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Periodic engine "stumble" I can't figure out

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by BadDog, Dec 29, 2001.

  1. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Periodic engine \"stumble\" I can\'t figure out

    Ok, I thought I had this thing worked out. At first, when I got it running, the idle was high and it ran little rough. I could find no vacuum leaks. I had very short exhaust (open pipe to back of tranny basically) so I thought maybe not enough back pressure causing trouble for the O2 sensor. Putting a cheap muffler and short pipe on it fixed that problem. Idled and ran great.

    However, it would periodically start sputtering and stumbling often accompanied by sporadic small backfires out the exhaust. If it didn't die, it would eventually (after 5-20 seconds or so) settle back down. Seems to me, too much fuel and/or cylinders not firing right. Plugs, wires and rotor button/cap showed signs of wear so I replaced them along with a new air filter. Runs better still but I still have this periodic "stumble". I will also replace the fuel filter but I wanted to wait till after a few tanks of gas run through. I don't have a fuel pressure gauge so I can't be sure if the filter is plugged. Seems like the problem usually manifests when I am above idle and add a bit more throttle. Other than that, I don't see any pattern, it just sometimes decides to do it. Most of the time, it runs very well. When it starts acting up, it will do it repeatedly for a while. Also, I've fooled with it a bit and it does not seem to do it until it starts to warm up, if that helps.

    I'm thinking maybe the coil. I may just run down and replace it today anyway. If that makes no difference, I'll have a spare for the trail. Maybe I'll go ahead and replace the fuel filter too. Anything else I should be considering? Is there some knowledge I'm missing of the, "Oh, on 89 TBI systems they do that when the ????..." type.

    I've never been a great mechanic when it comes to diagnosing engine problems like this. When it's broken, I can generally find it and fix/replace it but this "it mostly works but sometimes not quite" mess I fall short. Add in the new "unknown" (lack of experience) with EFI/ECM/Sensors and my diagnostic abilities become really suspect. On any of my old (non computer/EFI) toys I would at least feel comfortable that I had enough knowledge for poking around but there are so many unknowns here. I am basically to a point of replacing things at (more or less) random until it works. I also have to be concerned that there is some inherent flaw in my converting this thing to TBI which would mean that I am wasting time replacing components. Any help would be appreciated. TIA.

    [edit update]
    Right after I posted this, I got to thinking about the EGR. I pulled the hose and plugged it, then another test run. As of yet, it hasn't been reproduced. It is idling now, I'll continue fooling with it for a bit and see what happens.

    Russ

    85 K30 CUCV, 350 TBI, TH400, 205, D60/C14, 4.56 Locked
    Some day: 4" lift, 44" tires, massive cutting, shorter wb and rear overhang.<P ID="edit"><FONT class="small">Edited by BadDog on 12/29/01 11:12 AM.</FONT></P>
     
  2. tRustyK5

    tRustyK5 Big meanie Staff Member Super Moderator GMOTM Winner Author

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    Re: Periodic engine

    That sounds similar to my friends truck when his EGR went. His truck is an 89, TBI 4.3 V-6 in a full size pick-up. It took 3 different shops to diagnose the problem correctly.

    Keep us posted...

    Rene

    <font color=green>Dyslexics of the world...UNTIE!</font color=green>
    <a target="_blank" href=http://coloradok5.com/gallery/project_T2> tRusty pics...</a>
     
  3. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Re: Periodic engine

    I have not had another problem since I disconnected the EGR. All I did was unplug the vacuum line from the EGR head and stuck a bolt in it. No more problems. Looks like I'm buying an EGR valve.

    Thanks for the confirmation.

    Russ

    85 K30 CUCV, 350 TBI, TH400, 205, D60/C14, 4.56 Locked
    Some day: 4" lift, 44" tires, massive cutting, shorter wb and rear overhang.
     
  4. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Re: Periodic engine

    Ok, I just called about an EGR valve. Turns out there are two. One is $33, the other (the one I have) is $54. They bolt to the same mount (so he says) and there is really not any noticeable difference in the two. What might the difference be? Does it really matter? The price would seem to indicate that there is some difference. I would never imagine that there would be 2 different EGRs on the same model, same size engine, same application, and same EGR manufacturer. It seems quite happy with the EGR unplugged completely so I'm thinking I'll just stick the cheap one on there. I don't mind spending $20 when it matters but for something I will never notice a difference in, I would rather put that money toward other things...

    Russ

    85 K30 CUCV, 350 TBI, TH400, 205, D60/C14, 4.56 Locked
    Some day: 4" lift, 44" tires, massive cutting, shorter wb and rear overhang.
     
  5. 95 Silverado

    95 Silverado 1/2 ton status

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    Re: Periodic engine

    they may have a different spring on the diaphragm which would make it operate at a different vacuum setting, probably not much difference you would notice.

    '95 Chevy Silverado 1500
    5.7 V8-NV4500- 3.73 rears
    See My Toys: <a target="_blank" href=http://community.webshots.com/user/wdb172454>http://community.webshots.com/user/wdb172454</a>
     
  6. BadDog

    BadDog SOL Staff Member Super Moderator Author

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    Re: Periodic engine

    That I could understand. But $20 for a different spring rate seems nuts. I suspect you are right (or at least close) which is why I will probably go with the cheap one.

    Russ

    85 K30 CUCV, 350 TBI, TH400, 205, D60/C14, 4.56 Locked
    Some day: 4" lift, 44" tires, massive cutting, shorter wb and rear overhang.
     
  7. HarryH3

    HarryH3 1 ton status Author

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    Re: Periodic engine

    Most likely, the expensive one is the exact replacement while the cheaper one is a universal replacement. The universal ones come with a bunch of washers with different diameter holes in them. You match up to the one that's closest to the diameter of the opening in your factory EGR valve, then stake the washer into the housing and install the EGR.

    <font color=black>HarryH3 - '75 K5</font color=black>
    <a target="_blank" href=http://www.angelfire.com/super/ThunderTruck>www.angelfire.com/super/ThunderTruck</a>
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