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Engine Stumble

This is a discussion on Engine Stumble within the General Help forums, part of the LSx Technical Help Section category; In the past couple weeks I've been having an intermittent engine stumble. Car and mods are listed in sig. On ...

  1. #1
    The Bandit Wesman's Avatar
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    Engine Stumble

    In the past couple weeks I've been having an intermittent engine stumble. Car and mods are listed in sig.

    On occassion, while driving, I the engine will develop what feels like a misfire, the whole car will buck, and then it will smooth out again. Never does it for more than about 5 seconds at a time, and so far its only done it when warmed up. its done it about 4 or 5 times at random in the past 2 weeks. It starts up perfectly, idles smoothly, and I have no codes or pending codes. I installed new plugs and wires and changed the fuel filter less than two years ago.

    I'm thinking its a fuel issue. I'm going to try changing the fuel filter again and see if its plugged up, but it shouldn't be seeing as I only use 93 octane from BP. Is this a typical symptom of a weak fuel pump?? It has 91K miles on it now. Any ideas are helpful.
    1998 Trans Am WS6 SGM
    Lid, Pro 5.0, WS6 Short Stick, LS7 Clutch, TB Bypass, Detroit TrueTrac, Poly Mounts, Bilstein Shocks, LS6 Intake, SFCs, CTS-V Brake Upgrade, STB, Nitto 555's. SS Longtube Headers and True Duals w/ H pipe and Magnaflows. Tuned by Harris Speed Works. 341.4WHP/346.1 lb/ft.

  2. #2
    Slow'er'Ass Mr. Luos's Avatar
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    Fuel pump could be an option, but that normally is a constant thing.


    Tune changed? Weather changed significantly?
    Fuel filter could still be the culprit. Which one did you put in???
    Fram?....needs replaced every 2 years/25K miles.

  3. #3
    The Bandit Wesman's Avatar
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    Quote Originally Posted by Mr. Luos View Post
    Fuel pump could be an option, but that normally is a constant thing.


    Tune changed? Weather changed significantly?
    Fuel filter could still be the culprit. Which one did you put in???
    Fram?....needs replaced every 2 years/25K miles.
    Tune has been the same since the end of last summer, when I got it dyno tuned. Nothing different there. Weather has been pretty consistant. The car will run strong as hell most of the time, and then just randomly it will develop this stumble once in a while. Usually after a warm start.

    Fuel filter is a Purolator, I changed it about 2 years ago when I got the car. I'll pick one up from work tomarrow and see if it makes any difference. Thanks for the suggestions.

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    The Bandit Wesman's Avatar
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    Well I changed the fuel filter about a week ago, still did the same thing on a couple different occasions. Cleaned the MAF today before I went to the track, it was fine when I was running it hard, but when I went to leave it started to stumble when I was idling around in 1st gear.

    It only seems to do it when its warm, and it feels like a multiple cylinder misfire when it happens. Only does it for a couple seconds then its fine. So far no CEL. I've also noticed on warm starts it will do the same thing, the engine gradually stumbles to life instead of firing up like it does when its a cold start.

  5. #5
    Veteran 0rion's Avatar
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    I would look at that fuel pressure while it's doing it to at least rule out the pump.

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    Member fast enough Z28's Avatar
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    I think it has to do with the ignition system like a coil or something. I only think that by u saying after it warms up.

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    The Bandit Wesman's Avatar
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    Quote Originally Posted by 0rion View Post
    I would look at that fuel pressure while it's doing it to at least rule out the pump.
    What do you think is the best way to do that?? Just get a gauge and an a-pillar pod and hook it up inside the car?

    That would at least rule out the fuel pump if the pressure is steady when the stumble/hesitation happens.

  8. #8
    The Bandit Wesman's Avatar
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    Quote Originally Posted by fast enough Z28 View Post
    I think it has to do with the ignition system like a coil or something. I only think that by u saying after it warms up.
    I was thinking that also, since it only happens on a warm start after its been sitting. It has never done it on a cold start.

    If I invest in some tuning/data logging software that would be able to count individual cylinder misfires, I suppose that would be able to tell me which cylinder(s) is missing and I could diagnose it from there. If it doesn't pick up any individual cylinders misfiring, I suppose it would probably be a fuel supply issue.

    I was looking at the Autotap Diagnostic, anyone have any experience with that?? And would I need the GM enhanced perameters to get all the features of the program to work with my car??

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    Tell me about it...mine is doing the same thing!

    I changed my fuel filter and took the shrader valve off to reinstall my NX wet kit.

    I am thinking of putting the shrader back in, and changing out the fuel filter again.

    If it still does it, plugs and wires would be next, but after this, who the hell knows what it could be.

    I wonder if a local shop has the technology to diagnose something like this quickly?

    Thanks for the post...maybe we will get someone to help us figure out why our cars do this...91k miles too!

    Rich

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    O U 8 1 2 Spaz's Avatar
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    have you checked you air filter???

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    Bone it like you own it FORD RECOVERY EXPERT's Avatar
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    the fact that it happens mainly when warm would indicate to me that it has to do with something that is in direct contact with the engine... or with the fuel pressure changing once it heats up. Vato Zone has a check gauge that hooks up to the schrader valve on the fuel rail, you can then start the car, and play with the throttle under the hood to see and record your variations in FP. Let us know what you find.

  12. #12
    The Bandit Wesman's Avatar
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    Quote Originally Posted by Z28er View Post
    Tell me about it...mine is doing the same thing!

    I changed my fuel filter and took the shrader valve off to reinstall my NX wet kit.

    I am thinking of putting the shrader back in, and changing out the fuel filter again.

    If it still does it, plugs and wires would be next, but after this, who the hell knows what it could be.

    I wonder if a local shop has the technology to diagnose something like this quickly?

    Thanks for the post...maybe we will get someone to help us figure out why our cars do this...91k miles too!

    Rich
    I'm trying to get my friend to hook it up to his HP tuners to check everything out. If its a coil going dead, it should pick up the misfires and record them for that specific cylinder. You can also tell a bad input sensor by looking at the voltage readings of each sensor through the program.

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    The Bandit Wesman's Avatar
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    Quote Originally Posted by Spaz View Post
    have you checked you air filter???
    Yep. Air filter is good. Its a K&N, so I cleaned the MAF too, since the oil from the K&N can sometimes foul the MAF wire.

  14. #14
    The Bandit Wesman's Avatar
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    Quote Originally Posted by FORD RECOVERY EXPERT View Post
    the fact that it happens mainly when warm would indicate to me that it has to do with something that is in direct contact with the engine... or with the fuel pressure changing once it heats up. Vato Zone has a check gauge that hooks up to the schrader valve on the fuel rail, you can then start the car, and play with the throttle under the hood to see and record your variations in FP. Let us know what you find.
    Problem is it never does it at idle or without a load on the engine, its usually when I go to take off, somewhere between idle and 2500RPM. So I don't think the readings would help me out unless I got a fuel pressure gauge setup for the inside of the car, which I'd rather not do.

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