BRZ down on power....

A month ago I bought a ‘13 BRZ with a bad throwout bearing. Swapped it out, been driving it since - no problems. Car has intake, JDL header, midpipe, and HKS exhaust with an ECUtek pro-tune done when it was ~40k miles old. Car has 106k miles now.

About 200 miles ago I hit the gas in 3rd and got a backfire. I backed off, eased into the throttle and sure enough it felt down on power. Odd. Did about 150 miles commuting and figured maybe the station gave me 87 instead of 93? Got a fresh tank of gas, no change. Reset ECU - now it’s even more sluggish. The “edge” from cracking the throttle is gone and it really feels down on power. No CEL at any point. Since clearing the ECU the throttle mapping feels SO off, the first 1/4 throttle seems to do nothing. It feels 150hp slow, not 50hp slow.

Naturally, searching for BRZ and “low power” gets me nowhere.

I wish I had checked pending codes before I cleared the ECU. Whoops. Checked them tonight, nothing there. Live data shows TPS only ranges 15-80% which internet says is normal.

O2B1S2 shows near zero voltage but it’s the downstream sensor so it shouldn't matter.

Since the BRZ runs dual injection systems, that’s something I’m suspicious of. FRP PSI shows 583psi at idle. No clue if that’s right. All other parameters normal.....

... how do I even approach this? I was kind of hoping for a check engine light.

Electronic checklist:
IAT/MAP/MAF = possible culprit
TPS = looks good
Coolant Temp = looks good
Fuel injectors? = no idea. I’d imagine I’d have a misfire in a cylinder if it was bad.
AVCS/VVT actuators = no CEL, so not suspected
Knock sensor = wasn’t activated, no CEL
O2 upstream = 2.2V and up. 
O2 downstream = 0.003-.880V, typically around zero at idle?

Mechanical checklist:
pull plugs/check compression = this is a pain on a BRZ lol
check air filter = this is in the front bumper so will take some work
try cleaning MAF sensor

Naturally I just gave notice at my job today and am driving 3000 miles to Seattle in a few weeks. Time to get this sorted.

I wonder if the backfire (of unknown cause) could have damaged the MAF sensor?

rslifkin said:

I wonder if the backfire (of unknown cause) could have damaged the MAF sensor?

The MAF is the only thing I can think of. I need to figure out how to pull the MAF scaling numbers. Right now I have +3.9% for short and -8.6% for long (all my cheap OBD live data will show), which seems odd for a tuned car.

I'll definitely clean it tonight. Car runs and drives fine, just seems low on power.

Shine a flashlight through it as well while you're cleaning and see if there's any visible damage.  

Could the backfire have broken off scale in the exhaust which plugged up the cat?

the dead downstream sensor does matter.  It's a pretty key component in the fuel system, tweaking the fuel a little.

But that it's dead is really odd.  When you say dead- is it a solid zero, or something else and flat lined?  

When you plug in your OBD tool, can you ask it to check codes?  Some will set without lighting the light.  If the MAF is dead, that will light the MIL light almost instantly- so I'd bet it's something else.

Makes me kind of think that something happened to the catalyst, which forced some of the charge backward. 

Another possible issue is VCT- is it operating as intended or not?

Yeah, i'd be focusing first on that one thing on your list that "looks dead".

alfadriver said:

the dead downstream sensor does matter.  It's a pretty key component in the fuel system, tweaking the fuel a little.

But that it's dead is really odd.  When you say dead- is it a solid zero, or something else and flat lined?  

When you plug in your OBD tool, can you ask it to check codes?  Some will set without lighting the light.  If the MAF is dead, that will light the MIL light almost instantly- so I'd bet it's something else.

Makes me kind of think that something happened to the catalyst, which forced some of the charge backward. 

Another possible issue is VCT- is it operating as intended or not?

I've never had a car that used the downstream sensor. I've eliminated them in other subarus entirely and disabled the P420 code. Come to think of it, it could be elimited in this car too. I know I saw the upstream O2 but never looked specifically for the downstream.

By dead I mean showing 0.000V. As I stated above there were no pending codes (codes that are in the system but not setting off the light).

The MAF isn't dead, but it could be dirty. You can drive a subaru with no MAF if you unplug it but it goes into safe mode (MAP only) and it will throw a light fast.

VCT - maybe. No CEL so not sure. The ECUtek might allow active live data for stuff that that specific but I hadn't played with it yet.

Those trim numbers are okay.  8% long term is at the limit you want to see when tuning but that will not cause a power loss.  Generally you won't throw a rich or lean code untill you are above 20-+%.  Knock sensor won't throw a CEL either.

Possible that it's somewhere in the high pressure fuel rail/injectors.  Check the relief valve, etc... If your losing the super high DI pressure this will cause poor combustion which could have caused the backfire as well.  It will be much more pronounced at high load high rpm.  It may be inconsistent as well if a valve is just starting to go out.

In reply to Blaise :

You assume that it's not being used. It's been used as a fuel trimming device for about 20 years in the industry.  So it's not to be ignored.  It's failure can easily trip a failure mode that will make the car run poorly.

A dirty MAF won't cut the power that fast. If it's the cause of that much power loss, it would very much light a MIL light, as it would quickly be an emissions problem.  I know you can drive without a MAF, and I also know it won't do it happily.

But back to the second sensor- 0V on the rear sensor is bad if it's still there.  Something is very wrong.  Find out why, and I bet you find what is causing your power loss.

So rear o2 is def there and it does work although I’m not sure about the voltages.

Upstream: 2.2V and up. 

Downstream: 0.003-.880V. Usually around zero at idle. 

I also have torque pro to try as well.

In reply to Blaise :

The upstream sensor is a WB, so it's range will not be the same as the rear one.  That one needs to be looked at via a different parameter to see what it's out putting.  

The downstream voltages are interesting- normally it will be in the .60-.75V range, idle I can see it be low, but not zero unless the engine is not running.

But it's not dead.  So we can take that out of the symptoms.  Odd, but not dead.

I have had a similar issue when i had my audi a4 1.8t with a tune. I had a cutout on it at the bottom of the downpipe and when downshifting it would pop pretty hard, similar to a gun shot. and being young i did it pretty much every chance i got. one day it was waaay down on power, like hard to maintain 70 mph on the highway. turned out that the exhaust popping destroyed my cat, when we took it out the front half of the cat material had broken loose and had turned sideways greatly impeding exhaust flow. The car never gave me a CEL to give me any leads, i only found it because i was already planning on putting on a test pipe.

I dont know how easy it would be too get to the cat to inspect it on those cars but i'd put it on the list of things to check.

Yea I was thinking dead cat when I read the OP's post - you'd think that would show up with a wonky AFR (and probably a CEL). Also these cars' MAF is very sensitive, but you said you'd clean that & that its working so that's good.

Sounds like your car is probably in the infamous "limp mode", not sure what could've caused that, though. My first FRS did get into limp mode (I think I overheated the trans) and its no fun, feels like there's only ~50hp under the hood.

I have a pickup truck where the matrix in the after market converter came loose.   It was rolling around inside the housing so only stopped up the flow when it turned sideways and blocked the outlet.   

It was down on power while accelerating and it had me looking all over the place for the reason until I figured it out, changed the converter, and every thing was fine after that.    

Well, the header holds the cat and is under warranty. I could look to see how it looks, but only from the outlet side.

I do have an ECUtek and could bolt up the stock header and run a stock tune, I assume.

Kind of a long shot but maybe you could hear the catalyst material rattle around if you gave the header a knock? Assuming it's loose of course. 

In reply to thatsnowinnebago :

Of if he knows someone with one of those endoscope cameras That are available for cell phones and/or computers.

thatsnowinnebago said:

Kind of a long shot but maybe you could hear the catalyst material rattle around if you gave the header a knock? Assuming it's loose of course. 

You may hear it tapping the throttle also

You do realize you bought a BRZ right? 

Kidding, of course. Plugged cat seems reasonable to me.

alfadriver said:

In reply to Blaise :

The downstream voltages are interesting- normally it will be in the .60-.75V range, idle I can see it be low, but not zero unless the engine is not running.

 

What if the heater's gone? I know that widebands will show very lean if they're cold, how do narrowbands work? There's not a lot of heat post-cat to keep that thing lit up.

The classic, redneck but effective method of checking for a plugged cat is to loosen the exhaust flange somewhere before the cat and take the car for a drive.  Even if you do it before the upstream 02 sensor (or sensors) and it messes up the  O2 readings if the cats plugged you should get most of the power back.

Something about the initial failure makes me think that it's a timing issue of some sort.  VVT, timing belt or possibly ignition.  There's not much that can change on the ignition side of things in most modern cars so that ones a long shot.

^^ Hmm, well BRZ/FRS has timing chain so unlikely there. I guess it could be VVT related, OP should be able to see IAM and knock count values via OBD2 (which would indicate a knock-related issue).

EDIT: Also, wanted to say that I had a coilpack go bad on my FRS recently and the CEL *did* go on. Also pretty sure it comes on for a bad spark plug, too. So I'd check things easily diagnosable like OBD2 values ("Torque" app seems to work good if your reader can't pull all those values), and the cat first.

Keith Tanner said:

alfadriver said:

In reply to Blaise :

The downstream voltages are interesting- normally it will be in the .60-.75V range, idle I can see it be low, but not zero unless the engine is not running.

 

What if the heater's gone? I know that widebands will show very lean if they're cold, how do narrowbands work? There's not a lot of heat post-cat to keep that thing lit up.

If the heater is gone (on either sensor) it would light the MIL light- it' a pretty easy thing to detect.

WB's won't go alive until they are hot enough- so that's easy (there are internal check to the system).  NB's generally run at 0V until they warm up, and then they work fast enough.  Given it's importance to the fuel control, Subaru and Toyota would have put a good one in there.

It's the backfire that gets me.  What causes that?  Ignition timing usually in my experience.  I have nothing further to add, just that I think the cause of the backfire will lead to the solution of the issue.