humming noise

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Jan 8, 2014 | 09:59 AM

mediaman67

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Laughing Gas

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humming noise

Hi All

1996 Exp 4.0L OHV

wife drove it the other day, said made loud humming noise, then engine stalled.

drove it this am, did it to me too while idling in D with foot on brake... humms for about 1 min, and then stalls the engine...

if I rev the engine, was still doing it... but came and went when I was driving then till I got her home...

I know there is a black plastic cap that makes a similar noise but not as loud, this seems like something else... and the fact it stalled the truck is bothersome... it started up then fine, but still.

The only thing that is the same as when wife had it, is that both of these days are like under 10 degrees outside...

so, me thinks MAP? or something else that doesn't like these REALLY cold temps? or??

Thanks All!

Jan 8, 2014 | 08:11 PM

Mikeman

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From: PNW

Just throwing out an idea. Sometimes a vacuum leak can be heard as a humming or whistling (often it's a higher pitch). Running lean when cold can lead to stalling.

Jan 8, 2014 | 09:34 PM

mediaman67

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This is more like a deep growl - almost like an old style vaccum cleaner...

many have suggested it's the IAC, but I want to be sure first, before replacing it. -

Thanks :-)

Jan 9, 2014 | 11:46 PM

Mikeman

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From: PNW

Can you put one of those mechanic's stethoscopes on the IAC and see if that's your source of the noise?

Also, have you checked to make sure that all of the connections on the intake tube/hose between the airbox and the intake manifold are tight? Air sneaking in downstream of the mass airflow sensor won't be measured and throw off the computer.

Jan 10, 2014 | 06:57 AM

shorod

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From: Cedar Rapids, IA

My initial thought was also IAC, especially since the weather is cold and you have stalling at idle. The IAC is a fairly common issue, and it's also not uncommon for them to make a humming or buzzing noise when failing or binding.

You could unplug the IAC and see if the noise goes away. You might want to have someone helping though to keep the engine running at idle speed once the IAC is unplugged. Otherwise, unplugging the IAC might cause the engine to stall, even if it is not functioning quite properly.

An even better test of the IAC would be to use a professional scan tool to command the IAC to run the engine at a specific speed, then see if the IAC can achieve the desired engine speed and if it buzzes in the process. You'd probably need to do this when the truck and engine are cold though, in similar conditions as to when you're able to hear the buzzing.

-Rod

Jan 10, 2014 | 06:57 AM

mediaman67

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Laughing Gas

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Thing is, it only does this when the air temp outside is like under 15 degrees, and now, it's been in the 20's+ (feels like a heat wave) - so, she is not doing this...

Someone mentioned running lean would cause it to stall like that... so maybe the IAC is just old enough to not work properly when the temps really fall...

Im just curious what causes the cap to vibrate? - me thinks it's a sort of air release valve there, and if it comes out too fast, or at the right speed, the cap starts to humm...?

for those of you that are musical - it's humming a "D" above middle C - I have perfect pitch (curse and a blessing I guess)

I bought a new IAC... I figured for $40, what the heck, but I will also be sure all connections are tight everywhere... I don't hear any vacuum leaks.

Jan 10, 2014 | 07:05 AM

mediaman67

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Laughing Gas

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also, the new one has a lifetime warranty - since it's so easy to take off and on, I figured can't lose.

anyone know if this issue would throw a code? - the funny thing, is that I have a 420 come up all the time, cuz I think my CC is shot, but yesterday, the CE Light went out on it's own!! - maybe the air is so cold that the downstream sensor is registering a normal reading?

I found a $5 trick for that too... gonna try that when the weather is warm again, just to see if it works... I don't feel like buying a new CC if I don't have to, and being a 96, it doesn't have to pass emissions...