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  1. #1

    Default !994 Z71 Engine stumble

    First time user of this site.

    1994 GMC Z71, 350, TBI. 182K miles. Completely stock.

    Have had this truck since 1995 and has always run great. Plugs wires, cap and rotor are ~2 years old. Lately I've noticed a stumble under load, the tach also has a slight jump to it. It has gotten worse and now pretty much misses all of the time, worse after it warms up. At times it seems to almost clear up for a few miles but the miss will return. I checked the basics (plugs, wires, cap, rotor) Wire and plugs look great. I ended up replacing the cap and rotor and changed the fuel filter. No help. Coil was original so swapped that out, the tach is steady now but still misses. No light show under the hood. I read somewhere that the pick up coil or ignition module can break down over time. Given the age, I went ahead and installed a new distributer, set it for zero with the wire under the glove box disconnected. No help. I did a vacuum check and it pulls a steady 20 in. Checked fuel pressure and it was 12 lbs.

    No check engine light, codes have not been scanned for.

    What’s left? fuel injectors? I read somewhere to shine a timing light at the injector spray to see the spray pattern better. It looks cone shaped and isn't “dumping fuel”. It was during the spray check that I noticed that some of the time, when the engine misses, the timing light will not flash. Not always, but sometimes. I imagine the other times it's missing on another cylinder. Is the corresponding timing light miss mean I'm still chasing an ignition problem or can it still be fuel related?

    Sorry for the long post, wanted to get as much info as possible.
    Thanks in advance for any advice.
    Last edited by BJensen; 12-30-2008 at 03:52 PM.

  2. #2
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    dwill3015's Avatar
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    Welcome to the site!
    Darcy
    Washington State
    2006 Silverado 2500HD LT3 4X4 CC SB Duramax LBZ
    Tuff Country 6" lift, 35" Toyo M/T's on 20" Ultra Peacemaker wheels, Quadzilla Stealth2 programmer, Diamond Eye 5" cat-back exhaust, factory Special order color Yellow.

  3. #3
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    I had something very similar to this years ago with an 87 Jimmy. I did most everything you did but one evening I opened the hood and happened to see a faint blue spark go from a plug wire to the exhaust manifold . I reached out and grabbed the wire and quickly found out to shut the engine off stupid!!!The wire looked fine til you got it under some good light and could see tiny little pinholes, I believe some mice had tried to make a meal of it and found out that they just can't tear silicone like they can most wires.Hope this helps, just have the engine running and run your hands across the plug wires til you find it,LOL

  4. #4

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    I'll give the wires another look but I think I'll use a meter versus the hand method.

    Thanks for the tip.

  5. #5

    Default

    Well, the old girl is running smooth again. The probelm did end up being the plug wires. I had done a visual on them and they looked great, that plus they are only 2 years old made me think they were fine. But when I check them with the meter I foiund more than few that measured more than 200K ohms of resistnace.

    It really does pay to check the basics first.

  6. #6
    Jr. Mechanic
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    Makes ya just wanna go for a joy ride don't it.

  7. #7

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    Quote Originally Posted by mrfixdit View Post
    Makes ya just wanna go for a joy ride don't it.
    As a matter of fact I did. It ws nice to finely go down the road without it bucking all over the place.

  8. #8

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    On The left side of the fire wall there is a cover over your sending units and fuseable links. I had the same problem on mine and the fuseable links are going bad look for any green corrision> Or the fuel pump sending unit might be starting to go bad. Mine ran bad hard starting and it was the sending unit then it stopped last week and it was a fuseable link.
    Hope it helps. Jack

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