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Update: Why does my engine go put-a-put when I let off the gas at speed?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by Blue85, May 10, 2001.

  1. Blue85

    Blue85 Troll Premium Member

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    Engine runs good, gas mileage is fine (everything is relative, right?). The problem is that when I'm cruising on the freeway and I let off the throttle completely, the engine goes put-a-put-a-put as though there is some mixture burning in the exhaust. It is not backfiring. I have a stock Q-Jet, but no catalytic converter (please, no flames about this). It did this same thing a long time ago, but when I disconected the A.I.R. system (please no flames about this either), it worked perfectly. Now it does it again. If I rev the engine with no load (park or neutral, le'ts say) and then let off real fast, it does it then too. Does the timing need correction? Maybe my weights aren't snapping back or the vacuum advance isn't working quite right? I also heard about a solenoid that is supposed to prevent your throttle from closing all the way when you let off at speed. There is a solenoid on mine, but as far as I can tell, it only engages when the A/C compressor turns on, to bump up the idle speed. Should it perform this other function as well?

    Thanks for any help you can give me.

    <font color=green>"Do not replace belt while engine is running" --Warning on a Kelly Srpingfield package</font color=green>
     
  2. muddin4fun

    muddin4fun 3/4 ton status

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    Re: Why does my engine go put-a-put when I let off the gas at speed?

    Is it having trouble running or is it like little backfire sounding things in the exhaust pipe? What kind of mufflers are you running?

    [​IMG]
    <A target="_blank" HREF=http://k5chevyblazer.homestead.com/home.html>http://k5chevyblazer.homestead.com/home.html</A>
     
  3. Blue85

    Blue85 Troll Premium Member

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    Re: Why does my engine go put-a-put when I let off the gas at speed?

    No, it's not running bad, it's just sort of quieter backfiring sort of sound in the pipes, like you say. The exhaust is full dual with a X-over just behind the T-case and Flo-Pro 2 1/2" unidirectional mufflers (no fiberglass). It did the same thing when the stock exhaust was on (including the cat), it just didn't seem quite as loud because of the different muffler.

    <font color=green>"Do not replace belt while engine is running" --Warning on a Kelly Srpingfield package</font color=green>
     
  4. muddin4fun

    muddin4fun 3/4 ton status

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    Re: Why does my engine go put-a-put when I let off the gas at speed?

    Do you get any smoke out the pipes or is there a black coating on the insides of the pipes? That's weird that it did that with the stock exhaust. You might have a leak somewhere.

    [​IMG]
    <A target="_blank" HREF=http://k5chevyblazer.homestead.com/home.html>http://k5chevyblazer.homestead.com/home.html</A>
     
  5. flcpl4fn

    flcpl4fn 1/2 ton status

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    Re: Why does my engine go put-a-put when I let off the gas at speed?

    My 85' Jimmy does the same thing. I have full duals w/ 50 seires flowmasters, no cats, and i still have all of the emission stuff on it. My brother in law is a GM mechanic, and he says that is the way it does with aftermarket mufflers. I wouldn't worry about it

    '85 Jimmy in Jax, Fl
     
  6. Blue85

    Blue85 Troll Premium Member

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    Re: Why does my engine go put-a-put when I let off the gas at speed?

    muddin,
    A leak might be a good theory. I know that at times the exhaust actually has vacuum in it because of the movement of the gas and the pulsing nature of the exhaust. You can tell this because the A.I.R. system has check valves and if you feel the pump side of the check valves with the pump disconnected, it is a vacuum source.

    So if I get a little fresh air sucked into the exhaust, the heat could combust it with the unburned gas? I have already identified 1 or 2 very small leaks in the welds just down from the manifolds. Could that be it? I suppose I could also have a leak somewhere in what's left of the A.I.R. system. I'll also check to make sure that both manifolds are tight.

    It never blows black smoke except once in a while on a really cold morning after I've shot too much gas down it's throat starting it.

    <font color=green>"Do not replace belt while engine is running" --Warning on a Kelly Srpingfield package</font color=green>
     
  7. Blue85

    Blue85 Troll Premium Member

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    Re: Why does my engine go put-a-put when I let off the gas at speed?

    flcpl4fn,
    I'm not really worried about it, I just think that to other people, it sounds like the truck runs like crap or is backfiring, so I would like to know how to avoid it.

    <font color=green>"Do not replace belt while engine is running" --Warning on a Kelly Srpingfield package</font color=green>
     
  8. muddin4fun

    muddin4fun 3/4 ton status

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    Re: Why does my engine go put-a-put when I let off the gas at speed?

    It only takes one or two very small leaks to get that popping sound [​IMG]

    [​IMG]
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  9. Blue85

    Blue85 Troll Premium Member

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    Re: Why does my engine go put-a-put when I let off the gas at speed?

    Well thanks, man, maybe we have a winner. I'll check it out and post back

    <font color=green>"Do not replace belt while engine is running" --Warning on a Kelly Srpingfield package</font color=green>
     
  10. Blue85

    Blue85 Troll Premium Member

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    Re: Why does my engine go put-a-put when I let off the gas at speed?

    Well, I'm sorry that I didn't figure this one out before posting for help. The remains of the A.I.R. system were letting air into the passenger-side manifold. A plug in the hose and a hose clamp now have it running great. Strangely enough, I already did this do the driver's side about a year ago, but the passenger's side has been this way the whole time, but didn't start giving me trouble until recently.

    So is the A.I.R. supposed to stop pumping air when the throttle closes or is the vehicle supposed to go put-a-put-a-put-a whenever you lift the skinny pedal? Becasue my A.I.R. did this even when I still had the cat in. Maybe I just needed to tune the vacuum signal to the valve on top of the AIR pump all along. The thing is, you have to not only stop injection from the pump, but also prevent the exhaust vacuum from sucking air in. Maybe it's just supposed to make that sound. I would try tuning the AIR system to work the way I want it to, but from what I understand, it doesn't do anything for your emissions without the cat in place.

    <font color=green>"I don't have the brains for business. I want to go into politics" -Mao Xinyu </font color=green>
     
  11. HarryH3

    HarryH3 1 ton status Author

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    Re: Why does my engine go put-a-put when I let off the gas at speed?

    My engine is out of an '87 'vette, but it has an air divert valve that switches position on decelleration and diverts the air out a tube under the engine. The computer decides when to send air into the exhaust stream.

    <font color=black>HarryH3 - '75 K5</font color=black>
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