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Engine stumbles/stalls under heavy throttle

This is a discussion on Engine stumbles/stalls under heavy throttle within the General Help forums, part of the LSx Technical Help Section category; Driving back from the beach, car would "buck" when giving more than gentle throttle. Car is a M6 2001 Ls1 ...

  1. #1
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    Unhappy Engine stumbles/stalls under heavy throttle

    Driving back from the beach, car would "buck" when giving more than gentle throttle. Car is a M6 2001 Ls1 with flowmaster exhaust. Car was fine while crzuing, issue only came up while accelerating. No problems revving at idle at first. If I was going 50 in 5th gear and stepped on it, the engine would miss, but not throw a code. From a stop, the car will stall out and check gauges light come on. If I slowly accelerate in 2nd gear, I can get all the way to 4000 rpms with no issues.

    I changed the fuel filter, air filter, and added seafoam to the gas. Drove about 10 miles and can now repeat the issue in neutral.

    So basically, when pushing the gas pedal hard, it stalls/misses with no codes thrown. I'm open to suggestions!?!?!?!?

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    Spaz is My Mentor SMWS6TA's Avatar
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    Did you recently fill up?

    Check fuel pressure at the rail. Should read about 56 psi when you turn key on. Let us know if pressure drops or holds?

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    Was at 42 psi turning to "on" while cold. After cranking, fluttered around 58. Is it my fuel pump?

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    I'd check the fuel pump for sure. Codes being thrown?

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    Spaz is My Mentor SMWS6TA's Avatar
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    You missed my first question, when did you fill up prior to this issue?

    Could be something as simple as bad fuel.

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    Car was recently filled with new gas, had a full tank when I noticed the issue. Down to 3/4s tank, would filling it now help, or do I need to burn it down to a quarter tank?

    No codes being thrown, which is a pain.

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    Certainly sounds like a fuel issue. Does it crank a bit longer than usual before starting? Since the stumble can be duplicated in neutral, install the pressure gauge and rev on it a bit to see what happens.

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    Yeah...wasn't able to get the issue to repeat this afternoon with the gauge on it. No difference in start time that I noticed.

    Does the pressure I found seem normal?

    I know it's hard to troubleshoot from forum, thanks for everyone's input so far!!!

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    Senior Member JayTA98's Avatar
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    normal pressure is 58-60 psi at all times with the car running. If you rev it up and it drops then there is an issue.

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    The "42 psi" reading after energizing the ignition is not good. It should jump right up to around 58 psi and then vary by only a couple of psi up or down from there while running. I think the lowest fuel pressure I have ever seen on our car was 53-54 psi on a stupid hot day, in traffic, with the AC blasting. Other than that one occasion, it generally stays at 56 psi or higher no matter what.

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    Quote Originally Posted by bull1112 View Post
    Car was recently filled with new gas, had a full tank when I noticed the issue. Down to 3/4s tank, would filling it now help, or do I need to burn it down to a quarter tank?

    No codes being thrown, which is a pain.
    I asked because I was trying to eliminate the fuel as one of the potential problems. Everyone forgets that you can get bad gas, especially after a heavy rain.


    From the following posts and replies I'm with the other guys as well. Looks like your pulling your pump.

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    Thanks for all the quick posts. Any chance this could be a sensor somewhere or the IAC valve? I'm just curious as to everyone's thoughts before I cut a hole in the car or drop $600 on having someone else do the fuel pump?

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    IAC has no affect on fuel pressure.

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    Quote Originally Posted by bull1112 View Post
    Thanks for all the quick posts. Any chance this could be a sensor somewhere or the IAC valve? I'm just curious as to everyone's thoughts before I cut a hole in the car or drop $600 on having someone else do the fuel pump?
    It's going to either be your fuel filter clogged (which you replaced recently), a bad fuel pump motor or a bad leak on the fuel pump tubing(which is in the fuel pump itself).

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    Since this happened with a full tank, someone at work suggested bad gas. I pulled two plugs since that's easier than the fuel pump and their fairly fouled up (brown on one side). I'm going to pick up a can of octane boost, try to burn what's left of the gas, add some good gas, and the change the plugs. Problem is I have over half a tank to burn through!

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    you could disconnect the line at the fuel filter and put a bucket to collect the fuel. pull the fuel relay and use a small piece of wire to hardwire the pump in the relay terminals(you to find out the terminal that has power and the one that goes to the pump). when the bucket is getting full pull the wire out and put the next bucket in. this is the way I empty my tank when I was going to drop it.

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    Siphoning the gas isn't what I was concerned with, more what to do with 10 gallons of bad gas. If I do the pump, I may try your suggestion.

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    Skip the octane boost -- crap in a bottle that may raise the octane all of a tenth of a point. Better bet would be some isopropyl style dry gas.

    I agree though that you should drop the filter to see what is actually coming out of your tank if bad gas is truly suspected. We had someone accidentally dump diesel into the unleaded tank at an area gas station and one of my friends ended up with some. Also, call the station and complain -- see if they say anything about it. Could be there are others out there having the same issue and that would answer the question in a hurry.

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    I agree with Jeff, follow my suggestion and disconnect at the filter. get some fuel (just a little)out so you can see if it is contaminated or not. That will give you a visual confirmation.

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    There is also a filter sock at the bottom of the fuel tank(inside) connected to the fuel pump inlet. If it is clogged it will cause issues too.

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