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  1. #1

    Smile Rebuilt Vortec 5.7 runs rough after 15 minutes

    Hi all.
    I just rebuilt my 5.7 vortec engine in a 1996 chevy suburban 1500 4x4. Had the block all worked over by a machine shop. Had them redo the heads (valve job etc) and redid the rings etc. New oil pump everything pretty much new. This has a new upper & lower intake and new injectors as well.

    Problem: After first start up the suburban runs fine. Drive for 15 minutes or so and it acts like a plug wire is missing and tries to shake itself apart.

    What I have done: I took it to oreillys and borrowed their code reader. It said that cylinder 8 was misfiring. I took it home and checked compression in all cylinders. All are ok. I then took off the valve cover and readjusted the valve for cylinder 8. I put it all back together and had the same problem again. This time cylinder #3.

    I will be taking off the drivers side valve cover and checking valves on #3 and will retest.

    My Questions: 1. Can anyone think of a code or reset procedure or anything like it that I would need to do after rebuilding this engine? Someone was telling me that the throttle needed to be relearned and that I had to turn the engine on twice while putting my finger in my ear to make that happen
    2. I cant seem to get a straight answer on how to adjust the valves on this. What I have been doing is starting the engine, loosening the valve until it starts clacking. Then I tighten it until it just barely stops clacking. Then I take it 1/2 (one half) turn further. Is that correct?

    Thanks in advance.

  2. #2
    stephan's Avatar
    Join Date
    Sep 2010
    Far West, Oregon USA,
    Blog Entries


    Hey Jone, welcome to the GM truck club. I don't know about the "relearning of the ECU" Since your compression check was good on all cylinders, I don't think this is a valve adjustment issue. I would think either an electrical misfire, (plug or wire) or a fuel injector "lean misfire".
    Did you change spark plugs in it when you did the rebuild? There have been many discussions on here about the vortecs only likeing ACs'
    1988 Chevy C-3500 2wd (no pic)
    350 c.i. 5.7 L Stock Block, 4 Bolt Mains
    L-31 Vortec Heads, Edelbrock Cam & Intake,
    Holley 650, Flowtech Headers, Magnaflow exh.
    Jet Trans 700R4, B&M Ratchet, 4:10 gears,
    3" susp. lift kit "shadetree"
    No rev limiter, No speed limiter lol

  3. #3


    Thanks for the quick reply and the welcome.
    I put new ac plugs/wires. This was a complete rebuild. New distributor etc. Replaced cam and knock sensors etc. The only thing really not changed was vacuum lines. I also need to look at this thing at night while running and see if there are any sparks/light shows.

    THe lean misfire on the injector would probably be an airflow sensor or something wouldnt it? It is a new injector set at a whopping 350.00 bucks.

    I'm pretty sure I need to double check the valve adjustments though. I think they might have been tightened down too much when first installed. I also need to recheck compression on each cylinder and write down the numbers. I think a couple of them were kind of low for a newly rebuilt engine (90 lbs or whatever). Will a valve adjustment impact the compression poundage that shows up during a compression test?

    thanks again in advance for your help.

  4. #4


    if i may jump in, make sure u installed it without damaging the o rings on the injectors ( spider ) happens some times, and next time buy it on e-bay for 120 bucks, its the same as OEM !! lol

    AND yes, 90 is very low....should be around 140-160 IMO

    1996 Suburban K3500 SLT DUALLY
    1988 Camaro RS 383 stroker LT1
    1992 32' Aero-tek Race Boat, 600 CI BBC

  5. #5


    Thanks for the info on the o-rings. I'm not sure if I damaged them or not. How would I be able to tell? Pretty sure I got the lifetime warranty on the injectors now, so if they are bad i'll rip em out and take them back in. It isn't easy to get at the injectors under the upper intake manifold - alot of crap to take off. Is there a way to tell if I have a problem with the injectors without removing the upper intake?

    As for the compression - the engine was cold when I tested it last time. Do you know (or have a procedure) what steps I should take to check the compression. Sit at idle for 30 minutes or ? It's hard to remove plugs and put a compression tester in there when it is hot.


  6. #6
    stephan's Avatar
    Join Date
    Sep 2010
    Far West, Oregon USA,
    Blog Entries


    Yes if the valves are too tight it will cause low compression & your's IS low. Gas won't even fire much below 95-100 PSI. If the valves aren't closing, or barely closing, the air/compression can leak out when the pistons are trying to compress it. Did you do your preliminary V adj according to a shop manual? Also for an accurate compression check you need to have the engine warmed up to operating temp. 195 - 200 degrees. This is where it runs 99% of the time & you need to know what it is under these running conditions. It could also be a little low if you haven't run it long enough to break in the rings, but not that low.

  7. #7


    I knw this is a old thread but maybe someone out runs into a similar problem and can use this. I ran I to this same problem on my parents Vortec 4.3 blazer which is basically the same thing. And you knw who was to blame for cylinders not running the darn fuel injection spider system. When I swap these out I wish you could see how badly burnt the end where my god!! Luckily I found them cheap on eBay and I had a friend give me some very helpful advise. He ran compression tests we uncapped the engine and we could not find jack, 96-I believe 2000 uses the same spider system if u haven't already fixed this I'd give a shot a new fuel system will even ensure ur gas mileage is up to date.
    I'm doing the same here rebuilding entire 5.7 with a few lil goodies for my 99 Tahoe but this time it's getting everything new including the spider system trust me its a pain a change once the motors in the car why go threw all that to save $100. Hope that helps u guys a out a bit. Laters

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