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2006 Nissan Sentra won't start after sitting

2006 Nissan Sentra that went unused for about 3 months. prior to that the car worked perfectly. Now the car will not turn over. Interior lights work. When the key is turned it makes a single "thump’ sound and then nothing much. The battery was replaced (and even tried a second) so we know its not that. Shouldn’t be alternator or starter since they were fine prior. Someone suggested “hydrolock” that can happen when a car sits. Any ideas?

Remove the spark plugs and then try starting the engine.

If fluid shoots out the spark plug holes while cranking the engine over, that’s hydro-lock.

Tester

ok Ill do that in the morning. if water does shoot out, then what? Its brutally cold here. Would that water freeze?

Normally the cooling system would have a 50/50 mixture of water/antifreeze. So it shouldn’t freeze.

If water shoots out of any of the spark plug holes, it indicates a breached head gasket.

Tester

And then cars done? Could that go bad just by sitting?

You’re going to find out.

Tester

A car that worked fine then put into storage for 3 months, there’s not much reason to suspect that would cause a hydro-lock condition. Unless you have another reason to suspect a hydro-lock problem, I doubt that’s the cause of this symptom. More likely it is the starter motor is getting a little ornery. If it was near the end of its life anyway, sitting unused 3 months could allow enough oxidation of the various internal contacts to prevent it from powering up now. Starter solenoid contacts, armature, brushes, etc can all oxidize or corrode w/time. If you feel lucky, give the starter a quick knock with the end of a broomstick. It might jiggle it enough to free up the brushes or move it to a better conducting place on the armature contacts. Since you’ve replaced the battery, I presume the posts and connectors have already been cleaned and tightened. If not, try that first.