ECUTek Vehicle Wont Start, What Options Do I Have?

[Acanaceous]

I recently had my 2016 supercharged BRZ's engine rebuilt. The builder turned down the compression from stock compression to 9.5:1 without even consulting me. I believe this is a big drop in compression.

After receiving my engine, I reassembled the other parts and put the engine back in the car. My vehicle won't start but it does crank with the RPMs jumping around 350 when it's trying. There's a smell of gas from the exhaust, compression tests are fine, spraying starter fluid down the charge pipe doesnt work, splashed the fuel tank with fresh 93, crank sensor plate is installed correctly, battery is charged, ground wires are correct.

When cranking, sometimes a little rumble can be heard like it's about to start but that's it. I believe the problem is that the engine is running far too rich to even start because the vehicle was previously tuned for 93 on stock compression with the supercharger.

All things considered, I believe my option moving forward is to flash the ECU to a tune that could better accomodate the lowered compression. What options do I have? It was tuned with ECUTek prior. I would like to keep the license and I dont want to tow the car. I own no tuning equipment and I dont know anybody who does.

Thanks for reading this.

[DarkPira7e]

DI ecu has all its bolts?
Coilpacks firmly connected?
MAP sensor didn't change, right?

[Acanaceous]

Upon initial startup and failure to start, I scanned for engine codes and got a P0108 which said something about high voltage and the MAP sensor. I cleaned the MAP, put it back on, cleared the codes, tried again, same issue but the p0108 was gone. Still has the startability malfunction generic code p1604.

The MAP sensor is the same one used before. I tested it following this video's instructions. Readings from this

were as expected.

Ignition coils are firmly attached.

I am only aware of one ECU on the car which is by the passenger side port injector rail. It's fully connected to the wiring harness, fully bolted.

[Eaton PSI]

I would be pushing back on the engine builder. 9.5:1 is pretty low. It's going to use way more fuel when cruising, the DI system needs high comp to work properly. A change like that should not be made without consulting the customer.

[DarkPira7e]

I'm with Eatonpsi, that compression ratio is pretty awful and likely huge tune changes are going to be needed to get it to fire up if everything else is good.
Sounds like a Subaru builder who has never worked on our FA20 and lives in the EJ world

[Acanaceous]

Yeah the work was really cheap. That's probably a side effect of the cheap work that they just did whatever to the compression. The goal was to build this thing up and push more boost, guess the builder got carried away.
The builder has worked on FA20s before so idk if he did something different here or what.

Anyhow, a little update on what I've done so far:

Removed intake manifold, check connections, replaced the crank sensor.
- Noticed there was a disconnected hose underneath the intake manifold, reconnected it.

Checked ignition coils, they are being supplied voltage.

Tried starting with a spark plug removed from engine, sitting in coil, powered by vehicle. Grounded the plug to the engine and got no spark.

P0108 code is gone now and hasnt came back for a while.

The new codes are:
-B14A1 Sunload sensor circuit open
-U0100 Lost communication with ECM/PCM 'A'

I think it's some sort of electric problem. As if the wires get connected somewhere in the harness properly when the stars align and the engine fires for half a second like when I hear a pop every now and again, then immediately fails once it loses connection.


------------------------------------- update 10/26/2026 -------------------------------------

New code in body integrated module
-U0100 CAN-HS no-receive data

Old U0100 Lost communication with ECM/PCM 'A' still exist in keyless access power supply ECM, keyless access collation ECM, and "heating vent and air conditioning".

------------------------------------- update 10/27/2026 -------------------------------------

No more u0100 codes. Just the p1604 startability. Gonna drain the oil and inspect camshaft sensors after having inspected crankshaft sensor and found it to be fine.

------------------------------------- update 10/28/2026 -------------------------------------

"Camshaft" == VVT sensors were fine. Cleaned them out with MAF cleaner, hit it with compressed air to blow out the residual cleaner, reinstalled.

[Acanaceous]

Here's a little summary thus far:

Current Codes:
P1604 Startability malfunction

Previous Codes:
Code Description Reported progression
P0108 MAP sensor circuit high input Initially present; cleared after MAP cleaning
P1604 Startability malfunction Persisted after P0108 cleared
U0401 Invalid data received from ECM/PCM Appeared briefly
U0100 Lost communication with ECM/PCM Appeared after later work; subsequently disappeared


Here's a ChatGPT info dump on the troubleshooting I've done thus far:

1. Engine and vehicle configuration
Item Reported condition
Vehicle 2016 Subaru BRZ, push-button start
Engine FA20, freshly rebuilt long block
Pistons Wiseco forged
Compression ratio Reportedly reduced from ~12:1 to ~9.5:1
Forced induction Supercharger still installed
ECU Original ECU with previous EcuTek supercharger tune
Fuel injectors Stock
Engine harness Original, reused
Battery New
Spark plugs New
Ignition coils Original, reused
Engine oil system Primed; oil pressure reportedly established
Mechanical timing Set by engine builder; not independently reverified

2. Battery, charging supply, and grounds
Test Result
Battery voltage, ignition OFF Approximately 12.5–12.7V reported during electrical testing
Battery voltage during cranking Above 10V
Engine cranking speed Approximately 350 RPM
Engine-to-battery negative voltage drop during cranking 0.120V
Engine-to-chassis voltage drop during cranking 0.000V
Chassis-to-battery negative voltage drop during cranking 0.082V
Multimeter probes touched together 0.4Ω
Battery negative to ground eyelets Approximately 1.5Ω, before cleaning
Battery negative to suspected ignition coil ground terminal 3.0Ω, before cleaning
Corrected suspected coil-ground resistance Approximately 2.6Ω after subtracting lead resistance
Ground connections Reported securely attached
Ground eyelets and battery negative terminal Sanded/cleaned; resistance improved
Engine starting after ground cleaning Still does not start

Interpretation: Your voltage-drop measurements during cranking are favorable for the main engine/chassis grounds. The earlier resistance measurements were higher than expected, but we haven't established whether they reflected actual wiring resistance or meter/probe contact resistance.

3. Ignition system
Test Result
Spark plugs New
Ignition coils Original
Initial spark test No spark observed
Cylinders tested At least two reported in earlier testing
Ignition coil connector Three terminals
Bottom-right terminal, ignition ON 12.4V DC
Bottom-left terminal, ignition ON 0V DC
Top-middle larger terminal, ignition ON 0V DC
Top-middle terminal Continuity to ground reported
Suspected ignition-trigger wire, ignition OFF Approximately 0.7V DC
Suspected coil ground to battery negative, coil unplugged 3.0Ω, before cleaning
Earlier resistance check across individual harness terminals to battery negative Meter reportedly displayed 0Ω on all three
Coil power during cranking Not conclusively measured
ECU ignition-trigger pulses during cranking Not tested/confirmed
Spark after crankshaft sensor replacement Not conclusively retested

The three coil connector terminals have been tentatively identified as power, ground, and ECU trigger, but the exact Subaru connector pinout has not been independently verified.

4. Crankshaft position sensor
Test Result
Original crankshaft sensor resistance Approximately 2.0kΩ
Harness resistance during earlier testing Approximately 46.8kΩ
Replacement sensor Beck/Arnley 180-0571
Replacement sensor installed Yes
Reported voltage during sensor testing Approximately 0.8–0.9V DC
Dashboard tachometer during cranking Approximately 350 RPM, when registering
RPM after sensor replacement and intake hose reconnection Approximately 350 RPM
Engine starting after sensor replacement No

The 0.8–0.9V reading was not a verified crankshaft signal waveform, so it doesn't independently establish proper sensor operation.

5. Camshaft position sensors and VVT connections
Test Result
Bank 2 exhaust cam/VVT sensor connector C25 Oil contamination discovered
Initial cleaning MAF cleaner and compressed air
Connector condition Previously described as slimy/oily
All camshaft position sensors removed for inspection Yes
Contamination on later inspection Small amount of black residue on two sensors
All sensors cleaned and reinstalled Yes
Engine starting afterward No change
Camshaft sensor waveforms Not measured
Cam/crank synchronization Not confirmed

6. MAP sensor and intake system
Test Result
Initial MAP trouble code P0108 — MAP sensor circuit high input
MAP sensor 5V reference Present
MAP wiring continuity Reported good
MAP sensor Cleaned
P0108 after cleaning Cleared
MAP scanner reading at rest Approximately 29 inHg
Intake manifold gasket Previously jostled during installation
Intake gasket orange locating features Observed protruding
Intake manifold Removed again during troubleshooting
Disconnected intake hose Found and reconnected
Engine starting after hose reconnection No

The MAP code cleared.

7. Fuel system
Test or observation Result
Fuel initially in tank Approximately 1/10 tank
Additional fuel added Approximately 1 gallon of 93-octane gasoline
Fuel smell after cranking Present
Spark plug condition after cranking Wet with fuel
Occasional combustion sounds Rumble/pop/sputter
Starting fluid test Did not start engine
Fuel pressure measurement Not performed/reported
Injector pulse measurement Not confirmed
Actual air/fuel ratio during cranking Not measured

Fuel is reaching the cylinders, but the exact pressure and injected quantity remain unknown.

8. Relays and fuses
IG2 relay
Test Result
Relay coil resistance across smaller terminals 164.5Ω
Continuity across larger contact terminals with relay removed/unpowered Open circuit
Relay definitively verified under load No

An unpowered normally-open relay having open contacts is expected; that result alone doesn't establish a failed relay.
Engine compartment fuses
You reported checking the following groups:
- EFI Main 1, EFI Main 2, EFI Main 3
- ECU-B
- IG2, including small and large fuse positions
- IGS, including small and large fuse positions
- Other mini fuses below 25A
Results: Continuity good on the tested fuses, with approximately 12.5–12.7V measured at powered engine-bay fuse points.

Cabin fuses
Fuse/circuit Result
FOG 0V with lights off
TAIL 0V with lights off
PANEL 0V with lights off
HEATER-S 7.5A 0V with ignition ON
HEATER-S physical condition Visually intact
HEATER-S continuity Not conclusively established

The HEATER-S reading was noted but not established as an engine no-start cause.

9. ECU communication, OBD-II, and CAN bus
Test Result
OBD-II CAN resistance, pins 6 and 14 60Ω
Scanner communicates with ignition ON Yes
ECU live data available with ignition ON Yes
Scanner communication during cranking Frequently drops/disconnects
Scanner RPM during cranking Sometimes 0 RPM or unavailable
Dashboard tachometer during cranking without scanner Approximately 350 RPM
Dashboard tachometer with scanner connected Sometimes stops registering
Engine behavior with/without scanner No meaningful difference reported
ECU power during cranking Not directly verified at ECU terminals
ECU ignition command during cranking Not verified

60Ω across CAN-H and CAN-L is consistent with a normally terminated high-speed CAN network, assuming the measurement was made with the network unpowered.

However, the scanner communication dropout during cranking remains unexplained.

10. Diagnostic trouble codes observed
Code Description Reported progression
P0108 MAP sensor circuit high input Initially present; cleared after MAP cleaning
P1604 Startability malfunction Persisted after P0108 cleared
U0401 Invalid data received from ECM/PCM Appeared briefly
U0100 Lost communication with ECM/PCM Appeared after later work; subsequently disappeared

U0100 was reported across multiple systems, including the body integrated module, keyless access system, and HVAC-related communication.

The codes do not establish that the ECU itself has failed.

11. Starting behavior throughout diagnosis
Observation Result
Starter engages Yes
Engine cranks Yes
Cranking speed Approximately 350 RPM
Engine occasionally attempts combustion Yes, rumble/pop/sputter
Engine starts and runs No
Fuel smell Yes
Wet plugs Yes
Initial spark test No spark
Starting fluid No successful start
New battery No change
MAP cleaning No start; P0108 resolved
Intake hose reconnection No start
Crankshaft sensor replacement No start
Camshaft sensor cleaning No start
Ground contact sanding/cleaning No start

[DarkPira7e]

"Tried starting with a spark plug removed from engine, sitting in coil, powered by vehicle. Grounded the plug to the engine and got no spark."

You need to fix this. It is the problem.
Chase the spark and don't worry about anything else right now.

Builder didn't change anything about your wiring harness, right?