Vortec 454 vacuum leak/rough idle

Discussion in 'Engine Performance + Maintenance' started by 8T7K5, Sep 10, 2019.

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  1. 8T7K5

    8T7K5 Newbie

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    I'm gonna say first off I'm not a mechanic but I'm able to follow instructions and willing to learn. I've got a 96 K2500 Suburban I recently bought as a project. It had been sitting for at least 6 months maybe longer. Engine had a miss so I changed plugs cap rotor and plugs. After doing that, it idled fine but stumbled when I gave it gas. I figured it wouldn't hurt to change the fuel pump so I replaced that, the fuel filter, and fuel pressure regulator. Since I had the upper intake off naturally I replaced the gaskets. Rtv'd the gaskets and let it dry, not a lot just on the corners to hold them in place. Got it all buttoned back up and now it's idling up and down bad, from almost dying up to about 2500 rpm and back. When it idles up there's a high pitched whining noise coming from what sounds like the passenger side of the upper intake. I'm at my wits end on what to do short of either yanking the intakes and putting on an aftermarket fuel injection system or dropping a match in the gas tank. If anyone could point me in the right direction I'd be very grateful.
     
  2. andy396

    andy396 I'm Awesome

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    Did you remember to reconnect this guy?

    [​IMG]
     
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  3. thinger2

    thinger2 Newbie

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    Double check all of your vacuum conections for sure but, i suspect its a "relearn procedure" with the idle air control or the mass airflow sensor
    (IAC and MAF) when you google.
    A bad base gasket install usually results in a very high idle at cold start.
    look up how to reset both and go from there.
    good luck, let me know if that works
     
  4. thinger2

    thinger2 Newbie

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    Dont know if that posted properly so ill just do it again!
    Double check all of your vacuum connections for sure but i suspect its a "relearn" procedure with the idle air control or the mass airflow sensor
    (IAC and MAP when ya google it.
    A bad base gasket install usually results in a very high and constant idle at cold start.
    look up how to reset iac and map and go from there.
    good luck, let me know if that works
     
  5. thinger2

    thinger2 Newbie

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    Sorry, forgot to mention, dont use any kind of sealant on a base gasket.
    It will swell up as it cures and, if ya follow the instructions amd wait 15, it will leave an air gap causing a vacuum leak.
    Make sure the base of the throttle body and intake are very clean and flat. and bolt it down dry
     
  6. Supercharged111

    Supercharged111 I'm Awesome

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    Make sure you plugged in the vacuum reference line for the fuel pressure regulator too, I drove around for a year plus with that vented to the atmosphere. Your injectors are probably garbage as well, so once you solve this vacuum leak and the truck still isn't quite right you should probably set your sights in that direction.
     
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  7. andy396

    andy396 I'm Awesome

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    Is it possible you forgot a bolt on the lower intake manifold?
     
  8. 8T7K5

    8T7K5 Newbie

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    Yep that was one of the first things I reconnected.
     
  9. 8T7K5

    8T7K5 Newbie

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    I got new gaskets and haven't put them on yet so I'll leave off the sealant this time. I found a new problem with the upper intake now though. After cleaning all the old grease and oil off, I found a small crack at the base around a bolt hole. Doesn't appear to go all the way through the mainifold but given its age I'm worried it will only get worse with time and being torqued down.
     
  10. stutaeng

    stutaeng OBS Enthusiast

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    Darn, so many changed parts, and the problem may be elsewhere! Let's start with the basics:

    1. Do you have any codes?
    2. Check for vacuum leaks with a gauge. The Haynes manual has a good explanation of what the reading should be. YT has some videos as well. HF has a gauge for like $10.
    3. Check fuel with a gauge. I know you just changed the pump, etc., but still you still want to rule that out.
    4. Did you check your distributer? I know when I did my 4.3 Vortec intake manifold, I tried to reinstall the distributer back from TDC and failed. I had to call my mechanic to set it correctly. I was setting in on the exhaust stroke. What's recommended is to index it and reinstall it in the same location. I was feeling confident when I tried to install it from scratch.

    You replaced the upper intake, right, so didn't mess with the distributer? Still I would double check timing.
    5. Don't rule out compression test, especially if you just bought it.

    I hope this helps.
     
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