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Old 01-18-2021, 07:35 PM   #1
Spetnaut
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Default 2017 WRX Cold Weather and Cold Start Misfire

Whenever the temp drops below 40-45 degrees out, my 17 WRX misfires for a few seconds from a cold start. Throws a p0300 random misfire code. If the the engine is warmed up, it will not happen. I tried changing the VVT valves, did not fix the issue. Since it being a temperature sensitive issue, I also tried changing brands of fuel, did not work. Not really sure where to go from here. Any help would be appreciated!
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Last edited by Spetnaut; 01-18-2021 at 07:54 PM.
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Old 01-18-2021, 09:46 PM   #2
X49
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If you have an Accessport, take some datalogs. Look at Cylinder Roughness on each cylinder. A problem with just one cylinder might indicate an injector or ignition issue on that cylinder. Log fuel pressure as well.

Misfires are usually caused by too much fuel, not enough fuel, or poor spark. Could be related to the fuel system, spark plugs, other ignition system components, a bad MAF sensor, or a bad aftermarket tune.
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Old 01-19-2021, 04:58 AM   #3
Spetnaut
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As I stated in OP, it only happens in cold weather and is a random misfire across all cylinders. What you suggest would make sense if it happened in all weather.
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Old 01-19-2021, 06:49 AM   #4
Zak6182
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What mods?
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Old 01-19-2021, 08:29 AM   #5
Spetnaut
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Quote:
Originally Posted by Zak6182 View Post
What mods?

BigSF intake and Cobb tune, nothing more. Do not think this would effect it either, being temp sensitive.
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Old 01-19-2021, 08:54 AM   #6
undyjr
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Quote:
Originally Posted by Spetnaut View Post
BigSF intake and Cobb tune, nothing more. Do not think this would effect it either, being temp sensitive.
I would check you MAF extension harness for signs of corrosion. I learned to keep a spare harness.
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Old 01-19-2021, 10:52 AM   #7
WMTwrx15
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Originally Posted by undyjr View Post
I would check you MAF extension harness for signs of corrosion. I learned to keep a spare harness.

This is the reason I got rid of that horrible intake

I think mishimoto is the only one that keeps the stock maf location so you don’t have to use that dumb harness

But I’m sure that’s his problem..
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Old 01-19-2021, 07:44 PM   #8
su_ba_roooo
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There's a recent TSB for the stock ECU that is supposed to address hard to start/cold start issues. I don't know if AP installed tunes overwrite that part of the mapping or not, or how often Cobb and others update their maps to have non-performance related ecu improvements from Subaru integrated into their base maps.
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Old 12-26-2021, 09:21 PM   #9
Ghost1984
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Default Issue resolved

Quote:
Originally Posted by Spetnaut View Post
Whenever the temp drops below 40-45 degrees out, my 17 WRX misfires for a few seconds from a cold start. Throws a p0300 random misfire code. If the the engine is warmed up, it will not happen. I tried changing the VVT valves, did not fix the issue. Since it being a temperature sensitive issue, I also tried changing brands of fuel, did not work. Not really sure where to go from here. Any help would be appreciated!
Did you find a solution to this problem?
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Old 12-27-2021, 12:55 PM   #10
Elbert Bass
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Quote:
Originally Posted by Ghost1984 View Post
Did you find a solution to this problem?
If you are getting the P0300 code that is a random misfire not attributed to a specific cylinder. Whatever is causing the issue affects all cylinders. First thing I would try is induction/top engine service. Since the 17WRX has the FA DIT engine, carbon build-up on back of the intake valves can/will contribute to this problem.
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Old 12-28-2021, 09:15 PM   #11
mikemay
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Having the exact same problems as OP even same year car and some mods like OP has in cold weather. So far I cleaned MAF did spark plugs.added some fuel injector cleaner in and changing battery tomorrow.(due anyways)
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Old 04-05-2022, 10:37 AM   #12
NotViggs
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Default The issue is...

Hey everyone. I hate when people don't post the answer, so here it is.

I own a FA20 DIT 2018 Forester XT. It was having "airy thuds" and sputtering happening during cold starts. Turns out this is a cylinder misfire. The reason this was happening to my vehicle is because as a direct injection engine, carbon built up in the valves over time. The intake valves need to be decarbonized every 30,000-ish miles. I've been running a Cobb tune on my Forester for the last 30,000 miles. The issue started happening to me at 60,000 miles on the vehicle.

Take your car to a mechanic and tell them you want the intake valves decarbonized because cylinders are misfiring on startup. If a chemical cleaning doesn't do it, they can walnut blast the intake manifold to clear out serious carbon buildup. If the issue still persists, you will likely need to replace your fuel injectors.

Last edited by NotViggs; 04-05-2022 at 12:01 PM.
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Old 09-20-2022, 10:20 AM   #13
bassheadxt
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Did all the common things to fix this issue (2x valve blasting myself) and still have cold start misfire. Looked at my AP logs and other than seeing the stratified startup (retard timing, high fuel pressure) nothing else seems wrong. Nature of the beast?
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Old 09-20-2022, 11:51 PM   #14
Bigfella
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Quote:
Originally Posted by bassheadxt View Post
Did all the common things to fix this issue (2x valve blasting myself) and still have cold start misfire. Looked at my AP logs and other than seeing the stratified startup (retard timing, high fuel pressure) nothing else seems wrong. Nature of the beast?
FXT? You done the upper engine clean? If so it maybe the alternator causing it, there's an updated alternator to fix a P0300 problem.
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