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One inch (?) tube going from air cleaner to exhaust manifold...

This is a discussion on One inch (?) tube going from air cleaner to exhaust manifold... within the Technical / Maintenance forums, part of the General Discussion category; I was poking around under the hood today and noticed there was a metallic, one-inch (maybe 1 1/4) diameter hose ...

  1. #1
    I lie about my mpg's. 14GDHPs4ME's Avatar
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    Question One inch (?) tube going from air cleaner to exhaust manifold...


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    I was poking around under the hood today and noticed there was a metallic, one-inch (maybe 1 1/4) diameter hose going from my intake (just before the air cleaner) to the exhaust manifold on the passenger side. More curiously, it was undone on the exhaust manifold end, and I have no idea how long it's been that way.

    What is this? What does it do? How important is it that it is hooked up? What happens if it's unplugged?

    Sorry I'm still new at this, and the search didn't help much.
    1992 K2500 LD 4x4, 5.7, 700r4, 3.73 14-bolt, G80, Cleaner than sh!t.
    2001 Tahoe LS 4x4(the wife's), 5.3, 3.73, G80, third-row seats.

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    Registered User daved931's Avatar
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    Re: One inch (?) tube going from air cleaner to exhaust manifold...

    It's to heat up intake air for cold starts with warm exhaust gases.

    You can chuck it and not worry about it. I don't think I've seen an older vehicle with one intact.
    Last edited by daved931; 03-18-2010 at 12:23 PM.
    1999 2 Door Tahoe 4x4 w/ 14 Bolt SF & TruTrac. Corvette servo, Gibson cat-back, custom tune from Black Bear, AD244 alt. upgrade, Z71 skid plates, Escalade cluster, 2" block + 1.5" crank, 285/75R16 SOLD 1/2012
    2008 Silverado 1500 Z71, Borla catback, block and coil spacer

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    Registered User machinist09's Avatar
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    Re: One inch (?) tube going from air cleaner to exhaust manifold...

    seriously; chuck it. why would you want warm air going in?
    1999 Chevrolet K2500, Crew Cab, Short Box, 4L80E, 454 ci, 3.73 rear 14b FF 10.5"

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    Re: One inch (?) tube going from air cleaner to exhaust manifold...

    It is called a "chimney" and is part of the Thermac system. As noted above, the Thermac system was designed to deliver warm combustion air from a molded sheet steel collector that wrappped around the exhaust manifold. There is no actual exhaust gas in that riser (chimney) just fresh, preheated air.

    This warm air is available to the carburetor (via a vacuum actuated flap valve on the underside of the snorkel) during cold start conditions. As the engine warms, a sensor (usually screwed into the thermostat housing) gradually dumps the full vacuum down to the point where the flap in the intake snorkel has shifted to admit only cool air.

  5. #5
    JoshTheCanadian
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    Re: One inch (?) tube going from air cleaner to exhaust manifold...

    I love in Canada, Gets down to -35C here, I got rid off all of that Thermac stuff, truck starts EVERY time, even when not plugged in.

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    Hey watch this Colby 04's Avatar
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    Re: One inch (?) tube going from air cleaner to exhaust manifold...

    It's called a preheater hose and don't discard it unless you know local emissions inspections won't require it to be present.

  7. #7
    I lie about my mpg's. 14GDHPs4ME's Avatar
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    Re: One inch (?) tube going from air cleaner to exhaust manifold...

    I guess I'll just leave it disconnected (maybe plug it in before I get it smogged) and keep it in mind if I ever have cold start problems. Thanks for the information, guys.
    1992 K2500 LD 4x4, 5.7, 700r4, 3.73 14-bolt, G80, Cleaner than sh!t.
    2001 Tahoe LS 4x4(the wife's), 5.3, 3.73, G80, third-row seats.

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    0_o o_0 0_o o_0 98GreenMachine's Avatar
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    Re: One inch (?) tube going from air cleaner to exhaust manifold...

    My 84 with a 2 barrel carb starts fine in below 0 weather with no icing of the throttle blades after i got rid of the tube. You will be fine to remove the tube.
    08 HHR LS - 2.2 | 5 speed | 30 MPG

    98 K1500 Z71 - 204k | L31 | 60E | 3.73's | 3" BL | 33x12.5x15 Dick Cepek F-C II's | Dynomax Ultra Flo X DI/DO | 14 MPG

    84 S15 Jimmy 2dr 4x4 - engine/tranny all rebuilt

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    Re: One inch (?) tube going from air cleaner to exhaust manifold...

    I had a jeep CJ-7 with the six cylinder in it. It had the preheat disconnected on it. In the extreme cold the carb kept icing up and it didn't run worth chit.Might of been other factors involved but that was my result.

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