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Engine Stumble at idle.

Discussion in 'Vortec 4.2L I-6' started by Alec Venable, Nov 9, 2016.

  1. Alec Venable

    Alec Venable Member

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    Hello, I have a 2003 GMC Envoy with the 4.2l I6, and I'm experiencing a strange stumble at idle. Whenever the engine is warm and idles at 600rpm, if I go to turn the wheel and accelerate, the engine stumbles and acts like it wants to stall, then picks right up. It also happens when shifting from revere to drive, and trying to accelerate and turn the wheel. It has done this for along time, over a year, but it never stalled until one day a couple weeks ago; I had just shifted from reverse to drive. I replaced the vacuum line from my FPR to resonator because it had cracks but that didn't seem to solve my issue. Anybody out there had this and know a fix? I've replaced the upstream O2 sensor, cleaned throttle body, replaced VVT solenoid, cleaned map sensor, and changed the spark plus. The truck has 192,xxx miles. Could it be the cam position solenoid? Kinda stumped. Any help is appreciated. Thanks.
     
  2. shovenose

    shovenose Well-Known Member

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    Exact same boat here with the same mileage. Will keep an eye on this thread.
     
  3. freddyboy61

    freddyboy61 Well-Known Member

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    When you cleaned the throttle body and replaced those parts, did you reset the PCM by disconnecting the battery or pulling the PCM fuses for at least 30 minutes.
     
  4. Alec Venable

    Alec Venable Member

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    Yes I did leave the battery unplugged, both when cleaning the tb, and changing the vvt solenoid. I don't think I did for the spark plugs. But I didn't think I needed to for those.
     
  5. 07TrailyLS

    07TrailyLS Well-Known Member

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    Not sure if I misread. But did you change any of your coils? It sounds like your missing. When turning the wheel or shifting your rpms fluctuate slightly. One of your cylinders missing can throw the rpm increase out of whack and stall the motor. That's what happens to my buddies 01 dodge Dakota every time he turned the wheel if he didn't shift to neutral and apply extra gas and accommodate for the missing
     
  6. Alec Venable

    Alec Venable Member

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    Well, I wondered if maybe I had a moil missing. But I forgot to mention that I have no SES light on. And whenever it happens no light comes on, and when it stalled the one time no light came on. Is that still a possibility? If I have no light how would I test to find out which coil is missing?

    Oh and no I didn't change any coil packs.
     
    Last edited by a moderator: Dec 8, 2016
  7. Wooluf1952

    Wooluf1952 Well-Known Member

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    Do all the warning lights on the dash work? If the engine dies some or all the lights should light up.
     
    xavierny25 likes this.
  8. Alec Venable

    Alec Venable Member

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    Yes they do indeed work, they all light up when i start it.The other thing it does sometimes (and i hope it's related) is crank continuously, but then not start. It takes like 2 tries then it fires right up. But that happens like twice a year. Maybe the ignition switch? Or would the ignition switch not cause idle issues?
     
    Last edited: Nov 10, 2016
  9. 07TrailyLS

    07TrailyLS Well-Known Member

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    If it's cranking but not starting, you could have a possible fuel pump issue. It may not be turning on and moving the fuel through the line like it should.
     
    xavierny25 likes this.
  10. Alec Venable

    Alec Venable Member

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    But Wouldn't a fuel pump issue show up while driving at higher rpm's? Or are you suggesting an issue with the relay?
     
    Last edited: Nov 10, 2016
  11. 07TrailyLS

    07TrailyLS Well-Known Member

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    Maybe the relay isn't engaging on start up. For the ease of mind I would spend a couple bucks and just replace it for shits and giggles. If it works it works, if not, we move on to the next thing
     
  12. Alec Venable

    Alec Venable Member

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    Ok, well I think I might try that, and definitely try replacing the can position sensor. Will let you know if it works.
     
  13. Alec Venable

    Alec Venable Member

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    So I'm currently in the process of getting the camshaft position sensor replaced. In the mean time, does anyone have any experience regarding an issue like this? I'm really not sure what to think because I have no ses light on. I would hate to just keep throwing money at this till it goes away, but my options are pretty limited. Any more suggestions?
     
  14. Alec Venable

    Alec Venable Member

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    So I'm sorry for the late update, but I ran into some more issues that had to be taken care of first.

    At one point my engine stumbled again, then died. I started it back up only to find that it would run on 5 cylinders...I lost an ignition coil. So I ended up replacing all of them, believing that was causing the engine stumble. It appeared to have fixed the problem, but I later noticed it was still present. If nothing else, I have all new ignition coils now. :2thumbsup:

    I later did more research on weird issues at idle, to discover that the 4200's have been know to have low compression in one or more cylinders caused by ruined valve seats in the cylinder head. Ran a compression test on all 6 cylinders, and the compression values ranged from 170-180 psi. So no issues there. I am just now getting around to changing the cam position sensor, along with a new thermostat, coolant temp sensor, oil change and transfer case fluid change. :crazy:

    I will report back when the new thermostat, coolant temp sensor, and cam position sensor are in. My understanding is that a bad thermostat will cause a rich running condition due to the engine not up to operating temps. I'm thinking that could also contribute to the issue.

    To be continued...
     
  15. gmcman

    gmcman Well-Known Member

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    Did you change the plugs before or after the symptoms were present? Did you use Delco 41-103 plugs? Do the coil packs have their springs inside?

    Any driveability issues I encounter or hear about, similiar to yours...after a TB cleaning, ensure there are no vacuum leaks, I use a can of BG 44K....I swear by this stuff. It's not snake oil and IMO, it's the top of its class.

    Have you replaced your fuel filter?

    Check the large vacuum hose under the resonator, make sure it's attached.

    Make sure the vacuum hose is secured at the throttle body.

    Make sure all your intake manifold bolts are tight, use a 1/4" drive 10mm socket...gently snug them if they are loose, only requires 89 Inch pounds of torque.

    After that, let us know how it responds.
     
    xavierny25 likes this.
  16. xavierny25

    xavierny25 Platinum Supporter

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    That is a possibility but in my current situation where my thermostat isn't bringing my temps past 177 in the last week I haven't had a stumble but I do notice my mpg dropping pretty fast.
     
  17. Alec Venable

    Alec Venable Member

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    I replaced the spark plugs last year, and I yes I did use Delco 41-103's. Fuel filter was also replaced last year. I will have to examine those vacuum hoses you mentioned. What is the purpose of BG 44K? I've never heard of it. Is it used for finding vacuum leaks? Or is it like seafoam? Oh and the intake manifold bolts have been checked and tightened already. They were pretty much snug.

    Oh ok. Actually my parents Trailblazer is also running too cold, but it does not stumble either. Maybe that wasn't it. :Banghead:

    Moderator edit: Please use the edit button to add more info instead of multi-posting. Posts Merged.
     
    Last edited by a moderator: Dec 26, 2016
  18. gmcman

    gmcman Well-Known Member

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    It goes in with a full tank of fuel, will dissolve alot of carbon from the combustion chambers as well from around the valves. It's good preventative maintenance, I use this after the winter season and sometimes at the end of summer.

    Works very well, hard to see what's going on inside so I feel it's good practice to use at least once a year.
     
  19. Alec Venable

    Alec Venable Member

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    I'll have to look into that. About 2 months ago I ran some gumout fuel system cleaner through it. Would that still necessitate the use of BG 44K?

    Also, I completed the install of the camshaft position sensor, new thermostat, and new coolant temperature sensor. All of these maintenance items appeared to have improved the idle quality and throttle response to a degree, but the engine still stumbles when I try to turn the wheel and accelerate at the same time. One thing that is noteworthy, whenever I have had to disconnect the battery for any length of time, say when cleaning the throttle body or replacing a sensor, when I test drive the vehicle after reconnecting it, the issue seems to vanish, but slowly creeps back in within about 20 minutes of driving. Could it be that resetting the PCM causes it to forget air/fuel ratio and/or spark timing that causes the problem, only to slowly relearn what the running conditions were that caused the problem?

    I'm wandering if it has something to do with fuel trim values possibly affected by a vacuum leak as you mentioned, or possibly a partially clogged cat. I did check the vacuum hoses you mentioned and they appear to have been in order and properly connected. Unfortunately I have no scan tool to watch live data on, and they seem quite expensive.
     
  20. gmcman

    gmcman Well-Known Member

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    44K is an entirely different animal IMO, works quickly and is great preventative maintenance.

    What did you clean the MAP sensor with? I don't think you can spray things into the sensor, maybe electronics cleaner but I would doubt that also.

    Have you replaced your fuel filter? Located under the drivers side frame, about under the drivers seat. Pull fuel pump relay then crank engine, then remove black cap if it's still there and drain excess fuel into suitable container. All you need is a pair of pliers to squeeze both tabs then pry lightly with screwdriver. Just loosen the line then back away from it when it releases to avoid splashing into face.
     
  21. shovenose

    shovenose Well-Known Member

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    Gumout regane has pea in it which is the best thing for cleaning fuel system and combustion chambers. I'm assuming BG 44k is something similar with more concentration and thus more expensive.
     
  22. gmcman

    gmcman Well-Known Member

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    Yes, that's how I understand it.
     
  23. Alec Venable

    Alec Venable Member

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    I actually just used maf cleaner on it. Didn't change a thing. And yes I already changed the fuel filter. The old one was quite clogged. But that was almost 2 years ago.

    The Envoy has plenty of power and what not otherwise when accelerating, getting up to highway speed, or just passing. But it's like the idle isn't strong enough, although it idles smoothly. I'll look into running BG 44K through it. If you have any other thoughts I'm all ears.
     
  24. AzTruckGuy

    AzTruckGuy Well-Known Member

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    Had the same issues myself, do yourself a favor and change the fuel pump at 193k I am amazed it lasted that long I had to change mine out at 98k miles and those stalling issues went away....
     
  25. Alec Venable

    Alec Venable Member

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    I appreciate your input. I was reluctant to think that it was a fuel pump issue because I thought it would show up at high rpms if it was. Perhaps I can run a fuel pressure test.

    Were your symptoms exactly the same as mine? What I'm experiencing appears to be the engine's inability to support the load of the power steering pump at idle whenever I turn the wheel. When I turn the wheel at a stop, the rpms fall down to like 450, but then bounce way up to about 900, then down to 600. All this happens within a about 2 seconds. My parents have a 2007 Trailblazer and if I'm standing still idling in that thing, and I turn the wheel, the rpms are completely unaffected by it. I'm kinda stumped by it. Is that the exact same issue you experienced? Since I replaced my ignition coils, thermostat, and coolant temp sensor along with the cam position sensor I havent stalled out. I still need to run some BG 44K through the engine as gmcman recommended, we'll see if maybe that frees up some gunk anywhere.

    Thank you all for your replies and helpful information. Please keep em' coming!
     
  26. AzTruckGuy

    AzTruckGuy Well-Known Member

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    Do you really think a fuel pump is going to last 193k miles, if it hasn't been changed out?
     
  27. shovenose

    shovenose Well-Known Member

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    as far as i know the fuel pump in our 02 envoy with almost 200k is original. having the same problem so 1. i'm interested if it would solve the problem and 2. why you don't think it would last 200k? lol
     
  28. Mooseman

    Mooseman Moderator

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    Because they typically don't last that long.
     
    AzTruckGuy likes this.
  29. AzTruckGuy

    AzTruckGuy Well-Known Member

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    The same reason why batterys, alternators, brakes, etc go out....What you think you're driving a Toyota? Gm doesn't work that way
     
  30. gmcman

    gmcman Well-Known Member

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    Something else I meant to ask....what fuel grade do you use? When you're chasing driveability issues like this, with our platforms that run very well on 87 octane, I would strongly recommend using 87.

    I understand the small benefits of high octane with regards to ignition timing and high engine loads, but for 95% of the driving 87 is what will work best.

    Reason is, with gas prices the way they are, the 93 sits in the ground longer than the 87...and with being mixed with ethanol you can understand how it will be hit or miss.

    Same goes for 89, as 87 is mixed with 93 to get the 89.

    Just wanted to throw that out in case you don't run 87 on a regular basis
     
  31. Big B Jantzen

    Big B Jantzen Well-Known Member

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    I have exact same issue. I read most above, but not all.

    Check your rack and pinion unit. I replaced my steering pump and thought that would solve it, but it didn't. I just recently noticed some gear oil and grease that fell from steering rack when I turn the wheels while idle. If i gas through it it will not stall.

    I have not had the chance to tackle mine, but I noticed the oil and grease.

    EDIT** Just so you know this is after I have replaced the following:
    Fan clutch, thermostat, steering pump, idler pulley, belt, spark plugs, cleaned intake and throttle body, use 97 only, PCM Tune, new CV, New disconnect AWD, UAC's NOT flipped, alignment, new LCA...
     
    Alec Venable likes this.
  32. Big B Jantzen

    Big B Jantzen Well-Known Member

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    Most all these units start reporting gas level wrong when they are going bad. The float unit usually goes out first and you have this at 2 gallons left it says you have half a tank thing. Atleast what it did to me.
     
  33. MRRSM

    MRRSM Gold Supporter

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    @MAY03LT would probably counsel, “Test First...and Verify the Problem before changing anything...” In this first video, he shows the Two Methods for testing your Fuel Pump Pressure that depends upon which year vehicle you own:



    ...and if it comes down to cases and the Fuel Pump needs replacing "on the driveway"… His video and finishing remarks give an excellent lesson on the reasons for NOT using after-market Fuel Pumps:



    ...and as a second look at how it gets done with a Shop Lift showing additional good views of where things are located under the vehicle… this last How to R&R the Fuel Pump in a Trailblazer. This one has value for its Installation Instructions ONLY… Always Choose the Quality OEM Fuel Pump Bosch and Delphi Fuel Pump over any other less expensive brands!

     
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  34. Alec Venable

    Alec Venable Member

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    Sorry it has been so long again, just wanted to post back to update my progress. My motor mounts were just changed. Wow what a difference. And I was also able to test my fuel pressure. Key on, engine off: 49 psi. Engine running: 55psi. So the fuel pump appears ok. Seeming as how I have no codes, I'm not sure where to go with this issue. However, I will say this. After new coils, thermostat(yes, old thermostat contributed to the problem), and new motor mounts, the issue is greatly reduced. The new motor mounts didn't actually help the engine hold up better under the load of the power steering at a stop, however it minimized the vibrations and therefore it's less noticeable when it happens. (if that makes any sense). And even when it does happen, the rpms drop from 600-500, so nothing catastrophic. So I'm under the assumption at this point that pursuing the issue further would be an overreaction on my part. Unless the truck starts stalling again, I may just chock this one up and move on. Thank you all for your input and suggestions!
     
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  35. Realism

    Realism Well-Known Member

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    I've had this happen to me a few times in my 04 voy. It's been a while since it last occurred, but I think it might have been the power steering pump. I'm assuming you aren't heaving full right or left - Correct me if I am wrong, but turning the wheels means more power output from PS pump which. The load on the PS pump will cause a drop in output to alternator and the fluctuation is just the attempt to balance and maintain idle RPM and voltage. Similar to how when we reach over and hit more than one power window button at a time, we get that drop in RPM and light dim until we release it. I would say if it is persistent and dropping more than you think it ought to, you could try checking the pressure and then seeing if there is anything gunking it up and adding to an increased (above normal) load, which in turns transfers onwards.
     
  36. Alec Venable

    Alec Venable Member

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    I appreciate your input! I too had drawn the same conclusion. I thought it was the pump. However, I am beginning to think it is my steering rack and pinion. A close inspection of it revealed that both boots on each end are leaking (very slowly) fluid, and both have a small tear in them. Time for a new steering rack I guess? I also notice that when turning the wheel at slow speeds or at a stop, I notice a hum/groaning noise coming from the power steering system, that gets louder as I turn the wheel. For a long time I assumed it was the pump, but I seriously think it is the rack; it sounds like it's coming from underneath the vehicle. I found a post here about how to replace it, but am trying to decide on doing this myself, or hiring it done. Any recommendations for a new rack? Brand? New or reman.? Etc. Thanks!
     
  37. Realism

    Realism Well-Known Member

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    The noise you hear while turning your wheels is going to be from your power steering pump. If you have leaks in both bellows, it's a fair bet to say you've ran your pump dry or close to dry enough times it damaged it. Your rack and pinion though may or may not be fine. It could just be you wrecked the seals at either end and replacing those would solve that leakage.

    I've not had to replace a rack and pinion so I can't give you info on brands and all that, but I think the rodie made mention in a thread once what ones to avoid and which works best.

    I don't think they would be too terribly difficult to replace if one is fully assembled. But either way, I can guarantee you, if your PS pump is filled and still making that noise, it will need replaced.

    But that there is enough to cause for your load shift and drop in rpm
     
    Last edited: Mar 16, 2017
  38. Mooseman

    Mooseman Moderator

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    If it's leaking, might as well swap it. I'm personally not keen on Cardone anything and I don't have any experience with other brands except ACDelco for pumps, which is good. I got a reman from a Canadian rebuilder once. Look for a local rebuilder. I have one here and forgot to check them out first.

    I agree with Realism, you should replace your pump. Flush out your lines and cooler of old fluid before running it or flush them with the old parts in place.
     
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  39. Realism

    Realism Well-Known Member

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    I started thinking of what may have precipitated the leaking from both ends, and Mooseman struck on what to do to prevent a recurrence: Flush your lines and cooler. Blowing both ends out likely means you had some sort of blockage resulting in very high pressure in the lines.
     

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