Throttle Hiss on 3.2
12-19-2002, 06:08 PM
#1
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Throttle Hiss on 3.2
Has anyone here experienced a hissing noise, coming from the engine, which seems to be a function of throttle (independent of speed or RPM)position? It only occurs within a narrow range of peddle position, but that peddle position is probably the most common position as you accelerate through the gears. It does not affect driveability, but it is a bit annoying. It is markedly less noticeable when driving with the windows down, for some reason.
That peddle position is not down very far, but it's far down enough that if the car is in neutral, the engine is reving too high to hear the hissing. Or perhaps a load on the engine is needed to produce the noise. In any case, I cannot do the obvious thing and trace the hissing by standing by the engine and manipulating the throttle by hand.
Anyone else seen (heard) this?
Thanks
That peddle position is not down very far, but it's far down enough that if the car is in neutral, the engine is reving too high to hear the hissing. Or perhaps a load on the engine is needed to produce the noise. In any case, I cannot do the obvious thing and trace the hissing by standing by the engine and manipulating the throttle by hand.
Anyone else seen (heard) this?
Thanks
12-19-2002, 11:16 PM
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Bill's asking a good question - I have an Authority air meter with cone-filter and the throttle makes a whistling noise between 2-3K rpm. I think it's kinda cool.
12-20-2002, 11:49 AM
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One of the previous owners installed the Autothority hot wire air flow meter, which comes with the cone filter. Air induction noise is plausible judging my the hissing nature of the sound - maybe if a certain throttle position sets up some resonance. But I would think that resonance would also be air flow rate (hence RPM?)dependent.
The hissing is not a function of RPM. If I hold the peddle in the "hissing zone" as I accelerate through any gear, the noise is not affected as the RPMs build and stabilize. It only goes away if I push the peddle a bit harder, or let up.
If you picture the gas peddle movement as having maybe 30 degrees of arc, there is no hissing from 0 to 8, hissing from about 8 to 11, and no hissing again from 12 to 30 (floored). Unfortunately, the 8 to 11 zone is about in the middle of where my foot wants to be under most moderate acceleration conditions.
The only other possibility I have thought of is some mechanical buzzing brought about by the throttle position - maybe some piece of the throttle are brought in close contact with something in that range of position. But just from the nature of the sound, I have to think it is something in the induction.
If air flow is NOT RPM dependent or not totally, then it would make more sense that some king of resonance is occuring.
The hissing is not a function of RPM. If I hold the peddle in the "hissing zone" as I accelerate through any gear, the noise is not affected as the RPMs build and stabilize. It only goes away if I push the peddle a bit harder, or let up.
If you picture the gas peddle movement as having maybe 30 degrees of arc, there is no hissing from 0 to 8, hissing from about 8 to 11, and no hissing again from 12 to 30 (floored). Unfortunately, the 8 to 11 zone is about in the middle of where my foot wants to be under most moderate acceleration conditions.
The only other possibility I have thought of is some mechanical buzzing brought about by the throttle position - maybe some piece of the throttle are brought in close contact with something in that range of position. But just from the nature of the sound, I have to think it is something in the induction.
If air flow is NOT RPM dependent or not totally, then it would make more sense that some king of resonance is occuring.
12-20-2002, 01:13 PM
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Randal (& Bill); I have precisely the same noise in my stock '87, as you describe. I have tried to trace to a loose or leaking vacuum hose on the intake side - no joy. The hissing will not take place except under load, therefore I can't easily isolate the source, though it is clearly from the intake side in the engine compartment.
Current working theory is that the intake manifoild is leaking at the gasket to the head. I have tightened the bolts, but to no avail (couldn't reach all of them). I will look it all over very closely when I drop the engine for valve adjustment & clutch bearing upgrade this winter. Any opinions appreciated - thanks.
Current working theory is that the intake manifoild is leaking at the gasket to the head. I have tightened the bolts, but to no avail (couldn't reach all of them). I will look it all over very closely when I drop the engine for valve adjustment & clutch bearing upgrade this winter. Any opinions appreciated - thanks.
12-20-2002, 04:31 PM
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Kevin, if the hissing were a manifold leak, you'd probably hear the leak over a wider throttle range, wouldn't you? Also, a manifold leak would affect driveability, which is not the case for me (yet, anyway!).
Is the hiss you hear most pronounced when you have the windows up, and barely audible when they are down? Do you have the same Autothority air flow sensor system, or at least the cone type filter I have? If yes and yes, then we are probaly hearing the same thing.
Is the hiss you hear most pronounced when you have the windows up, and barely audible when they are down? Do you have the same Autothority air flow sensor system, or at least the cone type filter I have? If yes and yes, then we are probaly hearing the same thing.
12-20-2002, 07:45 PM
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A manifold leak can whistle/hiss in a variety a ways depending on where, how big, what shape. My drivability is fine, but a little leak can make very noticable hi freq. noise - perceived dB & negative impact are not necessarily linear (I don't think).
No & no - same noise regardless of windows up or down, Stock induction system, airbox, etc. However, that doesn't mean we DON'T share the same cause. We'll know that when either of us solves it!
No & no - same noise regardless of windows up or down, Stock induction system, airbox, etc. However, that doesn't mean we DON'T share the same cause. We'll know that when either of us solves it!
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12-21-2002, 02:06 PM
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I might try an experiment. I need the car to be still which I inspect the engine, yet I need the engine to be under load. So maybe I will get a friend to apply the brake, put it in first, rev the engine, and slip the clutch just enough to put the engine under enough load to produce the noise. It will have to be quick so as not to burn the clutch, but it might work...
By the way, my noise is definitely an unpleasant hiss - not a tuneful whistle...
And it is present whether the windows are up or down, it's just much more noticeable in the cabin when the windows are up.
By the way, my noise is definitely an unpleasant hiss - not a tuneful whistle...
And it is present whether the windows are up or down, it's just much more noticeable in the cabin when the windows are up.
12-24-2002, 01:11 AM
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Bill is wise...
The noise is the air rushing around the throttle plate during high vacume/part throttle conditions. It sounds like a vacume leak, not the pleasant throaty sound out of downdraft webbers or MFI throttles. You don't get the sound with the stock setup but the short couple from the open filter inlet to the throttle plate dosen't muffle the sound with the aftermarket MAF kit. You will notice it happens the most with very slight throttle openings during high vacume conditions, i.e; lightly accellerating up a slight incline. When you mash the throttle you open up the throttle plate, vacume drops to near zero and no whistle. You can't make it happen with the car in neutral because you don't have the engine under load. A vacume leak on the otherhand is easy to find with a stethescope.
I had this same condition, after going over every possible location and then having the shop look (listen...) and posting here and on pelican, my suspicions were confirmed. Some cars get the sound others don't(and some can hear it and others can't...). I know of one car that made the sound until the throttle body was bored out, which silenced the squeal.
The noise is the air rushing around the throttle plate during high vacume/part throttle conditions. It sounds like a vacume leak, not the pleasant throaty sound out of downdraft webbers or MFI throttles. You don't get the sound with the stock setup but the short couple from the open filter inlet to the throttle plate dosen't muffle the sound with the aftermarket MAF kit. You will notice it happens the most with very slight throttle openings during high vacume conditions, i.e; lightly accellerating up a slight incline. When you mash the throttle you open up the throttle plate, vacume drops to near zero and no whistle. You can't make it happen with the car in neutral because you don't have the engine under load. A vacume leak on the otherhand is easy to find with a stethescope.
I had this same condition, after going over every possible location and then having the shop look (listen...) and posting here and on pelican, my suspicions were confirmed. Some cars get the sound others don't(and some can hear it and others can't...). I know of one car that made the sound until the throttle body was bored out, which silenced the squeal.
