I was driving home the other night and noticed a hissing sound when I was driving out of the city garage. Then while driving I notice you can only hear this sound when you throttle at 50% to 100% throttle petal. You do not hear this sound at idle. The car does also have noticable power loss and a slight bog as you throttle. The engine is running good and getting go readings on all of the gauges. I ran the car to my mechanic who took a quick look at it. Initially i thought that maybe it was a vacuum leak. Mechinic checked all vacuum lines and could not find any leaks. Car was not put on the lift. Car is going in for a tune up later in the week so mechanic said we would deal with it then. So has anyone had this problem or does this sound like a possible exaust leak?

Probably your cat clogging or it breaking up and plugging the muffler.

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Red 86GT 4 Speed "MrDrift"

An exhaust leak would sound more like a ticking sound. If it sounds like a solid hissing sound, it would be on the intake side. Do you know exactly what area the sound is coming from?

It might help out a bit if you were to describe which side of the engine bay the sound seems to be coming from. Also, when you say you can hear the sound betweeen 50% and 100% throttle, do you mean when driving, at rest, or both?

If it only happens when driving, it could be a wind noise, but I'm assuming you'd know the difference. So, if it idles around 800 - 1000 RPM, then it's probably not a vacuum leak, although some vacuum lines only have vacuum applied when at highway speeds, like the EGR. But it's probably not an EGR vacuum leak either because EGR shuts off at WOT so the hissing would go away. Have a look at your fresh air intake ducting from the driver's side quarter panel inlet to the air filter, and from there to the throttle body. Any disconnection in the piping will generate a fair bit of intake hiss. If that's good, then it's possible you have an exhaust leak, although that's usually more of a puffing sound that speeds up with RPM.

Do you have the water diverter in place on the air intake?

From 1/2 to full throttle, there won't be much or obvious pulse at the exhaust exit , especially if it is a single exh--the piston strokes come too quickly to differentiate them. A clogged exhaust will make a hiss or even a whistling noise. It is also possible you have both a clogged exh (cat or muffler) and a leak upstream of the clog.

An intake side leak would likely result in high idle and lean condition often accompanied with glowing exh components.

Not being 100% sure but is sounds like its coming from the passanger side. As it seems to be even louder when passing parked cars, but you can hear it even on a wide open street. I did check the intake system since a friend of mine said to check that first and did. The mechanic did check the ERG valve and it was functioning. Same friend also mentioned that the cat might be clogged. I only make the noise while driving not stopped and at idle.

[This message has been edited by Radfiero (edited 09-26-2010).]

Probably your cat.If your cat. is on the way,you will lose power as you drive.

If you are low on power, then it's 100% the cat.

Heard it a few times in my life...

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Todd,
2008 Jeep Patriot Limited 4x2
2002 Ford Explorer Sport 2dr 4x2
2002 Ford Crown Victoria LX
1987 Pontiac Fiero SE / V6
1973 Volkswagen Type-2 Transporter

The noise a cat makes sounds more like the noise you get if you put your upper teeth against your lower lip and blow. It's a lower pitch hiss than a vacuum line hiss. It's also the only hiss that I can think of that gets louder as you open the throttle. A plugged cat hiss tends to make more noise as your looking for more power. The noise is hard to make happen with your just reving the engine in neutral.

quote Originally posted by Radfiero: You do not hear this sound at idle. The car does also have noticable power loss and a slight bog as you throttle.

As others have suggested, those do sound like symptoms typical of a failing catalytic converter. When the cat fails completely (and it unfortunately does not magically "get better" on its own), you'll notice it readilly because the top speed of the car then will become dangerously slow on any highway, no matter how much throttle you apply.


Ours necessarily are just long-distance diagnoses and guesses, so please keep us posted re what the problem actually turns out to be.

I'd think your doing so will benefit other Pennock's Fiero Forum members.

First thing you will notice, is a decrease in mpg, coupled with a loss of power. Just for reference, many "muffler shops" will not warranty a cat replacement if you don't replace the muffler at the same time. Crap from the failed cat 'can' (not always) end up in the muffler, and it's their view this leads to premature cat failure down the road--or maybe it's just their way of making more $. Hard to say.

Vacuum lines don't leak at 100% throttle. No vacuum to be had when the throttle blades are open. Leak would get worst when idling too.

Low RPM hiss = intake side of engine
On Throttle hiss = exhaust side.

my 2¢

I had that problem on my 2.8 (Rodney Dickman high flow cat w/ 7K miles) and it was the cat. When I pulled the cat out, the brick had broken into large pielces and pretty well blocked the outlet.
Jim

Well my car hisses because of my K&N intake.. sooo could be that your intake isnt on properly if its a stock one?