Vehicle: 2018 Subaru Forester F4-2.5L DOHC, Automatic Transmission/Transaxle

Mileage: 98,955 miles

Problem: The Subaru was dropped off at the shop because the malfunction indicator light (MIL) was on, and when starting each morning, the engine cranked too long and would occasionally stall after starting. For the rest of the day, the engine would start and run fine.

Case Details: The technician’s first thought was that the fuel pump check valve might be stuck open, allowing the fuel pressure to bleed down overnight. He connected a fuel gauge to check the fuel pressure and the fuel pump check valve. The pressure was at specifications and the check valve held line pressure for a few hours. He connected a scan tool and found the following diagnostic trouble code (DTC): P1604 – Startability Malfunction.

When the technician called ALLDATA Tech-Assist, the consultant recommended starting with the basics. He said to look at scan tool data while the issue was occurring, paying close attention to abnormal readings such as the engine coolant temperature (ECT), intake air temperature (IAT) and throttle position sensors (TPS). He also recommended checking fuel alcohol content then fuel trims to see if they are adding or subtracting fuel at idle (since the problem could be caused by a small vacuum leak). Finally, he advised checking and cleaning the mass airflow sensor (MAF) and the throttle body.

Confirmed Repair: As suggested, the technician reviewed scan tool data. All sensors were in specifications, fuel alcohol content was low, and the fuel trims were okay. Next, he cleaned the MAF sensor and throttle body. While the engine was cold, he cleared the DTC and the engine adaptations. The engine started immediately and ran perfectly on the test drive. Problem Solved!