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98 vortec 5.7 with problems

This is a discussion on 98 vortec 5.7 with problems within the Technical / Maintenance forums, part of the General Discussion category; i have a 98 5.7 in my GMC. the problems i have is #1,3,7 spark plugs foul out with a ...

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    98 vortec 5.7 with problems


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    i have a 98 5.7 in my GMC. the problems i have is #1,3,7 spark plugs foul out with a beige soot on them(only those 3), low on power, bad gas mileage, runs lean(pings). i already have the intake off and i found a slight problem on the lt rear gasket port(it goes nowhere on the intake, its flat, what would be a coolant passage) im concerned that it could be a head gasket. what are your thoughts?

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    potato gon' potate SlvrdoHDmax's Avatar
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    Re: 98 vortec 5.7 with problems

    It could be the lower intake gaskets. Does your coolant level change any?

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    Re: 98 vortec 5.7 with problems

    Quote Originally Posted by sw45 View Post
    It could be the lower intake gaskets. Does your coolant level change any?
    yes it does but its minor. i did a pressure check on the cooling system and it looses about 5 psi in about 10 minutes. no external leaks.
    Last edited by inscarguy; 06-26-2010 at 02:43 PM.

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    bikes cars trucks women vic_V8's Avatar
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    Re: 98 vortec 5.7 with problems

    coolant isn't showing up in the oil is it?
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    Re: 98 vortec 5.7 with problems

    Quote Originally Posted by vic_V8 View Post
    coolant isn't showing up in the oil is it?
    no, and the level is not reading higher either. no smoke out the exhaust, no CEL. passed emissions with flying colors last month. it has been doing this for about 9 months. just got around to it today.

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    Re: 98 vortec 5.7 with problems

    do you have dual cats?
    Selling a front Chevy Dana 60, 5.38s welded, $1000 never run, PM for more
    1992 K2500 6.5 Turbo Diesel/4L80e/4.10s (sas with Dana 60 just sitting around waiting for a rainy day)
    '84 K10, 6" Tuff Country, now 1 ton frame swap and Cummins swap have begun
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    Re: 98 vortec 5.7 with problems

    yes

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    Re: 98 vortec 5.7 with problems

    I was having some stumbling problems, did a full tune up and still had some, turns out I had a clogged cat.
    Selling a front Chevy Dana 60, 5.38s welded, $1000 never run, PM for more
    1992 K2500 6.5 Turbo Diesel/4L80e/4.10s (sas with Dana 60 just sitting around waiting for a rainy day)
    '84 K10, 6" Tuff Country, now 1 ton frame swap and Cummins swap have begun
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    Re: 98 vortec 5.7 with problems

    Lower intake gasket failures are common on these trucks. They will generally fail on the pass rear of the intake where you mentioned. They will leak down the backside of the engine or into the lifter valley contaminating the oil. In some cases, the leak can be slow enough that when the oil gets hot enough, it will turn the water to steam and will be burned off through the pcv system. Look under the oil cap for a light colored sludge. This is a sign of water contamination in the oil. The coolant passage and the intake passage are far enough apart that the coolant would have a hard time getting into the intake ports (the closest one to the sight of normal failure would be #7). The part # for the newest intake set is Fel Pro MS98000T. This is the latest design that uses rubber over steel instead of a rubber oring in plastic. This set is a little more expensive, but if you follow the installation proceedure, you should never have another issue with this leak.

    If you do suspect a clogged cat, unplug and remove the upstream O2 sensor on the same side of the effected cylinders (#1,3,5,7 are on the driver's side). Then run it and see if it feels any different. This is only for diagnostic, and it will set the light for the missing O2.

    Also, a cat doesn't just plug up. Generally, another issue in/on the engine led to the converters failure. Phosphorus, lead and silacone can coat the converters surface inside and eventually cause a clogged cat. Phosphours comes from oil (although, newer oil formulations have reduced amounts of phosphourus), lead from leaded fuel (not pump gas) and silacone from coolant.
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