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Thread: Sputter while driving, and MAJOR MPG drop 350 5.7L

  1. #1
    Senior Member stepsidejay's Avatar
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    Angry Sputter while driving, and MAJOR MPG drop 350 5.7L

    First off its a 92 350 5.7L TBI.... Ok well here is the problem, while driving down the road it sputter a little bit, not like a sputter u would get from a clogged fuel filter or bad plug, the RPMS actually go up! It only does it for like a sec, then stops, and does it pretty often, it only does it while driving over like 55 after the truck is warm, does not idle funny, or do it while driving slower speeds like in town, does not sputter leaving from a dead stop! also my MPG have dropped from 16-17 to ike 8-10. I took a look at the injectors, and there was a little drip along with the spray, and my EGR was out! SOOOO I figured it was due to that fact of too much gas or a bad EGR...NOPE, changed the EGR, and rebuilt the TBI! no change still have a drip and still sputters! all this started happening after I got the tranny rebuilt back in July, its the 700r4, and had a mild shift kit installed! I have no clue what it is....TOTALLY Crazy!! throwing me off!

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    Senior Moderator chevychase's Avatar
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    Do you have check engine light? Try the coolant temp sensor may be trying to flood some/too rich fuel mixture.

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    Senior Member stepsidejay's Avatar
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    ya the service eng soon light was coming on but quit after i changed the EGR valve.

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    Something else you might want to check is the Theromstat. If you have a gauge showing Temprature, look at it and see what it reads. If it's low, that may indicate a open Thermostat. Most Tempratures should be 190 to 195 if I remember correctly. Another indication may be if the heat in your truck isn't up to snuff. If the blower is sending out luke warm or cold air? Change the Thermostat.

    Something else I'd check. The fuel filter. How long has it been since the fuel filter has been replaced? If it's been quite a while, I'd replace it.

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    Senior Member stepsidejay's Avatar
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    ill Change both of those, was going to any ways, but do not think this will fix my problem! but we will see!

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    The thing about the thermostat (if it's indeed stuck open?) the computer will think the engine is running cold. What it does then is to run more fuel into the engine because it's trying to warm-up the engine, which never warms up.

    Does your truck have a temprature gauge? If so when the engine has run for about 15 minuets. What is the reading on it then? If you do have the gauge, and it's running below 190 degrees, this will tell you about your thermostat if it's doing the job or not?

    That Spitting you're talking about with the RPM's going up for a second almost makes me want to think it's either a vacuum leak under the throttle body? If you ever run into a problem where the RPM's stay high? Replace the gasket under the throttlebody.

    You might be getting into into the Ignition part of things. After checking or replacing the simple un-expensive items like the thermostat and fuel filter. I would make sure the cap and rotor are nice and clean looking, as well as the plugs and wires. If you think the wires are subject to arching out, you can take a spray bottle of water when it's low light outside and spray the wires to see if they arch to ground. You may also want to use some good fuel injector cleaner in your fuel tank. This drip you speak of may indicate the injectors are gummed up inside. Fuel should spray from the injectors when the engine is running.

    I'm sure you already know what I've said here. I'm just writing this stuff down so I can remember myself. Oh, about the Check Engine Light. Disconnect the Battery for a few minuets to re-set the computer. This will turn that light off. Unless there is another problem? If so, the light will just come back on. But you said it wasn't on untill you worked on the EGR?

    Good Luck!

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    Senior Member stepsidejay's Avatar
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    Well thanks WHITE... yes the EGR fixed the light, has not came on since! and I am also thinking of a vaccum leak, but I just rebuilt the TBI, new gaskets and o-rings and injector screens, but did not take the whole thing off just from the injectors up! so ill look and see if I can get the TBI off with out tearing down the injectors and all the other stuff off the top of the TBI cause all those gaskets are new! But as for the thermostat, i have not changed it yet, and the truck idles perfect, temp stays right around 190-195 only gets up to like 210 only if I'm driving the holy **** outta it!! lol which i have not done since I seen the needle hit 210 the last time I was FLYING lol...(dont do it often but had to prove a point to a buddy with a ford) so its like the truck tries to overheat only when racing on the interstate! but i figured that was cause i was pushing it so hard at the time. once I let outta the gas and go back to cruising speeds the temp drops right back to 190-195.

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    on my 92 the mpg would drop as the o2 sensor was going bad, but would not set a light
    4 wheels move the body
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    Senior Member stepsidejay's Avatar
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    i was gonna change the O2 any ways, and there is only one on the 92 model right?

  10. #10
    Senior Moderator chevychase's Avatar
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    Right - on driver side. If that does not do it . Try the temp sensor.

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