So about 2 weeks ago, I drove my civic in some heavy rain and hit some pretty deep puddles on the freeway and hydroplaned a bit. The next day, my car has a weird start up issue.

When the car starts, it usually will rev up to 1500 rpm and slowly drop until the car is warm and you can hear the intake whistling. Now the car will start up to 900 and immediately drop to 500-600 rpm. Until I rev the car to 2000 rpm, the intake sounds like it's not sucking air. When I drive it, the RPMs will not drop when engine braking, like I can coast at 1000 rpm in 1st through a whole parking lot. Also while slowing down, the car backfires a lot and has a bumpy idle around 2000-2500rpm.

I exhausted a lot of my options including checking many of the intake sensors and the air filter itself. Changed the oil too.

I'll provide a video link when I can
https://youtu.be/xThmeN6N05s

 

CEL codes?

And post a picture of your air intake system.

 

No CEL, Oil light flashes when revving it to get the intake to suck air then stays off

 

Flashing oil light is your greatest concern because it indicates low oil pressure that will damage the engine.

Do you have a splash guard installed to protect the air intake from sucking in water splashed up from below?

Have you opened the distributor to look for water inside?

Does the low idle return to a normal idle if you remove the air intake system?

 

I do not have a splash guard yet but I planned on getting one after that monsoon

The low idle isn't affected by the air intake system, it stays low at idle.

I'll be checking the distributor for sure later today and the oil system later

 

No water in the distributor

 

I would check all the plugs one by one and blow them out with air. I am willing to bet, without the splash guard. Water could potentially have penetrated any engine harness plug. They are supposed to be sealed plugs but with the age of the cars.....

Depending on what shorted out it could also damage the ECU.

 

Does the CEL work? The backfiring is almost surely caused by engine misfiring.

You may be able to fix the idle problem by cleaning the IACV (and also the FITV if your engine has one).

Your air intake filter was likely splashed with water. A wet filter or filter damaged by water will not allow smooth air flow, thereby causing the whistling. Replace the air filter.