Car diagnosis

[Theguywhobea]

33 minutes ago, bimmerman said:

Figure out what steps a technician would need to do to reproduce the issue, if you haven't already.

 

Sounds like some combination of MAF / O2 sensor to me, though at 18k miles that's definitely lemon law territory.

 

What octane fuel is it rated to use, and do you use it exclusively?

 

Could be fuel pressure related too. Lots of ways this could occur.

Right, I keep thinking it's some kind of sensor issue, which is why I both cleaned the old MAF sensor and tried that, and I replaced the MAF sensor, but that hasn't completely solved my problem.

 

The vehicle is specc'ed to run 87 octane. I always put in 87 octane, except a few times if I get stop and shop gas I'll put in 89 as I feel like they probably run some cheaper gas which could possibly run underspec, I don't have an octane sensor to prove it though so it's more of an opinion thing.

 

I feel like I can rule out it being a fuel system issue as the problem is reduced or eliminated simply by it being cold / low humidity outside.

[d4n1Xd34th]

On 6/27/2019 at 1:01 AM, Theguywhobea said:

Hopefully some of our more mechanically inclined members can give me a hint here. 

 

I've got a 2018 Subaru Impreza 5spd, which has an occasional hard stutter under acceleration, and a general loss of power at high way speeds when accelerating, the catch is that this only happens during the summer months, when it's hot and humid.

 

I've already tried different air filters, and a new MAF sensor to no avail. This car has about ~13k on it, taken it back to the dealership 8 times, made 2 cases with Subaru, no resolution or help.

 

Anyone have a suggestion?

Are you sure its not a fuel pump issue? Sometimes high temperatures can cause the petrol to vaporise in the tank causing pressure issues. Can be adjusted by fixing the problem or at ECU level

[Theguywhobea]

1 minute ago, d4n1Xd34th said:

Are you sure its not a fuel pump issue? Sometimes high temperatures can cause the petrol to vaporise in the tank causing pressure issues. Can be adjusted by fixing the problem or at ECU level

I am not sure, but I don't think it could be the fuel pump as I have this problem even when it's relatively cool out but with high humidity, as we sometimes get in the spring time around here.

[bimmerman]

Check your hoses then. Might have a vacuum leak where the higher humidity air is getting in somehow, though that should also occur at low humidity. If no sensor is throwing codes for mixture or airflow, you're likely looking at some combination of bad combustion-- fuel related, air supply related, and/or spark control related.

 

Another good place to search for issues is on Nasioc, though it's been a long time since I've been on subaru forums.

 

You said you have a manual-- when this occurs are you lugging the thing (WOT at low rpm and high gear), accelerating normally in midrange in a proper responsive peppy gear, or wringing its neck near redline? Does this occur at partial throttle or only wide open? If you unplug the MAF, will it still occur?

[d4n1Xd34th]

7 minutes ago, Theguywhobea said:

I am not sure, but I don't think it could be the fuel pump as I have this problem even when it's relatively cool out but with high humidity, as we sometimes get in the spring time around here.

A common issue is moisture under the distributor cap. But this is highly unlikely as it should go away after a few minutes of driving.

[Theguywhobea]

1 minute ago, bimmerman said:

Check your hoses then. Might have a vacuum leak where the higher humidity air is getting in somehow, though that should also occur at low humidity. If no sensor is throwing codes for mixture or airflow, you're likely looking at some combination of bad combustion-- fuel related, air supply related, and/or spark control related.

 

Another good place to search for issues is on Nasioc, though it's been a long time since I've been on subaru forums.

 

You said you have a manual-- when this occurs are you lugging the thing (WOT at low rpm and high gear), accelerating normally in midrange in a proper responsive peppy gear, or wringing its neck near redline? Does this occur at partial throttle or only wide open?

Yeah I have a very lengthy thread on this topic over at the Impreza5 forum, other members there have had similar issues but no one has gotten a fix.

 

No this is not when I'm lugging it, this happens mostly in the 2000-3000RPM range, and while the vehicle doesn't reach peak horsepower till about ~4500RPM, I don't believe this is the issue, it runs fine at these speeds under low humidity. This issue mostly happens in 5th gear at ~2800RPM when approaching a hill. Even if I put the car in cruise control at ~68mph (which is about ~2800-2900RPM) the vehicle will stutter and slow down until I loose enough speed that I have to downshift 4th.

 

I can however get this issue to occur in any gear, just this morning after driving for about an hour on the high way I was on a side road and going up a hill after turning around, I shifted into 3rd gear and hit the gas, the car accelerated to about ~3500RPM and then experience a hard choke before continuing to accelerate for a short while longer before I let off and shifted to 4th gear.

 

In total I have recorded about ~70,000 miles driving a manual transmission car so I am quite certain it's not due to poor technique, however this car has by far the worst manual transmission I have ever driven.

 

1 minute ago, d4n1Xd34th said:

A common issue is moisture under the distributor cap. But this is highly unlikely as it should go away after a few minutes of driving.

I don't believe a car this new has a distributor cap as it uses the coil on plug direct injection system.

[nbrowser]

1 hour ago, d4n1Xd34th said:

A common issue is moisture under the distributor cap. But this is highly unlikely as it should go away after a few minutes of driving.

I'm a mechanic...I haven't seen a distributor on anything newer than say a '00 save for the classic GM Vortec engines...distributorless has pretty much replaced the spinny thing under a plastic cap.

[Norwegiantweaker]

2018 model... not much you urself can do unless you know how to mess around with the ECU. and if the dealership and subaru wont or cant help you im sure there are some laws or rules on ur side where you can make a propper claim/complaint and have them fix it or replace the entire engine '? 

[Theguywhobea]

2 hours ago, Norwegiantweaker said:

2018 model... not much you urself can do unless you know how to mess around with the ECU. and if the dealership and subaru wont or cant help you im sure there are some laws or rules on ur side where you can make a propper claim/complaint and have them fix it or replace the entire engine '? 

I wish I knew more about this specifically though. Like I've looked over the lemon laws and I'd need to prove some kind of loss of usability. They don't protect against excessive wear or these types of intermittent problems. As many times as I've called Subaru to complain they seem content with just sending me to all the Subaru dealerships in the area and see if any will do anything. I'm strongly considering just trading in the vehicle and cutting my loses before my warranty runs out.

[dalekphalm]

1 minute ago, Theguywhobea said:

I wish I knew more about this specifically though. Like I've looked over the lemon laws and I'd need to prove some kind of loss of usability. They don't protect against excessive wear or these types of intermittent problems. As many times as I've called Subaru to complain they seem content with just sending me to all the Subaru dealerships in the area and see if any will do anything. I'm strongly considering just trading in the vehicle and cutting my loses before my warranty runs out.

If the issue remains unresolved by the time the warranty is nearing completion, I would definitely trade it in (in this particular case, I'd trade it into a non Subaru dealer) and get a different brand entirely.

[Theguywhobea]

1 minute ago, dalekphalm said:

If the issue remains unresolved by the time the warranty is nearing completion, I would definitely trade it in (in this particular case, I'd trade it into a non Subaru dealer) and get a different brand entirely.

Yeah, I am looking at Honda's right now, that's what the rest of my family has and what I've always had good experiences with. I would never buy another Subaru product.