Occasional hard start possible causes?

Question

I have a 2012 Subaru impreza, manual transmission. Most of the time it works great, starting with no trouble. However, from time to time it will have trouble starting. At those times, if I either a) keep cranking for 5-7 seconds, or b) stop cranking and wait 5-7 seconds before trying again, it will eventually start - I've never had it not start.

This does not appear to be temperature dependent - I've had the issue both in the middle of summer and the middle of winter. It does seem to happen most often when parked on a slope, however I haven't noticed if being sloped forward or back makes a difference. And of course it's too intermittent to be able to take it to a shop and say "see, it has problems!"

What are some possibilities of what could be causing a problem like this?

Answer 1

The fuel pump has a check valve inside to retain fuel rail pressure. If that valve fails, you have to crank the engine for around 5-10 seconds to refill the fuel line and rebuild pressure. You can test this yourself with this trick. Next time it doesn't start right away, turn off the key. Then turn the key to the RUN position for 2-seconds, then off again. Repeat 3 times. That causes the fuel pump to prime the fuel line 3 times. If it starts right up on the 4th try, then you probably have a failing check valve. A check valve can fail completely or intermittently.