Car diagnosis

[Theguywhobea]

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?

[dalekphalm]

20 minutes ago, 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?

I'm by no means a mechanic - though there are some car people on the forums - but are you running the AC during the issues?

[RGProductions]

I'm not super knowledgeable here, but to me the fact that it only happens when it's hot suggests it's oxygen limited. Are you at really high altitude, or anything else that would lower oxygen levels. 

[Theguywhobea]

5 minutes ago, dalekphalm said:

I'm by no means a mechanic - though there are some car people on the forums - but are you running the AC during the issues?

It happens with or without the AC on.

 

3 minutes ago, RGProductions said:

I'm not super knowledgeable here, but to me the fact that it only happens when it's hot suggests it's oxygen limited. Are you at really high altitude, or anything else that would lower oxygen levels. 

Yeah, that's been my thoughts the whole time, I was very shocked that the new MAF sensor didn't immediately fix it. I am just outside of Boston, so almost sea level.

[Monkey Dust]

I suspect you need to take it to someone who really knows what they are doing. Hot & humid weather causing the issue makes me think air intake system, might be as simple a split in the intake pipework. But ideally it needs to be plugged into a data logger when it stutters to pin down the problem.

[RGProductions]

3 minutes ago, Theguywhobea said:

It happens with or without the AC on.

 

Yeah, that's been my thoughts the whole time, I was very shocked that the new MAF sensor didn't immediately fix it. I am just outside of Boston, so almost sea level.

how bad is the issue?

[Theguywhobea]

1 minute ago, RGProductions said:

how bad is the issue?

The new MAF sensor has smoothed it out quite a bit around town, however it still loses power when trying to accelerate at highway speeds (65-70mph) after I've been driving for about an hour.

[Theguywhobea]

7 minutes ago, Monkey Dust said:

I suspect you need to take it to someone who really knows what they are doing. Hot & humid weather causing the issue makes me think air intake system, might be as simple a split in the intake pipework. But ideally it needs to be plugged into a data logger when it stutters to pin down the problem.

I have an odbii Bluetooth adapter and Torque Pro, the one thing I've seen is when it hard stutters, the engine timings go from being 25-35 degrees advanced to -6 to -20 degrees retarded.

[Monkey Dust]

26 minutes ago, Theguywhobea said:

I have an odbii Bluetooth adapter and Torque Pro, the one thing I've seen is when it hard stutters, the engine timings go from being 25-35 degrees advanced to -6 to -20 degrees retarded.

I'd say that's a symptom, not the cause. It needs to have everything, from every sensor logged.

[akio123008]

35 minutes ago, Theguywhobea said:

The new MAF sensor has smoothed it out quite a bit around town, however it still loses power when trying to accelerate at highway speeds (65-70mph) after I've been driving for about an hour.

You mean it only starts doing it after an hour, or that it does it despite you already being on the road for an hour?

[Theguywhobea]

3 minutes ago, akio123008 said:

You mean it only starts doing it after an hour, or that it does it despite you already being on the road for an hour?

It only starts after about an hour on the road. So after the engine is good an heat soaked. This is not a problem while driving for any length of time during the winter months though.

[mr moose]

Are you putting in decent fuel?  

 

If the ECU is retarding the engine when warm then it sounds like it is detecting knock or some other tempo related electrical issue.  It could even be a wiring loom connector not making a solid connection.

[akio123008]

Well the best I could come up with is that as the engine warms up, the intercooler becomes less effective, which means the density of the air entering the engine is much lower than it's supposed to be, which results in a loss of power.

 

assuming there is an intercooler.

[Theguywhobea]

12 minutes ago, mr moose said:

Are you putting in decent fuel?  

 

If the ECU is retarding the engine when warm then it sounds like it is detecting knock or some other tempo related electrical issue.  It could even be a wiring loom connector not making a solid connection.

Yes, I've tried regular from the major gas station chains, as well as mid-grade and premium with no difference in any of these issues.

 

8 minutes ago, akio123008 said:

Well the best I could come up with is that as the engine warms up, the intercooler becomes less effective, which means the density of the air entering the engine is much lower than it's supposed to be, which results in a loss of power.

 

assuming there is an intercooler.

No intercooler in this vehicle as there is no turbo.

[mr moose]

1 minute ago, Theguywhobea said:

Yes, I've tried regular from the major gas station chains, as well as mid-grade and premium with no difference in any of these issues.

 

No intercooler in this vehicle as there is no turbo.

Then it could be a dodgy knock sensor or related circuit.

[Theguywhobea]

Just now, mr moose said:

Then it could be a dodgy knock sensor or related circuit.

Yeah that was my next area to search, however I can't see knock sensor data from Torque Pro, and I don't have a check engine light on either that might give me some help...

[mr moose]

5 minutes ago, Theguywhobea said:

Yeah that was my next area to search, however I can't see knock sensor data from Torque Pro, and I don't have a check engine light on either that might give me some help...

The other thing it could be is coolant temp sensor.  If that is reading too low when the engine is warm the ECU might still be trying to run the engine as if it was cold.  

 

You may only get an error code on the knock sensor if it fails completely rather than the occasional false trigger.  The ECU won't be able tot tell the difference between a false trigger and genuine knock.

[Theguywhobea]

2 minutes ago, mr moose said:

The other thing it could be is coolant temp sensor.  If that is reading too low when the engine is warm the ECU might still be trying to run the engine as if it was cold.  

 

You may only get an error code on the knock sensor if it fails completely rather than the occasional false trigger.  The ECU won't be able tot tell the difference between a false trigger and genuine knock.

Yes, I think you might be on to something. I checked under my hood after driving home from work this afternoon and noticed that I could not see any coolant in the reservoir, which means it must be well below the low fill line. I guess I'll need to go find out what type of coolant I need and buy a bottle of that.

 

**You'd think the dealership would have topped that up for me after my 8th return trip....

[mr moose]

7 hours ago, Theguywhobea said:

**You'd think the dealership would have topped that up for me after my 8th return trip....

I can't believe you have a car that is only a year old and clearly has timing issues they can't diagnose/fix. 

[Slayer3032]

12 minutes ago, mr moose said:

I can't believe you have a car that is only a year old and clearly has timing issues they can't diagnose/fix. 

Dealerships don't diagnose or fix, they replace. If the computer doesn't tell them how to fix it, they just throw parts at it until the owner goes away.

 

7 hours ago, Theguywhobea said:

Yes, I think you might be on to something. I checked under my hood after driving home from work this afternoon and noticed that I could not see any coolant in the reservoir, which means it must be well below the low fill line. I guess I'll need to go find out what type of coolant I need and buy a bottle of that.

 

**You'd think the dealership would have topped that up for me after my 8th return trip....

Don't forget your OEM Subaru branded headgasket stop leak if you change your coolant! You'll void your warranty if you don't use it lmao.

 

[Theguywhobea]

9 hours ago, Slayer3032 said:

Dealerships don't diagnose or fix, they replace. If the computer doesn't tell them how to fix it, they just throw parts at it until the owner goes away.

 

Don't forget your OEM Subaru branded headgasket stop leak if you change your coolant! You'll void your warranty if you don't use it lmao.

 

 

9 hours ago, mr moose said:

I can't believe you have a car that is only a year old and clearly has timing issues they can't diagnose/fix. 

Yeah trust me guys, these dealerships suck. All they have done is have me sit in the waiting room for two hours while they "test drive" my car, then come back and say it's actually fine and they wont be trying any fixes. They did at one time admit there was a problem, but their only action was to reflash the ECU and send me on my way. I told them it didn't fix it and they said too bad, I could go somewhere else if I didn't like it.

 

The REAL amazing thing is that Subaru Corporate backs up the dealerships claims and says that there is nothing wrong with the car, or that it could be because I have a trailer hitch. They will believe anything the dealerships tell them even if I complain and make multiple cases. they just echoed that I could go somewhere else if I don't like it. I can assure you I will never buy another Subaru, I just want this one to run decent until it's economically feasible to trade it in.

[Theguywhobea]

9 hours ago, Slayer3032 said:

Dealerships don't diagnose or fix, they replace. If the computer doesn't tell them how to fix it, they just throw parts at it until the owner goes away.

 

Don't forget your OEM Subaru branded headgasket stop leak if you change your coolant! You'll void your warranty if you don't use it lmao.

 

Not planning to change the coolant, just top it up.

[harryk]

Sounds like a possible electrical/sensor issue which is the worse for diagnosing. I would search for a local Subaru-focused enthusiast shop. In my experience those who do performance mods and build race cars often encounter the most issues and will likely have different insight then the dealer. 

 

You should also look at your state's Lemon Law. If the dealer/manufacturer is failing to fix the issue then they are required to buy the car back. 

[Theguywhobea]

10 minutes ago, harryk said:

Sounds like a possible electrical/sensor issue which is the worse for diagnosing. I would search for a local Subaru-focused enthusiast shop. In my experience those who do performance mods and build race cars often encounter the most issues and will likely have different insight then the dealer. 

 

You should also look at your state's Lemon Law. If the dealer/manufacturer is failing to fix the issue then they are required to buy the car back. 

Yeah, I have found one shop but they're about an hour and half drive from me.

 

I have looked into the lemon law, however it seems the dealerships can skirt the law pretty easily by not admitting there is any issue, and since the car hasn't come to a halt and quit working I can't prove any damages or loss of vehicle use. Lemon laws unfortunately don't seem to cover these types of issues.

[bimmerman]

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.