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HCFFHC
2017 Forester 2.0XT
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253 Posts
Discussion Starter · #1 ·
So today after washing the car, I was pulling out of the stall and into traffic. So I gave it a little gas, it hesitated, and then I heard the dreaded rocks against metal (detonation). So I was easy on it the rest of the way home. Did not throw any codes. I have taken it back out and got into a little boost, 7lbs or so, and no detonation. Anybody ever run into this after washing your car?


And my car has 11976 miles. It has had an oil change Mobil 1 full synthetic three times since I have owned it. Once at 4839 miles just because I didn't trust the dealership, 8236 miles 6 month oil change, and 11071 6 month oil change. I do 6 months or 6000 miles
 

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2016 XT
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282 Posts
My 2016 MY (same as your 2017 MY) started doing this from around 16000 kms every now and again, (no washing) I took it back to the dealer a few times just to be blown off with No fault found and or they updated the ECM, still to no effect.. So I got it a mild etune and its never done it again (touch wood) in around 15000 kms.
 

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HCFFHC
2017 Forester 2.0XT
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253 Posts
Discussion Starter · #3 ·
Only mod I have is high flow resonator and nameless muffler deletes. I wouldn't think it would affect performance. Maybe I need to break down and buy a Cobb tuner
 

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2016 XT
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282 Posts
Mine was completely stock and it was still happening. Just to be clear.. My Etune involved data logging also.. Not just an OTS map. :thumbsup:
 

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2015 Forester 2.5i Premium 6-speed manual
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177 Posts
Did you wash the engine compartment? If so how? Get water in any part of the intake system? What brand and grade of fuel? Was the engine cold when you left the car was. Was it a cold, cool, moderate, or hot day?

I understand that by way of the oil change intervals you have driven less than a thousand miles a month. Correct?
 

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HCFFHC
2017 Forester 2.0XT
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253 Posts
Discussion Starter · #6 ·
No, engine compartment was not washed. 91 Octane Shell gas. Roughly 60 degrees.


I usually average 500 miles or less each month. Way less in the summer cause the Harley comes out.
 

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2015 Forester 2.5i Premium 6-speed manual
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177 Posts
My guess is that you had a transitory intermittent lean misfire, in the absence of any other evidence, in a high compression turbocharged motor. It happens in normally aspirated motors too, but with low octane fuel - or the cheap stuff. Possible vacuum leak due to a loose hose or fitting upstream of the intake valves, but I think not likely, but possible.

I hear what you describe all the time from cars on the roads. Like when the when they pull away from the curb, driveway apron, or passing someone walking in my neighborhood and they want to go a bit fast than is prudent in a residential neighborhood. There is a name for it: "Low-speed pre-ignition (LSPI), also known as stochastic pre-ignition (SPI)."


Read more here:

https://www.oronite.com/about/news/low-speed-pre-ignition.aspx

https://en.wikipedia.org/wiki/Low-speed_pre-ignition



Nothing really to worry about, but I would put in some driving time to get your motor out an breathe and operate at operating temperatures for a while to "clean its internals" if you will, from the spark plugs to the oil. You must spend a lot of driving short distances at low speeds with very little time at full operating temperatures. Though far more complicated than this, turbo motors by their nature as machines need to stretch their legs a bit. Short distance low speed driving is actually severe driving.
 

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HCFFHC
2017 Forester 2.0XT
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253 Posts
Discussion Starter · #8 ·
Its got a 3200 mile round trip coming up in September. And I try to get her out on the highway whenever possible just to get her into boost.

I live 1 1/2 a mile from work. So alot of driving is from home to work, vice versa. Working 12hr shifts kind of kills the day for you. But here in the next month she will be a garage queen. Only coming out to run errands with the wife. Harley doesn't have enough storage to do grocery shopping. YET (Trying to talk the wife into the touring bags.....lol)
 

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2015 Forester 2.5i Premium 6-speed manual
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177 Posts
@RuckU2,

If your motor is anything close to my Golf Alltrack 1.8TSI, you are maybe in boost more than you may realize. Being in boost at low RPMs was one of the aural pleasures of both my former 2012 Golf TDI and 2012 Touareg TDI Lux. That and high MPG's, respectively for each, and the vast amount of torque on tap when needed, especially at long distance Interstate speeds of 65-90 mph and at 2100 - 3,200 rpm in the Golf.

Your September trip will do your Forester good.
 

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2016 XT
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282 Posts
Does the noise sound like this?


If so It needs addressing.. or one day you may see the insides of you motor were they shouldn't be.
 

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2016 and 2020 Foresters
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1,574 Posts
This was an automatic car wash? Did you leave the engine running during the wash? It may have simply inhaled some water.

You should be tuned for those exhaust mods.
 

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2018 X3 M40i / 2016 X3 xDrive35i
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1,632 Posts
I think there's a fairly simple explanation related to heat soak. If you took any time at all to wash the car and you turned it off, it was sitting there heat soaking the intercooler.

On a hot start, it's going to take a bit of time running or driving to get enough air to cool down the intercooler. If you rolled into the throttle right after pulling out of the car wash, you likely didn't give it that time needed.
 

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‘14 FXTT; '15 Legacy FB25
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This was an automatic car wash? Did you leave the engine running during the wash? It may have simply inhaled some water.

You should be tuned for those exhaust mods.
Based on how things are routed, this is VERY unlikely.
 

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‘14 FXTT; '15 Legacy FB25
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I think there's a fairly simple explanation related to heat soak. If you took any time at all to wash the car and you turned it off, it was sitting there heat soaking the intercooler.

On a hot start, it's going to take a bit of time running or driving to get enough air to cool down the intercooler. If you rolled into the throttle right after pulling out of the car wash, you likely didn't give it that time needed.
Even if the OP came into the car-wash with an already heat-soaked IC, it's very unlikely that he would leave with an even worse condition. It also sounds like the OP left the wash and immediately accelerated. These things combined lead me to believe that EXCESSIVE heat-soak isn't likely.

Heck, I've been in stop-and-go traffic, then, when things cleared up, stomped on the go pedal, only to receive substantially less go than usual. I've never experienced actual knock during these times.

The only time I experienced knock was during a random acceleration onto an on-ramp, where the CVT seemed to stick with an erroneously low RPM. After a second of what can only be described as LSPI-induced rock-tumbling, I let off the accelerator and never, ever experienced the same thing again. I did have the dealer flash the ECM soon after....again....to resolve this, so maybe that did something.

One thing of note: my current '14 FXT is NOT the same beast I purchased in 2013. It starts every time now and doesn't knock at all, but it hasn't been the same since the nth number of reflashes. Solution: Cobb...
 

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HCFFHC
2017 Forester 2.0XT
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253 Posts
Discussion Starter · #15 ·
In this carwash you leave your car running. I did sit in line running for 30 minutes and 5 minutes in the wash. And the exit is right into traffic. So yeah I immediately tried to throttle into traffic but got knock instead. It has not done it since. If it does then a trip to the dealer may be necessary
 

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2015 Forester XT CVT
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27 Posts
I have had a similar experience to gathermewool in that I have only had one really noticeable LSPI occasion. I had started the car from cold and driven only about 1/2 mile and accelerated slightly when I heard the can of blots rattle. I normally drive in S# but had not switched over so I was in I. The temperature was about 65 F and it was dry. I was on a vacation and had driven about 4,000 miles in the week before. I think the low rpm in I was part of the problem but it has not happened again and that was 3 years ago.

I have an STI catback exhaust as the only mod and run regular 87 gas all of the time. I like the torque and do not normally race at high rpm so do not need the premium cost here in Canada.

So, from my experience, this can happen without there being an obvious cause and has not, in my case, caused any problem.
 

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‘14 FXTT; '15 Legacy FB25
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I would love to see a log of your engine running on 87AKI. I wouldn't ever risk that and I for-sure wouldn't want to be the second owner.

Not only is there a high risk of inaudible detonation occurring, there could also be additional risks due to things like fuel dilution, since I'd imagine to mitigate knock, Subaru's ECM logic probably dumps additional fuel and/or pulls timing to keep things cool and calm.

I honestly don't see any benefit to the practice of using a fuel octane lower than what the ECM is capable of maxing-out on. This logic seems akin to buying an STI, then buying ultra-low rolling resistance tires to save some gas money.

With that said, I don't know the cost difference between 87 and 93 AKI there. Here, the additional several dollars per fill-up is easy to justify...as was my decision to purchase a turbo instead of an N/A Subie.


I'll be glad, however, when you post up that you've surpassed 100k, 150k, 200k miles without incident.

/rant
/OT
 

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2015 Forester 2.5i Premium 6-speed manual
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177 Posts
It might be a good idea to read the known facts provided by the vehicle owner in this thread. And then recall the TV show Dragnet. Just the facts.

This is a vehicle that is driven apparently under 5 miles a day on a typical day. On the day in question the ambient air temperature was 60 degrees Fahrenheit. There is no heat-soak in the true sense of the phrase in an automotive technology context in this situation to have impaired the intercooler in anyway and induce detonation - be it excessive heat soak or not. This is true, whether, as the owner stated, they idled for 30 minutes and spent 5 minutes washing the car and then entered urban street traffic shortly thereafter - or 30 minutes with the engine off, the ran the engine for 5 minutes while washing the car and then entered urban street traffic shortly thereafter.

It is just, based on the known facts, a case of brief LSPI and nothing more.
 

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2018 X3 M40i / 2016 X3 xDrive35i
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I'm not sure what experience you have in monitoring heat soak to be able to make that claim but I spent years racing with probably 100+ passes at the drag strip in the exact same conditions and on busy nights where I had to wait 20 minutes in line between passes I had to mitigate heat soak or I would detonate. Faster passes back to back would be no problem...but that cool 60 degree air doesn't do any good when your underhood temps are seeing no air movement and your intercooler is soaking all that heat in.

I mean....the reason you feel power loss even without detonation events in these conditions is because the factory tune is attempting to scale back timing, boost, commanded torque, etc to try and proactively prevent detonation. Sometimes it doesn't get it right. It's not a big deal...but it's also not a good idea to roll onto the throttle after sitting idling for 35 minutes based on my fairly lengthy experience with ice packs, propped hoods, captured logs, and timing slips.
 
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