership tonight for a fog light repair, I thought I'd ask to see if I should mention this too.

For the past two mornings when I first start the car, the RPMs will drop to the point where the car shakes a little and I wonder if it's going to stall. It then recovers itself by revving the RPMs high (2500 to 3000 RPM, I think), after which it settles down to a normal idle.

I've seen this on rare occasions since I bought the car 18k miles ago, but this is the first time I've seen it two days in a row. It's possible that this is happening because when I filled the car last weekend, I used premium gas with 0% ethanol instead of the regular unleaded with 10% ethanol that I normally use. So my theory is that the computer is re-calibrating itself to the new fuel type, but that's just a guess.

Has anyone else experienced this? Is it normal, or should I mention it to the dealer tonight?
I got this after a major change in weather, temperature or humidity and still parked outside. I saw this after an 80, turned 40 degree day, again after a nice dry day to a humid, rainy morning. I attributed it to condensation in the intake tract. Mine actually stalled a few times.
Since I purchased a house with a nice, insulated garage, it hasn't happened again.
So just go buy a house with a garage! ;-)

But yes, it's worth mentioning at the dealership just to have it documented.
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Old 08-13-2013, 10:06 AM   #3
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This has happened to me a couple times.

Usually happens on the rare occasion Im driving someone, I turn on my new car and the engine sounds like its elderly. Great way to impress the ladies...
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Old 08-13-2013, 10:41 AM   #4
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This has happened to me a couple times.

Usually happens on the rare occasion Im driving someone, I turn on my new car and the engine sounds like its elderly. Great way to impress the ladies...
I would argue that if your goal is to impress the ladies, then you bought the wrong car. I like my Impreza, but I wouldn't exactly call it a chick magnet.
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Old 08-13-2013, 11:51 AM   #5
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Mine does when I start it and the A/C is on. It takes the computer a minute to take the readings and adjust for it.
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Old 08-13-2013, 12:27 PM   #6
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I would argue that if your goal is to impress the ladies, then you bought the wrong car. I like my Impreza, but I wouldn't exactly call it a chick magnet.
lol subaru's "diverse" advertising might mean the car attracts women... who are attracted to other women!
"they cant help it theyre just built different"
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Old 08-13-2013, 01:06 PM   #7
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Mine does when I start it and the A/C is on. It takes the computer a minute to take the readings and adjust for it.
Interesting observation. I can't remember if my A/C was on both times when this happened. I'll start paying closer attention to this.
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Old 08-13-2013, 01:09 PM   #8
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lol subaru's "diverse" advertising might mean the car attracts women... who are attracted to other women!
"they cant help it theyre just built different"



as far as the stumble/rough idle. My 09 does this also. I was told its normal due to the afr's being high at startup. Then goes away when car reaches temp or like you said a little push of the gas pedal.
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Old 08-13-2013, 01:14 PM   #9
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My car started doing this regularly when I had to start using the A/C this spring and summer. It caught me off guard as it sounded like the car was going to stall. I just leave the A/C on always, so my car does this almost every time it starts up.

Mention it to the dealer and see what their explanation is. Curious why it does that.
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Old 08-13-2013, 01:24 PM   #10
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as far as the stumble/rough idle. My 09 does this also. I was told its normal due to the afr's being high at startup. Then goes away when car reaches temp or like you said a little push of the gas pedal.
What are AFRs?
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Old 08-13-2013, 01:36 PM   #11
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What are AFRs?
Air/Fuel ratio. That's what determines the amount of fuel the computer or ECU will deliver to achieve a safe level. It is the lean and rich fuel conditions. . A stock ECU will normally base things off the parameters of the AFR setting in factory program. Usually 13-14 and change. With aftermarket ECU, or mine anyway, you look at it backwards. You program to achieve your parameters you want and stay within a certain AFR range. The stoichiometric A/F is 14.7:1 I think. That's 14.7 parts of air to 1 part fuel. The higher the number the leaner it is, lower number its rich. The leaner it is, the higher temps you'll see. Mainly EGT, which for a boosted engine is not good. So when tuning you look at AFR for fuel settings, for timing settings etc.
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Old 08-13-2013, 01:39 PM   #12
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Originally Posted by 79letour View Post

lol subaru's "diverse" advertising might mean the car attracts women... who are attracted to other women!
"they cant help it theyre just built different"
Hey, Another perk of having a Subaru. ;-)

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Old 08-13-2013, 08:56 PM   #13
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The dealership checked for fault codes and didn't find any. So they suggested I monitor the situation, and if it persists to leave the car with them overnight so that they'll be able to witness the situation themselves first thing in the morning. They said they would provide me with a loaner car, since I would be leaving my car with them.

I love my dealership. Very fair and honest.
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Old 08-13-2013, 09:12 PM   #14
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The dealership checked for fault codes and didn't find any. So they suggested I monitor the situation, and if it persists to leave the car with them overnight so that they'll be able to witness the situation themselves first thing in the morning. They said they would provide me with a loaner car, since I would be leaving my car with them.

I love my dealership. Very fair and honest.
That's kind of them, but its a very normal condition for a small engine vehicle. My hemi does it too, you just don't notice it as much since being a much larger engine it recovers before its noticeable. Hopefully that's your situation and your car doesn't have cardiomyopathy.
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Old 08-14-2013, 07:46 AM   #15
sgoldste01
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Well, the engine stumble happened again this AM, even though the A/C was off (and I even confirmed that the ventilation wasn't on one of the defrost settings). So this behavior has nothing to do with the A/C compressor.

My theory continues to be that I last filled with premium 0% ethanol gas, which is not my normal gas selection, and that this change is causing the computer some initial calibration confusion at start-up. My next fill up will be with regular unleaded 10% ethanol gas, and then we'll see what happens.
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Old 08-14-2013, 08:26 AM   #16
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Well, the engine stumble happened again this AM, even though the A/C was off (and I even confirmed that the ventilation wasn't on one of the defrost settings). So this behavior has nothing to do with the A/C compressor.

My theory continues to be that I last filled with premium 0% ethanol gas, which is not my normal gas selection, and that this change is causing the computer some initial calibration confusion at start-up. My next fill up will be with regular unleaded 10% ethanol gas, and then we'll see what happens.
Were you feeling fancy at the pump that you wanted premium? higher octane does retard the burn so the mixture is throwing the spark timing off from what your car is normally used to at startup....
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Old 08-14-2013, 08:49 AM   #17
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Were you feeling fancy at the pump that you wanted premium? higher octane does retard the burn so the mixture is throwing the spark timing off from what your car is normally used to at startup....
I didn't buy premium because I wanted premium. I bought premium because that's the only grade of gas that comes with 0% ethanol (at least where I live). I would have preferred to buy regular unleaded with 0% ethanol, but that's not an option. If I want 0% ethanol, then it has to be premium.
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Old 08-14-2013, 09:09 AM   #18
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Then it was the change between it, as I run premium no E on every tank and this happens when I have A/C on. Regardless its the same thing. It can happen if the air is not on, but the air temp changed a notable amount from what it was expecting. It's just that it needs a second to recalib its settings. There is a delta somewhere, mine normally happens when I leave a/c on
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Old 08-14-2013, 10:45 AM   #19
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Mine had a quick stumble this am, no A/C but its 66 deg this am and was 89 when I came home lastnight. But interestingly enough my blue light was only on for less than 10 seconds.
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Old 08-14-2013, 11:43 AM   #20
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I would argue that if your goal is to impress the ladies, then you bought the wrong car. I like my Impreza, but I wouldn't exactly call it a chick magnet.
Well that's because you bought a hatch. haha jk

Mine rarely stumbles, and I only ever used 87oct 10e

Last edited by myrt1987; 08-14-2013 at 12:38 PM.
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Old 08-14-2013, 11:49 AM   #21
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I put gas in every time it gets near half a tank. I run any kind of gas and have never had this issue.
It gets better mpg's with the 90 non-ethanol, but it runs smooth and quiet on 87-93 octane.
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Old 08-14-2013, 02:39 PM   #22
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Happens with my '12 Impreza now and then. Engine stumbles, then catches, with revs fluctuating for a few moments. Doesn't stall though, and quickly settles into a steady high idle. No relation to gas I'm using, nor weather, I don't think. I think it's just a quirky way it deals with "choke", along the lines of the AFR thing mentioned by Echodeltaseg. That high idle is rough, too. I asked my dealer about that and he said it's the way they all are, related to achieving the (ridiculously named) PZEV specification.
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Old 08-15-2013, 10:05 AM   #23
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This happened to my 12 cvt sport limited this morning. First cool morning start in a while, rumbled a little then warmed up as normal. No fault codes or anything hope it's just cold start related.
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Old 08-15-2013, 11:22 AM   #24
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And ironically, this AM was the first morning in 4 days that my Impreza didn't stumble upon start-up. Go figure.
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Old 08-15-2013, 04:13 PM   #25
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Yeah, it acts like some cold-start sensor they use works a bit clumsily. Or maybe there's variance in the manufacture of it, since it seems common on these cars, but not to the same degree. I always have the rough idle on cold start, but the stumble has only happened a few times in 26,000 miles. I haven't tried using non-ethanol gas though - maybe that confuses it more than usual.
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