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What is that roar on startup from the intake?

Talyn

Talyn

NAXJA Forum User
Location
Radford, Communistwealth of Virginia
I'm sure all of you notice that roar on a cold start that comes from the intake, much more noticeable with an open element cone. Its not the fan and its not just from the higher RPM on start up, as after start up you can bring it up to the same RPM and the sound is not there. So, what creates the roar?
 
Most engines run on different maps upon cold start-up (different fueling, timing, etc). Once the engine reaches a certain temp. the ECU changes or interpolates between maps. These difference make the change in sound.
 
2x on the fan clutch.
 
vt98xj said:
Most engines run on different maps upon cold start-up
Click to expand...
I am aware of this, but it wouldn't account for the sound.

98stocker said:
I think it is the fan.
Click to expand...
Its not the fan. That you hear when you pull out of your parking space, or driveway. Its different. But there is the fan roar, but its different.

hubs97xj said:
The IAC, or at least the shape of the port.
Click to expand...
I'm leaning toward that as the sound would be different with extra throttle opening to achieve the same RPM.
 
Mine sounds like an air leak as opposed to a roar, but our filter setups are very different. Since I only hear it at cold starts in cooler weather, and the throttle is always closed at startup, I figure the IAC (specifically how much it's open) is the only thing that changes between hot and cold starts.
 
I guess if you really care, you can take the fan off and see if the noise goes away. My 98 xj doesn't make any other noises no startup, but now that I think about it, my 98 suburban make a noise on startup that sounds like some old dude blowing his nose into a hankie.
 
Mine has a super noisy intake roar when I first start it, I have a cowl intake.

And I don't HAVE a mechanical fan.
 
My girlfriend's 98 makes plenty of noise with a stock air box / normal paper filter. The only alteration to her motor / exhaust is an unidentified aftermarket muffler.

My 2000 has an AFE 'CAI' and a Thrush Turbo muffler and it makes a nice roar at startup and then it proceeds to purr. I personally love the sound, it's pretty beastly.
 
hubs97xj said:
The IAC, or at least the shape of the port.
Click to expand...

Yes this - mine is particularly noticeable after a cold start, and shifting into gear - it immediately gets louder. This became Really noticable when I swapped in the 62mm TB.
 
it's not a jeep specific thing either, my saturn does it even worse.
So did my motorcycles. mostly when the choke was closed with a cold start. i assume it's related to the air restriction.
 
Vanimal said:
it's not a jeep specific thing either, my saturn does it even worse.
So did my motorcycles. mostly when the choke was closed with a cold start. i assume it's related to the air restriction.
Click to expand...

I never notice it on any of our motorcycles (YZF750, SRX250, RM125 x2, KX250, etc) but its very noticeable on all of our cars - My mom's 06 Durango is the loudest, its there in my dad's 04 accord and his 08 JKU, along with my 2000 XJ and my girlfriend's 98.

I know this doesn't answer what it is, but it seems pretty normal to me.
 
Vanimal said:
it's not a jeep specific thing either, my saturn does it even worse.
So did my motorcycles. mostly when the choke was closed with a cold start. i assume it's related to the air restriction.
Click to expand...

It is the air rushing through the IAC valve. This is also why you can tend to get an intake whistle with some aftermarket intakes and throttle bodies. If you think about it, the throttle plate is closed upon start up, but typically the RPM's rush to 1200+ due to the modern rich start. That rush of RPM creates alot of velocity (along with noise) in the IAC tract while also creating substantial vaccum on the head side of the manifold.

I hope that helps. But it is a modern EFI version of a "choke"

Another thing to mention is that you could also be hearing the crankcase windage caused by the crank and rods rapidly spinning. Naturally the physical motion of the internal components also creates noise through the air it's displacing.
 
Last edited:
I don't hear it in my 98 when the stock airbox is hooked up.

The other day I was starting it up with the airbox disconnected after changing out the water pump. I kept shutting it off to look for some problem with my work.

After several times I left it run & listened around with a piece of garden hose to locate the source. It was definitely coming from the air intake hose.

I wasn't sure exactly what was causing it, but figured i just never heard it before since the airbox was always connected.
 
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