That Hissing from Your F-150 Engine Bay Really is No Big Deal

Modern air conditioning systems like the one in the F-150 hiss when powered down, but it is normal.

If you have ever shut off your Ford F-150 on a hot day and as you walked away you heard a hissing coming from the engine bay, you may have wondered if this was normal. Well, F-150 Online forum member “Lumadar” kept hearing that noise when he parked his truck, so he posted in the 2004 – 2008 F-150 section for advice on whether or not this sound was caused by something that would need a mechanic’s attention.

Fortunately, as many members were quick to point out, the odds are good that this hissing sound is a normal event in most modern vehicles, and it isn’t anything to worry about in the Ford F-150.

The Introduction

When the OP first posed his question to the forum, he provided the following information.

I’ve got about 7k miles on my 05 5.4 and I notice after I turn the truck off it sounds like the engine is letting off some sort of pressure. Even if I just move it in the driveway it still makes a fair amount of noise for a few minutes after shutdown.

Any ideas on what could cause that? Vacuum hose leaks?

also, not sure if it is related, but I get a clanking/knocking sound when I start my truck…sounds bad, and randomly (about 1 in 10 to 1 in 15) when I attempt to start the truck it fails to turn over all together. Dealership claims nothing is wrong.

It’s the A/C

Shortly after the initial question was asked, several members posted to explain what is causing this problem.

“etrevino103186”:

the hissing sound do you notice is when the a/c is on? If so it is probably due to the freon draining out. No biggie it happens to almost every vehicle.

“cwelk150”:

Thats probably it. Actually it’s the A/C equalizing the pressure.

But the OP refuted that information, explaining that he wasn’t using the air conditioning.

Actually, no lol. I usually DO NOT run the AC. I typically have something on…but especially this time of year AC is very rare in my truck. I prefer windows down…

So any other ideas? Heh

Seriously, It’s the A/C

Even though the OP pointed out the fact that he wasn’t using the air conditioning, two more members shared information on how the refrigerant system works, reaffirming the suggestion that the hissing noise is the air conditioning system resting.

“ChrisAdams”:

The compressor only runs when you are using the A/C, or the Defrost. The A/C is of course to cool the truck. The defrost is to remove moisture from the air.

The hiss is normal. It’s the high side leaking down into the low side. Everything does it.

We don’t use Freon anymore. We use R134. It’s far more dangerous than Freon, but more politically correct. It’s a sealed system. If it was leaking enough to hiss it would be empty in less time than it takes to read this.

“Osbornk”:

It is leaking from the high pressure side of the AC to the low pressure side.

The compressor runs all the time on vehicles with automatic temperature control (I think Lariat and higher). Otherwise, it works when the little AC switch is pushed (mine is on a lot of time and I don’t realize it) and when the defroster is on. Look for the little light on the switch.

While there are things in the engine bay of your F-150 that could fail and cause a hissing noise, if your truck is running correctly when you hear this hissing noise on shutdown, the odds are good that it is just the air conditioning system depressurizing.

Join the F-150 Online forums now!

Patrick Rall

"Before I was old enough to walk, my dad was taking me to various types of racing events, from local drag racing to the Daytona 500," says Patrick Rall, a lifetime automotive expert, diehard Dodge fan, and respected auto journalist for over 10 years. "He owned a repair shop and had a variety of performance cars when I was young, but by the time I was 16, he was ready to build me my first drag car – a 1983 Dodge Mirada that ran low 12s. I spent 10 years traveling around the country, racing with my dad by my side. While we live in different areas of the country, my dad still drag races at 80 years old in the car that he built when I was 16 while I race other vehicles, including my 2017 Dodge Challenger SRT Hellcat and my 1972 Dodge Demon 340.

"Although I went to college for accounting, my time in my dad’s shop growing up allowed me the knowledge to spend time working as a mechanic before getting my accounting degree, at which point I worked in the office of a dealership group. While I was working in the accounting world, I continued racing and taking pictures of cars at the track. Over time, I began showing off those pictures online and that led to my writing.

"Ten years ago, I left the accounting world to become a full-time automotive writer and I am living proof that if you love what you do, you will never “work” a day in your life," adds Rall, who has clocked in time as an auto mechanic, longtime drag racer and now automotive journalist who contributes to nearly a dozen popular auto websites dedicated to fellow enthusiasts.

"I love covering the automotive industry and everything involved with the job. I was fortunate to turn my love of the automotive world into a hobby that led to an exciting career, with my past of working as a mechanic and as an accountant in the automotive world provides me with a unique perspective of the industry.

"My experience drag racing for more than 20 years coupled with a newfound interest in road racing over the past decade allows me to push performance cars to their limit, while my role as a horse stable manager gives me vast experience towing and hauling with all of the newest trucks on the market today.

"Being based on Detroit," says Rall, "I never miss the North American International Auto Show, the Woodward Dream Cruise and Roadkill Nights, along with spending plenty of time raising hell on Detroit's Woodward Avenue with the best muscle car crowd in the world.

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