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Thread: Engine misfire

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    Jr. Apprentice gmcman101's Avatar
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    Default Engine misfire

    I have a 1997 GMC (5.7) with a miss that I can't figure out. A friend of mine hooked his engine computer up and it says cylinder misfire. I could tell that by the way it runs. Changed the spark plugs checked the cap and rotor (look good) and it still runs terrible. It sounds like it's running on 6 or 7 cylinders. The engine shakes at all RPM's. Any suggestions on what I should check next?

  2. #2

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    Was the misfire code specific to a cylinder (P030x) or was it a generic misfire code (P0300)? If the misfire can be isolated to a specific cylinder that might help narrow down the diagnosis to things specific to that cylinder.

    Did the change the wires when you changed the plugs? If not, any sign of a bad wire (spark leak or high too high resistance)?

    I've heard of a few cases where the distributor gear wears funny, creating ignition problems. If you are careful so you can get the distributor back in right, it might be worth the effort to pull it and check the gear.

    Fuel delivery up to spec (fuel pressure up to spec, all "injectors" firing)? I know our vintage 5.7's have issues with the stock poppet valves, to the point that GM has redesigned the injector spider to use real injectors. I'm not familiar with the diagnostic procedure, but it may be worth looking into.
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    I'm not sure on the 97's but my experience with the misfres on the Vortec engines has mostly been a result of a bad coil pack (on misfiring cylinder).
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    Jr. Apprentice gmcman101's Avatar
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    I thought of a coil pack too, until I started getting into it and found that it has a coil.
    It must of been a generic misfire code because it didn't tell which cylinder was not working.
    As far as injectors not firing, it think it has a throttle body. The truck ran good one day and the next day it just ran the way it does now. Bad. Thanks for your replies.

  5. #5

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    Did you try a compression test on all the cylinders?
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    I had to replace a bad coilpack on our 2001 suburban because of a random mis-fire code that kept coming back even efter replacing the plugs and wires so I thought it was worth a shot. You could check for intake manifold leaks to see if it might be a vacume problem. Fuel filter maybe?

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    Jr. Apprentice gmcman101's Avatar
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    No, I didn't do a compression test or check for vacuum leaks. As far as the fuel filter, that was changed about a year ago and I can't afford to buy parts it don't need. Thanks for the replies, I was hoping it might have been a common problem that I didn't know about and someone would say check your...

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    Jr. Apprentice gmcman101's Avatar
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    OK, let try this question. I was told by a friend of mine that it does have injectors and they are located under the intake. One for each cylinder, is this true? I was told by another friend that it only has one injector that runs to all the cylinders? Which is true? Does anyone have a picture of a intake that removed? Thanks again.

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    You have 8 "injectors". The guy who says only one is probably confusing your '97 with the TBI setup that was used on '95 and older 5.7's. They are inside the intake. If you end up replacing them, you'll want to get the newer, upgraded spider that uses real injectors instead of "poppet" valves.

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    Jr. Apprentice gmcman101's Avatar
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    Thanks MrShorty, thats sounds like a lot of work. So the guy at the parts store will know what I'm talking about when I tell him I need the upgraded version? Do I need a Central Port Injector Assmembly? Not sure what to ask for.
    Last edited by gmcman101; 09-16-2008 at 02:53 PM.

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