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Hey guys,

It's my first time posting here and hoping to get a little help. I purchased my mustang about 2 weeks ago. It's a VERY cold day and i start my mustang, I then go out to clear snow out of the windshield(took a while). When I come back in my engine starts a VERY loud humming sound, when it stops the oil pressure tanks all the way down and then the rpm's tank all the way down(all on idle). I turn it off then on again and the rpms range between 400-750 sporadically(idle). I turn it back off and check the air filter then turn it on again and just drive off.

I've heard this noise(almost sounds like a cruise ship taking off) before and one time was able to open the hood to try and check where it was coming from. Originally I thought it came from the left side but it seems to be coming from under/around the air filter on the right. Nothing seems to be wrong with the air filter or any blockages.

Two more things to note, the mustang does have a fuel leak I've been meaning to get fixed. Whenever I park my car up hill and the tank is decently full it will leak near the tank. Also I believe the previous owner used 89 octane fuel, I used it for a while but then a friend of mine told me it runs better on regular so I recently switched to that about a week ago. Maybe its in my head but I feel like it has been riding better. How bad is it that i changed up the gas?

I appreciate any help I can get! Thanks a lot!
 

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87 is what the ford recommends for that motor. 89 and 93/92 runs "better" in the motor because its a higher grade octane and better quality gas. Have you done an oil change recently? The humming sound is possibly comming from an inoperative or failing IAC valve. Since your idle is sporadic I believe that is the cause. Fuel leak is possibly a leaking fuel pump gasket? Maybe? Not entirely positive on that. If a vmburns (I believe thats his name) he will probably give you a decent diagnostic procedure to follow.
 

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+1 on idle air control valve. The dropping oil pressure gauge thing is because the engine is dying; engine isn't dying because of lack of oil pressure so don't sweat that part.

The fuel leak sounds to most likely be the fuel filler neck grommet. That name is fancy technical talk for a glorified rubber ring that seals where the filler neck goes into the tank. New one costs about $10 at the dealership. Gotta at least partially drop the tank to install it; I recommend doing it with the tank as close to empty as reasonably possible for the obvious reason that it will otherwise be heavy.
 

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Discussion Starter #4
87 is what the ford recommends for that motor. 89 and 93/92 runs "better" in the motor because its a higher grade octane and better quality gas. Have you done an oil change recently? The humming sound is possibly comming from an inoperative or failing IAC valve. Since your idle is sporadic I believe that is the cause. Fuel leak is possibly a leaking fuel pump gasket? Maybe? Not entirely positive on that. If a vmburns (I believe thats his name) he will probably give you a decent diagnostic procedure to follow.
I looked up the noise an iac valve makes and it's pretty consistent with what I hear. I've also heard the noise while I'm driving(not on idle), would that still indicate that it's the IAC valve? Thanks a lot guys!
 

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The IAC still works as you drive around. On my v6, my iac sits at about 40% and can get into the upwards of 60% as I drive. So yes the iac constantly works even during you drive. That was something new I learned when I started modifying that v6 and dataloging it. Good luck with your car.
 

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May also be worth investigating the possibility of a collapsed pcv hose, that can make a bit of a funky noise as well.
 
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