Motor will not stay running when car is started, intermittent problem

I have a 2004 Subaru Outback. This fall I started having intermittent problems starting the car. It would run like a champ, no problems. Then every once in a while when I would try to start it, typically happens after it has sat all night, it would turn over just fine, motor would catch, then the motor would quit running (almost immediately upon start). It is kind of like the old days when your carburetor was not adjusted quite right and you would have to keep one foot on the brake the other on the gas while stopped at a stop light to keep the motor running. The first time it happened we added “heat” as we thought it might be water in the gas line somewhere. It started, then ran just fine. This winter the problem started recurring more frequently. We added injector cleaner, it seemed to help a bit, but did not solve problem. We took it to the shop, and they could not figure out what was going on as nothing is registering on the computer and the car would start just fine for them. Brought the car home it worked OK for a week or so, then started up again. The motor would catch then would quit running almost immediately. We were able to get it running good enough to get it to the shop (kept one foot on gas at stop lights, etc so would not kill). My car has been at the shop for two weeks now. It finally misbehaved on their watch, and they replaced a sensor, but they are not sure it fixed anything. They are keeping it so see if they can get a better diagnosis. My first thought was the gas pump, but they said they had never seen a subaru gas pump fail and usually when a pump goes out, it goes out. They thought it could be electrical, but not sure as the problem is so intermittent. I would like to get my car fixed and back home. It’s a great car; when it is running it runs great. Do you have any ideas what the problem could be? The shop we work with is very good, and they are reluctant to start just replacing parts if it is not going to fix the problem. Anyone have any ideas?? Thank you!

My opinion is that this problem could be due to either an Idle Air Control valve problem or an intermittent lack of residual fuel pressure.
The former is a device controlled by the PCM (computer) that regulates the idle speed. A problem with this unit may or may not set a trouble code for diagnosis.

Offhand, it sounds like an Idle Air valve problem. Cleaning the IAC and throttle body may, or may not, help.
I have no idea what sensor they replaced but if it was an Engine Coolant Temp sensor or Intake Air Temp sensor, neither one of those is likely to be the cause of the problem.

The stalling problem might be caused from a faulty Idle Air Control valve.

The IAC valve allows the engine to idle anytime the accelerator is released. Because you have to step on the brake and the gas at the same time to keep the engine idling points the IAC valve.


On the occasional residual fuel pressure issue mentioned by @OK4450, I’d also add that I find the shop’s comments about fuel pumps to be weird. Fuel pumps fail. Its not a weird thing, and there are no magic Subaru fuel pumps that don’t fail. It’s also the case that one failure mode for a fuel pump is that it will just stop working altogether and that’s it. But that’s not the only way that a fuel pump can fail (or under-perform). Very odd of them to tell you that.

You might ask your mechanic to verify the EGR is working correctly. If they stick in the on position, the engine will idle poorly or stall.

I just checked in with our shop and he indicated that the first thing he did was check and clean the IAC. Because it is so intermittent, it is hair pulling. He thinks that the computer might require resetting, but not sure.

Replacing the fuel filter should be done if that hasn’t been done already. Perhaps even the Main Relay and fuel pump relay should be checked also.