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truck missfire

Discussion in 'Chevy Trucks' started by karl klein, Feb 24, 2002.

  1. karl klein

    karl klein Senior Member
    Messages: 557

    i own a 1976 chevy c-20 with a 400 in it. lately i have noticed that when you are driving with a load or up hill the engine seems to stutter or missfire

    i was wondering if anyone new what this could be thank you
     
  2. 75

    75 PlowSite.com Addict
    Messages: 1,382

    M-a-n-y possibilities here, the most obvious one that comes to mind is how recently has there been a tune-up and is the fuel filter(s) and fuel pump in good shape?

    If the fuel filter(s) are getting clogged there will be enough fuel flow for idle and cruising, but not enough for the engine under heavier loads like going uphill or a full load in the truck as you describe.

    That's probably a good place to start anyway.
     
  3. karl klein

    karl klein Senior Member
    Messages: 557

    do you think it could be the plugs
     
  4. 75

    75 PlowSite.com Addict
    Messages: 1,382

    That goes back to the question of how recently has there been a tune-up? Plugs certainly could be a factor, I have a feeling it may be a combination of several items.
     
  5. karl klein

    karl klein Senior Member
    Messages: 557

    i just bought it about amonth ago the guy said he replaced the plugs wire and distributer a year ago but the plugs do look pretty bad on the outside
     
  6. 75

    75 PlowSite.com Addict
    Messages: 1,382

    I think they all say that............................

    What is the overall condition of the engine? If it's in decent shape then a tune-up may be the first thing you should try. Plugs/wires/cap/rotor/fuel filters are a good place to start if the engine is in decent shape mechanically.
     
  7. thelawnguy

    thelawnguy PlowSite.com Addict
    Messages: 1,011

    Misfire under load is almost always electrical...cap, rotor, wires, or plugs...
     
  8. karl klein

    karl klein Senior Member
    Messages: 557

    if it is almost always a electrical problem why does it work fine with out a load on it
     
  9. karl klein

    karl klein Senior Member
    Messages: 557

    i pulled the plugs today and one of them was very gunked up the others looked fine so i replaced all of them and drove it about 5 miles and so far so good. but i was wondering if it really is fixed
     
  10. Chuck Smith

    Chuck Smith 2000 Club Member
    from NJ
    Messages: 2,317

    Karl, the many times I have had the same problem it was spark plug boots. Once when I was trying to find the miss, I found out I had no rear brakes in the process.
    Get yourself a squirt bottle, like an old windex bottle and fill it with water. Put it on "mist", have a helper get in the truck, start it up, put it in gear, hold the brake down, and give it a little gas to put a load on it. You spray the spark plugs and plug wires with the water mist. The water will bring out any arc problems. Believe me, I thought I had none until I tried this at night once, man it looked like lightening under the hood, arcing all over the place once I added water.

    What happened one time with my rear brakes was a line let go while we were doing the mist test. Luckily the truck didn't lurch forward. I was sitting there in park after testing one side and seeing an arc, waiting to do the other side. I put my foot on the brake to put it in gear and the pedal went to the floor. My buddy said he saw a stream of brake fluid go squirting past his leg. Needless to say we replaced only the one bad spark plug boot. Luckily I was at his shop, and we were able to fix the brakeline there.

    It seems (in my cases) that the 90° spark plug boots develop cracks, I would guess from heat. Inspect the boots carefully, mine were only about a year old when I had to replace a boot on both my trucks. In 13 years I have replaced boots more than plug wires.

    ~Chuck
     
  11. karl klein

    karl klein Senior Member
    Messages: 557

    what do you think the gunk build up is from

    thanks
     
  12. plowjockey

    plowjockey PlowSite.com Sponsor
    Messages: 622

    If it was black gunk the odds are that that plug was fouled out with oil.

    Bruce
     
  13. John DiMartino

    John DiMartino PlowSite.com Veteran
    Messages: 2,154

    Spark plug wires are the main cause of a miss under heavy load,the higher cyl pressures require more KV at the spark plug to light off the spark,if the wires are weak,electricity takes the path of least resistance,the spark energy will find it easier to jump right thru the insulation of the plug wire,usually by the hot exhaust mainifold,and ground out to the mainifold,than it is to fire the plug under pressure,the instant it does this,you feel the motor miss.The wires may be only a year old,but if the motor leaks oil,and they got soaked,the insulation doesnt last as long,and if they were routed poorly,happens a lot,they might have rubbed thru on the manifold somewhere.if he used cheap wires,ive seen them last only a year too.A majot tunr up is your best be,and should cost around 85-110 bucks for that truck in parts.Your plugs are all full of stuff from oil fouling,a carb/choke problem running rich,or possibly you do a lot of short trips,and light loads where the enigne temp never gets hot enough to burn anything off the plugs,and out of the cylinders,short trips,and light loads load engines up with carbon deposits.take is out ,get it fully warmed up,and work it a little ,once in a while,clear its throat,so to speak.
     
  14. karl klein

    karl klein Senior Member
    Messages: 557

    so assuming the missfire was my plug wires how do you explain the gunk on the one plug
     
  15. thelawnguy

    thelawnguy PlowSite.com Addict
    Messages: 1,011

    If the plug isnt firing it will become fouled...but seeing as you have a 25 year old small block the valve seals are probably gone...