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Thread: What does an exhaust manifold leak affect?

  1. #1
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    Default What does an exhaust manifold leak affect?

    I have a slight exhaust manifold leak and was curious to know if it will hurt anything to not replace the manifold? The leak isn't bad...barely noticeable tapping sound under acceleration. Can't hear with the radio on, even at extremely low volume. So, do I really need to replace the manifold? Am I suffering from power loss, etc. by having the leak?

  2. #2

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    Although I know nothing for sure, I have a few guesses:

    1. Should not affect anything while driving a steady speed, like hwy driving.

    2. When you take your foot off the pedal & at idle, I believe outside air is drawn into the manifold, the 02 Sensor then instructs the gas mix to become more rich than necessary. This races the engine rpm slightly & perhaps enough to keep outside air from entering the manifold, then the rpms will fluctuate as the 02 Sensor goes a bit crazy with the mixture.

    3. Guess #2 will cause the 02 Sensor to throw a 21 code, especially when releasing the throttle after hwy driving.

    4. Outside air gets into the manufold overnight, causes the )2 Sensor to race the engine (1100-1400rpm) for a couple minutes.

    5. Since exhaust mostly still exits the rear, no need for Carbon Monoxide concerns.

    6. So leaking manifold is really only a noise issue.

    Yup, I was bored enough to write this.

  3. #3
    Registered Bolderboy's Avatar
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    Bobelmore makes some good points about creating a vacuum that sucks in air on the idle reverse stroke(b/c of low back pressure) and engine racing.

    Problem is:
    1. A leak will only get bigger as the area around it deteriotes (spelling?) and sucking in unflitered, hot engine compartment air can't be good for your engine.
    2. If the engine starts racing because of the leak --it will do it on cold starts which will eventually burn up piston rings, plugs, etc while wearing out the engine.
    3. It will fail Colorado emissions.

    Most muffler shops can weld a patch over it --Midas (Boulder) wanted $200 for the labor, IMO buy an after market (Banks, Thorley are 2) $400 and install it yourself --an afternoon job that doesn't require any speciality tools. Or for a temp fix --wrap high temp tape around it til you decide what to do --

    any of these options are cheaper than replacing burned-out pistons, or worn-out engine.
    2002 Laredo, 4.7, Selectrac, D44A w/tow package, Kolak 4" inch BB-OME with BFG ATs 265/75/16's. JKS track and quick disco's, Addco rear, Tera Adj. LCA's, Bilstein 5100's, OME SS and Stillen rotors. Aussie Locker. And a Flowmaster 70.
    NO LONGER A DAILY DRIVER
    Don't ever call my Jeep an SUV.

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    All of you are giving me some great arguments I can use to tell the wife why I need to put on a Banks!

    Oh BTW, it passed Colorado emissions last month with the leak.

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    -=- originally posted by Bobelmore -=-
    Although I know nothing for sure, I have a few guesses:

    1. Should not affect anything while driving a steady speed, like hwy driving.

    2. When you take your foot off the pedal & at idle, I believe outside air is drawn into the manifold, the 02 Sensor then instructs the gas mix to become more rich than necessary. This races the engine rpm slightly & perhaps enough to keep outside air from entering the manifold, then the rpms will fluctuate as the 02 Sensor goes a bit crazy with the mixture.

    3. Guess #2 will cause the 02 Sensor to throw a 21 code, especially when releasing the throttle after hwy driving.

    4. Outside air gets into the manufold overnight, causes the )2 Sensor to race the engine (1100-1400rpm) for a couple minutes.

    5. Since exhaust mostly still exits the rear, no need for Carbon Monoxide concerns.

    6. So leaking manifold is really only a noise issue.

    Yup, I was bored enough to write this.
    WRT #2/#3 Depending on where the leak is, extra air may be introduced into the manifold due to acoustic resonance. This extra air fools the o2 sensor into richening but wouldn't effect the idle speed for a couple of minutes after an overnight sit. I got the 21 Code usually after sustained idling. I was ignoring the exhaust leak until I couldn't pass CA smog. Did so much stuff trying to make it pass that I couldn't tell what did what.

    Good Luck to You,

    D.J.
    '93 ZJ 4.0L Laredo, Stock &
    '02 WJ 4.0L Limited, Stock

  6. #6
    *****es KillerZJ's Avatar
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    So it doesn't effect gas mileage or performance I am assuming?
    Harry


    mine on the left, friend's on the right
    formerly a 5 speed swap ZJ with 33" TSL's

  7. #7
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    I've cracked two manifolds on my TJ. Both time I had a noticable drop in power, and my gas mileage from from bad to worse. From reading everything in the TJ forum your best bet would be to get a Banks and be done with it.

    Only reason I didn't go with Banks is because they were both warrentied and I got a new one. If I crack another one I'm asking the parts store to credit me and I will pay the difference for the Banks.
    Current: 95 YJ - 2" BDS, 31" procomp MT's, Warn M8000, homemade rear bumper/tire carrier
    07 WK
    Previous: 76 CJ5, 93 YJ, 98 TJ

  8. #8
    Registered RazorZJ's Avatar
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    -=- originally posted by KillerZJ -=-
    So it doesn't effect gas mileage or performance I am assuming?
    a rich fuel mixture will hurt milage because it's dumping more fuel into the motor than it realy needs.
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  9. #9

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    -=- originally posted by RazorZJ -=-
    a rich fuel mixture will hurt milage because it's dumping more fuel into the motor than it realy needs.
    Agreed, but if it is only rich at idle then mpg is generally unaffected unless you idle time exceeds driving time.

    I get 21mpg (hwy) when not pulling the trailer & on 235/75s. I get a 21 code every day except during winter. Idle jumps to 1,500 every morning, except Winter, and always settles to a rock-solid 750 after warm-up. I even have an under dash switch to turn-off/on the Check Engine light.

    edit: I feel sure my manifold leaks as it sounds louder at startup -but not loud enough for the wife to complain :winky: . I have looked for cracks but have not found one yet. Performance is much better after it warms, as strong as ever, so reluctant to spend much time/money to fix.
    1994 ZJ/1995 4.0; NP231; LS 3.73; Aux.Trans cooler; MSD/Magnecor; Headlight Harness; K&N; EdelbrockIAS/ProCompES9000 F/R

  10. #10

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    JB Weld only works for about 2 hours. Lookin to n-rig up some header-wrap to try again. Damn thing is gittin too loud.

  11. #11
    Registered RazorZJ's Avatar
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    -=- originally posted by Bobelmore -=-
    Agreed, but if it is only rich at idle then mpg is generally unaffected unless you idle time exceeds driving time.

    I get 21mpg (hwy) when not pulling the trailer & on 235/75s. I get a 21 code every day except during winter. Idle jumps to 1,500 every morning, except Winter, and always settles to a rock-solid 750 after warm-up. I even have an under dash switch to turn-off/on the Check Engine light.

    edit: I feel sure my manifold leaks as it sounds louder at startup -but not loud enough for the wife to complain :winky: . I have looked for cracks but have not found one yet. Performance is much better after it warms, as strong as ever, so reluctant to spend much time/money to fix.
    sounds like yours isnt a big crack yet, the metal expands when it gets hot and closes the crack, dont worry it'll get bigger with all the expanding and contracting.
    Democracy will cease to exist when you take away from those who are willing to work and give to those who would not.

    -Thomas Jefferson





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