2017 Subaru Legacy - Engine stmbles

Stumbling starts after a few stops and starts when the engine is warm. 2.5l with 51k. This seems to happen after driving the car for most of the week, so on Friday for example and after short trips and starts and stops seems to get worse after the forth or fifth start… Engine turns but struggles to get fully started.

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have your alternator and battery checked.

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Having had alternators go in two Subarus, a Legacy and a Forester, I agree with WW. It sounds like the symptoms I had prior to the alternator going. And it weakened the battery so that also died (first). I had the Forester’s alternator replaced last year. The dealer quoted $1K to do the job. So I’d suggest not using a dealer. Subaru replaced mine because there was a TSB related to frequent Subaru alternator failures and I appreciated that goodwill since mine was out of warranty.

Does it turn at its normal cranking speed? If so, the battery and alternator are not the problem. A failing crankshaft position sensor is more likely. They fail intermittently when hot, and eventually fail full time.

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Hi there,

Yes, it does turn at normal cranking speed. I just had a new battery installed. The car was taken to the dealer to see if they can figure out the issue. They did a capacitor discharge and idle relearn, after I told them a new battery was installed and suggested if that does not stop the stumbling warm starts then replacing the spark plugs and air cleaner and preform a top end clean and idle relearn since the car is close to it’s 60k service, it’s at (51k). I think the tech is just throwing darts and doesn’t know.

I don’t know much about a crankshaft position sensor but that makes sense to me, the car starts fine when cold and the first few times after that, then the stumbling starts. I do a number of short trips during the day for work.
Let me know your thoughts.



I agree. Kinda scary, especially coming from a Subaru dealer’s mechanic.

Is this stumbling you speak of just during the start cycle or does it stumble while you are driving/accelerating?

The stumbling is just during start cycle, it’s being referred to as extended start. The car runs great the rest of the time.

Extended start could be a couple of things, but if the starter is spinning the motor at normal cranking speed, that rules out the battery. If it is cranking slower, even just a little slower, then the issue is that you are not charging the battery enough on your short, low speed trips.

This happens during peak weather (hot or cold) when running the fan on high. The fan uses more power than the alternator can provide under those conditions. It slowly drains the battery during the day. The battery chemically recharges itself over night, but that severely shortens the life of the battery.

The other most common reason is that for some reason, the extra shot of gas that the injectors are supposed to supply to start the vehicle is not being provided. The amount injected is usually determined by the outside temperature. Just a guess but I think that could be due to the intake temperature sensor. That is part of the MAF (mass airflow sensor). You’d expect a code for that but as long as it is providing some sort of signal, I don’t think the computer would detect that the signal provided is not accurate.

Edit: Does the dash display for outside temperature look reasonably close to you?

Additionally, if the OP is using the seat heaters, those devices use a lot of electricity.

I have just replaced the battery in November (on my third one) I rarely use any unnecessary devises, and leave the lights off while driving during the day, to the point of leaving the radio off as well.
The car starts fine when the engine is cold. So if I let the car sit for a while and it cools down it will start up as normal. It’s when the engine is still warm after a short stop and start. I drive the car for work so there are a number of starts and stops during the day. I’m having to manage this due to the starting/stumbling issue fearing the car may not start at some point.

well, nobody seemed to ask… is there a check engine light on or any codes stored?

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also is there a aftermarket alarm system?

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No, no after market alarm system.

you might want to delete your cell number and email. this is an open forum and you never know who comes here.


Not sure if this could be the case, but I’m still thinking temp sensor. If the engine is warm but the air is cold and for some reason the computer doesn’t see the engine temp, it could inject too much gas on start up, effectively flooding the engine. But that should be causing other problems and give you a check engine light.

Edit: I think I may have it, you have an air bubble in your cooling system that forms around the ECT (engine coolant temperature) sensor so on hot start, the engine temp sent to the computer is incorrect. The computer would not detect this.

Subaru’s are notorious for being difficult to bleed the cooling system. It takes special equipment that the dealer has. They do NOT self bleed.

Edit: Cooling system service required at 11 years or 110,000 miles. Have you passed 110k miles and had the cooling system serviced recently?

No, check engine light on.
I had the car into the dealer in December to get this addressed, and they were able to experience the starting issue, and performed a relearn on the idle after I told them I had the battery replaced, but really had no idea what was causing the starting issue. They did not mention any codes that came up.
The tech then suggested that if that didn’t fix the problem then suggested replacing the plugs, and doing a top end clean and idle relearn since i’m close to the 60k service (currently at 51k) but in the next sentence said that this may not solve the issue and further diagnosis & repairs maybe required. :confused:

I’m starting to think, it’s a sensor that is starting to fail and does so when it’s hot.
The crank speed turns fine it seems like there may be some lack of fuel possibly? It does turn over but clearly struggles. Then once the engine is running it’s all good.
One person suggested a failing Crankshaft position sensor that can intermittently tend to fail when it’s hot. This is a possibility as well. I have an Appt with the dealer on the 27th. I want to have some better ideas of what could be going on, b c i’m not sure they will?

No check engine light is coming on. It seems more like a lack of gas then too much.

just a thought… your vehicle has a fuel sock and also a fuel filter in the tank attached to the fuel pump. I believe your owner’s manual says to change it every 60k miles. (The dealer will probably change the whole assembly then)
maybe it’s causing your problem. or the pump heats up causing the hard start. this video will show the 2 filters.
most vehicles usually just have the sock.

Subaru Fuel Filter Change (in tank) - YouTube