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ENGINE PROBLEM

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by MA87K5, Nov 10, 2000.

  1. MA87K5

    MA87K5 1/2 ton status

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    When I'm traveling at around 30mph, and the gas pedal is depressed enough to keep the speed, the truck starts to buck. I can faintly feel it at high speeds. The engine even does it when the truck is in park and I gradually give it gas. I have replaced the fuel pump, fuel filter, plugs, wires, cap and rotor, I checked the coil and the ecm, map sensor, throttle position sensor, the sensor that sits on the side of the TBI. The timing is correct. I am stumped. Any help would be greatly appreaciated. Thank you.
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Have you checked fuel pressure? If the fuel pressure is too low you will get a surge at a steady light throttle cruise. I seem to remember 9-12 psi being the tolerances, but I would be aiming for 12-14. I beleive the pressure can be adjusted on the throttle body itself but I've never done it.

    Rene

    [​IMG]
     
  3. dumbfounded

    dumbfounded 1/2 ton status

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    Sometimes the O2 sensor can cause this to happen change it they are relatively cheap. Also feel the exhaust pipe to see if it pulates then give it gas it should blow pretty hard (to check the cat). One more thing if your truck has alot of miles or even "hard" miles it could be the timing chain or a lifter. Maybe even a bad valve. A compression check can check the valves.

    Love, Peace, and Hair Grease...........
     
  4. mrivan

    mrivan 1/2 ton status

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    sounds like a fuel problem go to a garage with an exaust gas anilizer they can tell you a lot in a couple of minutes might be as simple as a plugged catilitic converter if theres one on it...
    Mrivan.....
     
  5. MA87K5

    MA87K5 1/2 ton status

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    The cat, and the O2 is about 30k miles old. It popped a code 33 a few days ago. The ECM checked out to be good. I'm going to replace the fuel pump since it has a lifetime warrenty on it from autozone it is an AC Delco one, I think. What else could be the prob if it isn't the pump? It only has 144,000 miles on it, but it was owned by a company before I bought it.
    I am all out of ideas.
     
  6. MA87K5

    MA87K5 1/2 ton status

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    Fuel pump is now ruled out. No change in problem. Maybe the injectors are bad at certain pressures.
     
  7. Tonyraca

    Tonyraca Registered Member

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    I don't know about your specific situation, but in my car I had a similar problem once, and it turned out that I had a bad spark plug wire and I had one cylinder misfiring.

    Tony's Rusty 79
     
  8. BurbinOR

    BurbinOR 3/4 ton status

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    Check the basics first, as mentioned above check all 8 of your wires, cap and rotor. Could also be a tank of bad gas, that is not unheard of.

    <font color=black>'79 - 406 TPI -<font color=orange> K5#5 - <font color=blue>See it at---&gt;http://www.blazzinor.coloradok5.com
     
  9. Blazer1970

    Blazer1970 1/2 ton status

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    My '88 pickup is doing the same thing, and I believe that it is because of an exhaust leak that I have at the donut right by the O2 sensor (or the manifold might be cracked). This allows a little air to get to the sensor under light throttle, and even worse when you lift off the throttle abruptly. The air causes the O2 sensor to return a false lean condition, which makes the computer make the engine run real rich. I am procrastinating on fixing it, because all of the studs are going to break on the manifolds, and if I have to take the manifolds off, I am going to have to replace all of the AIR piping that is attached to them. I have had it apart once before, and I know it's not coming loose again without damage.

    Tim

    1970 Blazer CST 4X4 350 SM465 NP205
    1987 Suburban 4X4 350
    1988 Chevy Pickup 4X4 350
     
  10. MA87K5

    MA87K5 1/2 ton status

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    Can slightly clogged injectors cause this problem? They do have small drips with the fine spray.
     
  11. Blazer1970

    Blazer1970 1/2 ton status

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    I think that could be a possibility. My injectors are original at 170,000 miles. It seems like mine drip a little too, but it is hard to see. I did just rebuild the throttle body, and that didn't help much.

    Tim

    1970 Blazer CST 4X4 350 SM465 NP205
    1987 Suburban 4X4 350
    1988 Chevy Pickup 4X4 350
     
  12. 90K5

    90K5 1/2 ton status

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    Dont get a fuel pump from AutoZone unless you want to replace it every year. That is how my alternator is, and any otherparts you get there. get a good pump from NAPA or GM. The only things I buy from AutoZone now are oil, spark plugs, cleaners, basically anything thay didn't make or rebuild.

    90K5
     
  13. sosamantx

    sosamantx 1/2 ton status

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    <font color=blue>Have you run a compression test? Maybe one cylinder is a little low. I would personally put in a good fuel system treatment, maybe some octane boost as well as some good gas and see what that does. Someone else mentioned maybe bad gas, it could also be moisture in the gas tank, there are additives for this as well. - Steve</font color=blue>

    <font color=blue>Steve Sosa a.k.a. "sosaman"</font color=blue>
    http://sosaman.home.texas.net/carpics.html
     

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