Rough Idle

[Krawl]

Guys... need some help with this one...

Backstory:
2013 BRZ rebuilt title. Bought the car 5 years ago. Showing 87K on the motor. Ran and drove fine. I bought the car as a "father son" project. I taught my kids to drive it (manual). We put about 1K miles on it (really stretching that number to be honest) and then started working on the car. Fast forward to 4.5 years later, sons have 0 interest in helping dad.. and I have the itch to race... so I put the car back together. Over the past 4.5 years, I have started the car and let it idle for a bit but not what I should have.

So fast forward till now. Car is back together but idling rough. Moving it around in the shop, if you let off the gas, it will die. It idles about 450-500 rpm which seems low. When I started the project, it had about 3/4 tank of gas in it. I put a bottle seafoam in it. I also just did a oil change with full synthetic. It also recommend putting a couple ounces in the oil so I did that as well. I do have the magic black box thing on there but I am also getting a CEL. Nothing was done motor wise to the car since it sat. I did put in a new battery last week. That is really it.

I have a nice Snap-On computer (2 actually) and both are saying "cant communicate with vehicle". Something about it is not getting enough voltage from the car? I am not sure there so I can't check the CEL codes.

There is a gas station half a mile from my shop. I just ran it down there (first time on road in almost 5 years). It is a straight shot and speed limit is 45. It did fine going. As I slow down to get in the turning lane, it dies. Fire it up, pull in and as I am pulling to the pump, dies. I put just shy of 5 gallons in it. Leave and it seems okay. Coming back (up to temp now) I try to let it stretch its legs some and notice it wont really go over 4K RPM. That does not seem right when redline is what, 6.5K? I pull back into my shop and it bogs down but does not die. Pull up to the bay door, dies.

Any ideas? Sucks I cant get the computer to read the CEL's. I thought about the MAF sensor but that guy is a $100 gamble. I tried cleaning the throttle body with some throttle body cleaner.

So, I thought I would ask the experts. Let me know if you have any ideas!

-Jason

[sly]

First thing that comes to mind is a vacuum leak. Although it could be anything. I would start by getting the codes read. Since you have a CEL, that's a good indicator of where to start. Check all fuses both under the hood and under the dash. Make sure you don't have a fuse blown to the OBD II port. If it will still drive, take it to Advance, AutoZone, O'Reilly, etc and see if they can read the codes.

You say you put the car back together. How far apart was it? Still leaning toward a vacuum leak... You could also clean the MAF with MAF sensor cleaner. Don't use anything else or it could be damaged. But getting to the bottom of being able to read your engine codes is where I would start.

The fact that it woudn't rev past 4k indicates that the ECU may be in limp-home mode

[Krawl]

Well like I said, I have almost $10K in Snap-On computers that SHOULD read the codes but the OBDII is not putting out the voltage. I didnt think about the fuses. I can check that. I agree, that would tell me a lot.

Didnt do anything really electrical to the car, or major. Painted it, pulled the bumpers, all the glass, all the interior (minus dash), new radio, that black box thing for the track, welded cage, seats, harneses... that is about it. Outside, we did new brakes, wheels and tires. That is about it.

How do you get it out of limp mode? It made it up to speed (45mph) but would not get over that 4K limit.

-Jason

[sly]

Basically we need to find out why it's in limp home mode and address it. Knowing why the CEL is on would greatly improve odds of fixing things, or at least tell where to start. Since the car sat for a long time, you could have bad ported injectors. Maybe they have varnish on them. Basically you need to check air, spark and fuel. But before diving too deep into it, knowing why there is a CEL would be the way to start.

Here's chatGPTs recommendations, to take with a grain of salt:
If your Subaru BRZ has the Check Engine Light (CEL) on but your code scanner can’t read any codes, the issue is almost always in the communication path between the car’s ECU and your scan tool — or in the way the light is being triggered.

Here are the main possibilities:

1. OBD-II Port Power/Communication Issue
Blown fuse — The OBD-II port on the BRZ is powered by the same fuse as the cigarette lighter/12V outlet (Fuse #5 “Backup” or similar). If it’s blown, your scanner won’t communicate.

Corroded/loose OBD-II pins — Bent pins or corrosion can block data.

Aftermarket electronics — Remote starters, stereo installs, or security systems sometimes disrupt CAN bus communication.

Check:

Use a multimeter on pin 16 of the OBD port — should have 12V with ignition ON.

Check fuses in the interior fuse box.

2. Incompatible or Low-End Scanner
Some cheap OBD-II readers can only pull generic powertrain codes, not manufacturer-specific Subaru/BRZ codes.

If the CEL is for something stored in another module (ABS, TCM, body control), a basic scanner won’t see it.

Fix:

Use a scanner with CAN protocol + manufacturer coverage (e.g., Autel, Foxwell, Launch, or Subaru’s SSMIII software).

3. The Light Isn’t Actually the CEL
On the BRZ, the yellow “malfunction indicator” light can be confused with other warning icons (VSC, traction control, etc.).

If it’s a “Service Required” light, it won’t store OBD-II codes — just a mileage-based reminder.

4. ECU/Powertrain Communication Fault
A bad ECU ground, damaged CAN wiring, or a failing ECU can cause no comms + CEL.

Rare, but if the car recently had jump-starts or battery swaps, voltage spikes can damage ECU circuits.

5. Dead or Weak Battery
Low voltage during startup can light the CEL but cause incomplete code storage.

On the BRZ, anything below ~11.5V during ignition can make modules misbehave.

Quick Troubleshooting Path:

Check OBD-II fuse and power at port.

Verify scanner works on another car.

Try a higher-end scan tool with Subaru protocol.

If still nothing — inspect CAN wiring and ECU grounds.

[Krawl]

Okay cool. That 16 pin should be easy to spot (just have to find how it is numbered). Also, I can check the fuses. I know when I plug in the scanner (the larger of my 2), the plug on the port does light up green. I will have to check it this weekend but your right, we are just guessing at what it could be if we dont know code is.

-Jason

[blsfrs]

I had an issue with a Snapon reader not being able to communicate with my ecu. It may have been "blocked" by my ECUTEK program. My cheap Actron code reader worked just fine. A second scanner at my friend's shop wouldn't work either.

[NoHaveMSG]

Without knowing exactly what you had done recently to the car it is kind of hard to make a suggestion. What I can say is that from my experience, along with others, these cars will pop random CEL’s when they sit for a while. I had a bunch that disappeared after a short time. They all seem to be a high/low voltage on a circuit type of code.

[callisto]

The OBD port should be outputting CAN signals if your reader tools are able to do that; or, there’s supposedly a direct CAN tap behind the radio if that’s preferable (in case you want to get an alternate diagnostics tap). The gen1 seem to respond to Toyota CAN/OBD protocols for extended access to various things. It’s possible the OBD port needs to be replaced; check the official service manual for the procedure to test that it’s electrically functioning, perhaps?

[Krawl]

Okay back at the shop today messing with this. I thought I was getting somewhere till I wasnt...

Seems to be a short or something? I dont know if I read it here or where but someone said they had to slightly pull on the OBDII to get it to read. It worked. I had tried to clear the codes thinking if it would work, great before I could read them. When I FINALLY was able to read them, I got a U0073: Control Module Communication Bus "A" Off. I thought this might be related to the short on the OBDII. I cleared it again. I started it and it was perfect! LOL well... for the most part. I could watch the live data through the OBDII. Idle was mid to upper 600's. No codes no nothing. The ONLY thing, (and I was sitting still so dont know if that is the reason), I could not rev it past 3500/3600 range. BUT, when I would rev and let off, it would come back to 600ish and idle out. I could not get it to die!

Then I got to thinking... I could not hear my fans for the radiator. As mentioned, I have the black box thingy that everyone suggested. I did buy it used so not sure if that is the issue. I talked to the guy that makes them when I installed it and sent him pictures but he seems to think everything was fine. There has always been a question on what does what. His pictures and documentation seems to differ what I have. I put the fans on my second switch when I installed it. Switch 1 is traction control. It turns the light no the dash on and off. Prior, the fans were running all the time so I thought maybe they were wired or set to "always on". Well, after resetting the CEL, the fans were off but it ran fine. I could cut the fans on and the would come on but this is the odd part, the live data on the scanner, would freeze? I turn it off, and it would pick up where it was at? Okay... well I dont need live data all the time but the engine seems fine now...

Unplug the scanner. Start the car and right back to the same thing! Rough idle and fans on even though the switch is off. Plug up the scanner and look at the live data. It is low. I turn the car off, check the codes (same) and do another reset. I start the car, it fires up an dies. Okay, I do it again, same. I check the codes again and now I have a P1604 which is a Stat Ability Malfunction. I check the "Code History", and I new see a P0172 System Too Rich Bank 1 code that is stored but not present.

Now the car wont start. Cranks and dies. Nothing more. Think it could be an issue with that black box? Someone asked above what all was done to the car when it was apart. Once again... nothing engine wise. Electrical, we did the black box, a radio, wired a standalone switch system (which the black box is controlled from and interior lights), and put in a new battery. That is it. Nothing with the motor, trans, exhaust, fuel, nothing. Nothing else was touched while the car was down and it ran fine prior.

-Jason

[Krawl]

Okay quick update...

I walked up here, made that post, walked back to the car.

I cycled the keyless alarm thingy (I dont think it is factory but was on there in the beginning and never been an issue). I cycled it to just reset it in case. So, I get it. Lock, Unlock, start (scanner not hooked up), and it starts and idles but does not die. It is back to that same rough start we started with. I turn it off and back to run, no cranked to check the codes. Still the same U0073 but now a P0500 code is showing. That is a VSS code and dont think it would be the problem as we are not moving.

So, back to square one...

-Jason

[Krawl]

Last update for a bit...

So the track box thingy is behind the radio. Hope you guys know what I am talking about... I pulled the radio to access it. There is a harness on the output side that you plug into a harness on the car. I dont remember but I dont think it was ever plugged in anywhere. Then there is a input side that comes from the switch panel. I unplugged the output side to where it goes to the car. Started it and no change. I cleared all the DTC's again and started it and it seemed fine. Fans were off and it ran. Still not over 4K on rev but would not die. Turned car back off, and then back on. This time, idled rough and fans were back on. Seems like when the fans are ON, the car ides rough and dies. On the live data, idling rough I am seeing roughly 495-510 on the RPM side. When it seems to be doing fine and the fans are not on, I am at roughly 670-680 range but this has only happened these two times.

-Jason

[smackrel]

Man, this sounds like it is going to be a weird one. I don't have anything to offer except encouragement.

Keep going, hopefully something will click or someone here will have had a similar experience

[Krawl]

Yeah I hope. Still messing with it and still no luck. Still running like crap...


-Jason