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94 chevy w/5.7 runs rough after new exaust...any ideas?

This is a discussion on 94 chevy w/5.7 runs rough after new exaust...any ideas? within the Technical / Maintenance forums, part of the General Discussion category; hi guys, just bought a 94 chevy truck with a 350 in it that a guy had , the truck ...

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    94 chevy w/5.7 runs rough after new exhaust...any ideas?


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    hi guys, just bought a 94 chevy truck with a 350 in it that a guy had , the truck was running great but with an exhaust leak in the factory "y" pipe and wouldnt pass inspection......he replaced everything with new headers, high flow cat and dual exhaust out the back and now the truck runs rough and wants to die and pops some...since I havent seen the truck in person, and other than the obvious like broken plugs or wires or #5 and 7 crossed, could the o2 sensor go bad or not work with the new exhaust? what else could I look for? I have to go pick up this truck and drive it home...3 hrs away...need to bring everything with me to make it a good runner to make the trip home. thanks for the help!!
    Last edited by mikes78; 01-03-2006 at 08:58 PM.

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    Re: 94 chevy w/5.7 runs rough after new exaust...any ideas?

    Plug wires backwards,not fully connected, o2 sensor not connected? It's probably gonna be something obvious. Maybe a plug got broken when the headers went on if they were not removed.
    Don't forget to check the oil and coolant levels before you drive ie home!
    Last edited by Bigblack90; 01-03-2006 at 11:34 PM.

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    Re: 94 chevy w/5.7 runs rough after new exaust...any ideas?

    did the 94 use a 1 wire 02 senser? If it did you will need to upgrade to a 3 wire 02, the one wire 02 uses exhaust heat to reach it's operating temp, with headers or cat back it usually wont so upgrade to a 3 wire is necessary, the only diff from the 1 wire to a 3 wire is a 3 wire has it's own heater so you plug in the sensor wire the same and then hook one of the white wires to switched 12v and the other white wire to ground.....if the 94 already has a 3 wire just forget I was here.

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    Re: 94 chevy w/5.7 runs rough after new exaust...any ideas?

    Good ideas guys, wher will I find a 3 wire o2? Is there any application I will ask for at the parts counter for that? Thanks!

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    Re: 94 chevy w/5.7 runs rough after new exaust...any ideas?

    mikes78:

    Go to Advance Auto Parts or whatever company you prefer and ask them if your truck came with a 3 -wire O2 sesnor. They can look it up on the computer. If there was an option for a 3- wire O2, simply buy it and wire it like the instrucitons show.

    My 1995 is a 3-wire, and I don't think GM changed a lot from 1994 to 1995, so you may already have a 3-wire? I changed mine not long ago...old one had 117,000 miles and was really bad. Helped the engine a lot, but I also had some EGR- vacuum hose issue than a Chevrolet dealership recently fixed.

    PS: You might just drive to a GM Parts dealer and ask them about an O2sensor. They'll lookup what you need and tell you if its 1 or 3-wire. You can always decline their price, then take what they told you to Advance, NAPA, O'Reilly's, AutoZone, etc.
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    Re: 94 chevy w/5.7 runs rough after new exaust...any ideas?

    I Have A 94 Chevy W/ 5.7 Tbi Just Like Yours. Mine Has A 3 Wire O2 Sensor. Colors Are Black, Purple And Red. You Can Go To Your Local Parts Store And They Should Have An Oem And A Universal. Go With The Oem, You Will Pay A Little More But You Will Basically Just Plug It In. I Bought The Universal Which Has A Black And 2 White Wires. The Instructions Said Black, White And Grey So I Had To Do A Little Guess Work The First Time Hooking It Up. Go With Oem.

    Also, you said the truck now "pops"....is it backfiring? If so, check the timing if you have a light. Make sure you disconnect the idle...located under the passenger dash, it is a brown wire w/a black stripe and has a black connector. Turn on engine and check your timing. Try advancing it to 4. Turn off engine. You will also have to reset the computer (disconnect battery for 10-20min) once you do this before starting engine again and hook up idle connector. My truck is set at 0 degrees but it is also stock. If it is not backfiring...disregard what I said.
    Last edited by 94CHEVYPU; 01-05-2006 at 12:09 PM.

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    Re: 94 chevy w/5.7 runs rough after new exaust...any ideas?

    that is very similar to whats happening ot a buddy of mines truck, we have had it in the shop since last fall, doing lots of body work, and mechanical work to it, it is also a 94 chevy w/ a 5.7. all we did was put in a new air filter, remove the ac compressor, and put in an eliminator pully from an other 350, and and put a new exhaust, y pipe back all the way to the tail pipe, didn't change o 2 sensor though( had to cut the 3 wires and splice back together), and we put in a new fuel pump, and sending unit. now the truck runs really rough, back fires through the throttle body, and if you rev it up, when you let off, the rpms drop so low it will almost stall, then after a couple seconds it comes back up, since our problems are similar, maybe any advice you give hime will, help my buddy.
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    Re: 94 chevy w/5.7 runs rough after new exaust...any ideas?

    Quote Originally Posted by kenman1717
    that is very similar to whats happening ot a buddy of mines truck, we have had it in the shop since last fall, doing lots of body work, and mechanical work to it, it is also a 94 chevy w/ a 5.7. all we did was put in a new air filter, remove the ac compressor, and put in an eliminator pully from an other 350, and and put a new exhaust, y pipe back all the way to the tail pipe, didn't change o 2 sensor though( had to cut the 3 wires and splice back together), and we put in a new fuel pump, and sending unit. now the truck runs really rough, back fires through the throttle body, and if you rev it up, when you let off, the rpms drop so low it will almost stall, then after a couple seconds it comes back up, since our problems are similar, maybe any advice you give hime will, help my buddy.

    Typically when a motor backfires especially out the TBI is usually a timing issue. Check your timing, and reset your computer. With the new exaust you may need to adjust your timing.

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    Re: 94 chevy w/5.7 runs rough after new exaust...any ideas?

    Hey guys I swapped an engine in my truck and had this issue. JUST fixed this a few days ago after almost 5 months later, replacing hundreds of dollars worth of parts... it was a distributor that was the culprit.

    94CHEVYPU ... That brown wire is not an 'idle' wire is actually the 'EST' (Elecronic Spark Timing) wire. :P :) Jsut to let ya know.

    Stefak
    Last edited by MStefak; 01-07-2006 at 03:04 AM.
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    Re: 94 chevy w/5.7 runs rough after new exaust...any ideas?

    Quote Originally Posted by MStefak
    Hey guys I swapped an engine in my truck and had this issue. JUST fixed this a few days ago after almost 5 months later, replacing hundreds of dollars worth of parts... it was a distributor that was the culprit.

    94CHEVYPU ... That brown wire is not an 'idle' wire is actually the 'EST' (Elecronic Spark Timing) wire. :P :) Jsut to let ya know.

    Stefak

    Thanks.....sorry, I couldn't remember what it was called, just knew you had to disconnect it to set timing.......now I won't sound like such a moron thanks to you.........

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    Re: 94 chevy w/5.7 runs rough after new exaust...any ideas?

    Quote Originally Posted by 94CHEVYPU
    Typically when a motor backfires especially out the TBI is usually a timing issue. Check your timing, and reset your computer. With the new exaust you may need to adjust your timing.

    will check the timing, we have reset the computer a acouple times since, and the battery has been out since last fall anyway.
    2003 Chev Silverado 4x4 4.10 gears and it has a 4.8L
    1997 GMC Sierra 4x4 4.3L 4L60E KOSS CD player,16mpg,3.73 gears z71 skid plates <<<sold

    4th year apprentice at Chev Dealer

 

 

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