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Losing power at speed (pulsing)

Discussion in 'Chevy Trucks' started by eamims, Mar 5, 2004.

  1. eamims

    eamims Junior Member
    Messages: 20

    Hey guys, I'm in a bind. I'm in the middle of driving my 1977 Chevy c-10 (350 v8 q-jet... all stock) about 700 miles. Things were going smooth for about 300 miles, then without warning (and in the middle of Atlanta rush hour!) I started losing power as if I was taking my foot completely off the gas then putting it right back on, like a pulsing..about a 1 second long pulsing that would continue if I gave it more gas but if I took my foot off it would go away. It would inevitably do it again, whether it was 15 seconds later or a few minutes later.

    So I took it to a place in the middle of nowhere Georgia, where I am now. We spent all day trying to figure it out (me and some really nice mechanics without enough to do). Fuel filter, check. Fuel pump, check. Alcohol to get rid of water, check. Debris in lines, none. Kink if fuel line, none. We'd get in and drive around trying to repeat the problem and would drive for 20 minutes with nothing happening until it would do it out of the blue (often when accelerating HARD, but it's happened when going up slopes, down slopes, no slopes, and everywhere from 30 mph up.).

    Any other ideas? I hope to get Florida by Friday night (tonight) so any quick help would be greatly appreciated. Thanks guys!
  2. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    It definitely sounds like fuel starvation under load.

    I'd check the fuel tank venting. It might be plugged and preventing the fuel from drawing through the lines. It would only show up when the fuel pump created enough vacuum to prevent pulling an adequate amount of fuel through the lines.

  3. Dan S.

    Dan S. Member
    Messages: 31

    Check the fuel pick up in the tank for a screen or strainer, I had a truck that had the same kind of problem it would run for about ten minutes then die, let it set for a while and it would run for another ten minutes and then die.
    Found that strainer loaded up with crud it would flow a little bit then plug up enough that the pump could not pull it out of the tank.
    Good Luck
  4. leaveitto

    leaveitto Junior Member
    Messages: 1

    Our 77 C20 had about the same problem you're describing. After disassembling and cleaning the carb twice, I finally figured out that the fuel flows through the INSIDE of the fuel filter, therefore you can't see whether it's dirty or not. I cut open the filter, and sure enough, it was full of gunk. So I cleaned the gas tank, put in a new filter, (plus an extra in-line filter for good measure), and have had no more fuel starvation since.
  5. streetsurfin'

    streetsurfin' Senior Member
    Messages: 770

    If you have dual tanks does it start doing it after switching from one tank to another. Ive driven trucks where switching valve was bad and caused same symptoms.
  6. eamims

    eamims Junior Member
    Messages: 20

    Thanks for all the info. It's a single tank. We talked to a guy and he recommended changing the modulator under the distributor cap saying it could be shorting out. It was cheap so we tried it and I drove the truck for about 80 miles no problem. Then I stopped at a rest stop for about 5 minutes and when I got back in and really accelerated to get back on the interstate it started to do it again (as if it was a heat related problem). I let off the gas and eased back into it and then surging went away. 600 miles later, it hasn't done it again.

    I do have a filter sock on my fuel sending unit, but what a pain to check..I actually did check it out when I changed my fuel tank last year and everything was fine.

    75gmck25...by fuel tank venting, do you mean the capped vent on the sending unit could be clogged?

    It's working pretty good, but we're also driving pretty conservatively. I'll change the fuel filter and cut that old one up to check better.

    thanks guys
  7. 75gmck25

    75gmck25 Senior Member
    Messages: 119

    About fuel tank venting. - based on the year of your truck, I don't know what type of tank venting you have. You might have a vented fuel filler cap, or you might have a vent hose that runs to a vapor canister under the hood.

    If the vent on the sending unit is capped, and you don't have vented fuel filler cap, air can't get into the tank to replace the fuel that is pumped out. This will cause fuel starvation after the engine has run long enough to create a vacuum in the fuel tank.