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Tl;dr: Car is having trouble starting, and I'm wondering how to determine if it's the alternator or the starter. I don't experience the longer turn over times of a typical starter malfunction, but I also don't see the dimming lights or no engine rpm of a bad alternator, even after a week's use.

Background info My car had trouble starting ~8 days ago our timeline starts there

Mon, day 1: The engine tried to turn over once then refused to try again. First time car has ever had trouble starting. Headlights still worked, so did radio and other electrical doodads. Seemed to start a little slower after work, but in the coming days...

T-F, days 2-5: No difficulty starting or driving. My work is ~30 minutes away and I take the interstate.

Sat, day 6: I had trouble starting it late at night and needed to jump it. Same style of thing as on day 1. It had been sitting since Friday afternoon.

Sun, day 7: Car sat for the whole day

Mon, day 8: Trouble starting same as before. Advance ran their electrical diagnostic and did not think it was an alternator or battery problem, though they noted the battery needed charging even though I drove ~30 minutes to get there. Advance seemed to think it was the starter because of low cranking amps, even though the battery drains while the car is off. I had them fully charge the battery. I also ran the car for a bit at night.

Tues, day 9: No difficulty starting, but now check engine light is on.

I was hesitant to think it was a starter problem, since I've never experienced longer turn over times, but also doubt an alternator issue because I was able to run the car for so long after the first jump, and it seems to start well if I just drive it regularly.

Is the situation I've described more indicative of a bad alternator or bad starter?

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I would blame the battery. A weak battery can hide that it is at fault as long as the engine starts quickly. The capacity of the battery could be getting lower and lower over a long time, but you wouldn't know because the engine starts and then the alternator takes over. However you start to reach a point where the battery starts to struggle to start the engine due to low capacity. Sometimes the engine will start immediately, then other times it just talks a little longer to start, but the battery isn't able to supply the power, then you are in the situation you are in now.

What you need to do is start the engine, turn on the lights and the heated rear screen then check the battery voltage. If it is 13-14 volts, then the alternator is fine. Then get a new battery.

  • I ended up doing exactly that and I got a little more than 14 volts. The alternator is charing the battery while running at least. – DeepDeadpool Nov 5 '15 at 18:02
  • 14.4v should be the maximum, so your alternator is working fine. I would go for a new battery. – HandyHowie Nov 5 '15 at 18:30
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To address your question directly, I'd say neither - I would suspect the alternator. If your alternator is faulty, it would not provide enough 'juice' to power your spark plugs while driving, which would drain the battery and make it more difficult to start the car. A faulty alternator is typically intermittent, which would explain the intermittent issue you've described.

Barring that, and assuming the battery and starter are indeed fine, I would next check the spark plugs and finally fuel injectors.

  • If the vehicle runs fine while it is running, why would you suspect the spark plugs or fuel injectors have anything to do with a starting issue? These are completely unrelated. – Pᴀᴜʟsᴛᴇʀ2 Nov 3 '15 at 23:31
  • The OP didn't really say that it ran fine. He said he had 'no problems' driving to work, but that's not to say that it was not running sluggish. Besides, these were simply fallback suggestions, in the case that the alternator, battery, and starter are all fine. – wesanyer Nov 4 '15 at 16:39
  • It was not running sluggish, but thanks for the suggestion. I'm actually just about to check for parasitic draining, which may be the alternator I think – DeepDeadpool Nov 5 '15 at 2:25

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