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  1. #1

    Default Strange starting/rough running issue - '96 5.7L

    Hi all,
    I'm a newb, appreciate any help. Been fighting a problem for a long time in my 96 Yukon 5.7, 2WD. It has about 180k miles.

    The only mod I've done to the truck is the installation of an injector spider upgrade to the Auto Shop independent injectors (which I highly recommend). I did this mod in part because of the problems that I'm having, in hopes it might correct some issues. It definitely helped, but did not solve everything. I also have an all-aluminum high capacity radiator (replaced this after some heating issues that finally took out the old radiator).

    Over a year ago I started blowing up starters - the engine would kick back so hard during starting, it just blew the starter head up. The timing was going way retarded, creating a kick-back. After much frustration (2 or three starters, new flywheel, crank sensor, distributor, etc.), the problem was found to be that the crank damper key had sheared, and the hub had turned about 90 degrees on the crank snout (I now wonder whether the broken key might not have been the cause, but was instead caused by some deeper issue).

    After fixing the crank hub problem, the engine did start and run okay. Sometime after that, I started noticing sort of rough performance, sputtering, etc., and that's when I broke down and put in the injectors. The fuel pump and filter are also less than a year old.

    Now, I'm back to the engine kickback on startup. It was also running like a rag intermittently. First thing my local mechanic wanted to do was replace the flywheel (grrrrr). Long story short, I did go ahead with new, heavy duty flywheel (he said the old one was cracked because of all the stressful starting and the HD one was needed because of this issue) and starter. Of course, that didn't solve the underlying problem. After running checks, he said both my head gaskets were blown (or, perhaps cracked heads or block). He said he got coolant blowing out of two spark plugs, and coolant system pressure test failed (needless to say, I'm still pretty ticked that he couldn't have got the truck started and done this diagnosis before installing a flywheel). Took the truck home. I was recommended something called Blue Devil. Supposed to fix cracks, gaskets, etc. I know, there are no real substitutes for real repair when it comes to something like BHG. But, I wasn't convinced that was the problem (I guess I'm at the point of second guessing my mechanic). I did experience an overheating episode a while back, due to bad water pump, so, I can't throw out the possibility of bhg or cracks.

    After applying the Blue Devil. The situation seems to have improved, but on cold start, I still get kickback (usually on hot start, it's okay). Now, I took out all the plugs, and I don't see one drop of water coming out. I have no white smoke (mechanic said it was blowing white smoke, I never saw any, ever). No other symptoms of bhg. I bought the idea of water in cylinders causing the kickback, and the knock sensor retardation of the timing. But, I don't really buy it now. There's no water there. But the timing IS being retarded by something. I noticed that with even ONE or TWO spark plugs installed, the engine kicks back on start.

    I am getting ready to try disconnecting knock sensor for diagnostic purposes. When I do this, do I need to ground the wire, put a resistor to ground, or what? How do I fool the ecm to thinking it's working ok?

    I'm also confused as to why the engine ran like crap from time to time, but now is fine.

    Any ideas and recommendations are greatly appreciated.

  2. #2
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    Default

    Welcome to the Club!!!

    Hopefully this bump will put some more eyes on your thread and someone who is familiar with your circumstances will respond shortly.


    Mike (Denver, CO) - 2008 Sierra 1500 Z71 SLE 5.3L 4WD

    SUSPENSION: Rancho 4" Suspension Lift; Rancho RS9000XL Shocks; Rancho Skid Plates; Rancho MyRide Wireless Shock Controller;
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  3. #3

    Default

    Quote Originally Posted by mfleetwood View Post
    Welcome to the Club!!!

    Hopefully this bump will put some more eyes on your thread and someone who is familiar with your circumstances will respond shortly.
    Thanks! Just an update...it looks like the knock sensor is okay (at least it gives the right resistance).

    What about O2 sensor, could it cause these symptoms? Doesn't seem like it would. Beginning to wonder if my Ecu isn't fouled up.

  4. #4

    Default

    Wow what a nightmare, I hope I can help. First, your timing is being advanced WAY too far, not retarded, as that would have it sparking after TDC. It's advancing so far that it's fireing on the compression stroke, not retarded to the point its waiting too long. Recheck the damper keyway, since that was bad before, give the truck a compression test, cold, all plugs out, watching the gauge as you crank, watch for pump up as oposed to straight high reading, and post the results. Check all grounds and connectors for contamination. If it's running rough, it should throw codes. Water in cylinder bores can easilly bend rods, crack pistons, pop head gaskets, and so on, but is usually seen as starter lockup, not kickback. Kickback violent enough to break starters is caused by extreme timing advance. If cold start timing is wandering that far off, then I'd say either the distributor advance is bad, the TPS is bad, or the ECM is schitzo. When I hydroed a 350, I had water in the crankcase, your oil should show this if you still have coolant leaks. Make sure the air intake isn't fouled, same for exhaust. Try runnin g the truck with the gas cap off. They can plug up and cause a vacume in the fuel system. I'll be praying for ya, and let us know how it goes.
    Brian L.




    Brian Larson CST. KC9DAK.
    Janesville WI.
    87 GMC G20 6.2 diesel.

  5. #5

    Default

    Thanks for the suggestions Brian, and the prayers.
    Here's an update. Shortly after I posted that first description, I did pull the damper and check that out. As I suspected, the damper slipping could not have caused the problem (except that it could fool you when trying to find TDC, or using a timing light). The CPS is triggered by a disc that sits between the damper and the sprocket, and it has its own key (it's on the sprocket key, pretty sure). I think all the kicking and jumping the motor was doing actually caused the damper to sheer its key. Anyway, all looked well as best I could tell when I removed the damper (hard to see the condition of the chain though).
    I have heard that the knock sensor can cause so much retard that it effectively behaves like advance (but I haven't thought deeply enough about it to see if the math adds up).
    I have looked for all the signs of BHG that I know - water in oil, oil in water, white smoke, wet plugs, etc. I don't see any. But I know these things can sometimes not show themselves and still have a bhg or crack. My mechanic said it failed a coolant system leakdown test.
    How would fuel tank vacuum affect this? My fuel pressure looks good at the manifold.
    The distributor is only a year or two old. What is it that actually goes bad to cause "bad advance" from the dist? Do you know if the TPS can troubleshot? I can't remember seeing a testing procedure in my Haynes book, but maybe there is.
    I'll run the comp test and post - I'm out of town at the moment, so I'll probably get to it on Sunday.
    I've read about a cam/crank calibration and relearn procedure. I understand that has to be done by someone with the right scanner. Thinking about taking it to my local GM dealer.
    It did throw a code or two, but mechanic said it was erasing them or something. I really don't know what to make of his information at this point, it's very suspect. I've heard the "relearn" code won't cause a CEL.
    My most immediate problem at the moment is that I can't get it to start because it keeps kicking back, and the timing is so jacked up it doesn't want to fire up. I'm afraid I'm going to blow up another starter, or worse yet, break the block.
    Thanks again.
    Keith

  6. #6

    Default

    I'm no expert on the newer trucks, due mainly to the cost of diagnostic equipment, and the increased use of aluminum. I pulled a spark plug out of an aluminum head once, and the threads came with it. The other three came out fine, but I stick to cast iron now. Working with only one arm has it's issues. That said, If I can offer info, I like to try and help all I can. The spark retard is controlled by the ECM, the knock sensor is only a trigger. You can unplug it, and the motor will run, but you may get a CEL. That's ok, if it solves the poor performance, but even if you retard the spark to the extreme, you just lose power until the engine stalls, or you hear it popping out the tailpipe due to ignition with open exhaust vavles. No way can that cause the motor to buck against the starter, let alone enough to break stuff. In a perfect world the ignition would fire at TDC, Top Dead Center, the very top of the compression stroke. In the real world we need a few degrees of advance so the explosion happens at the right time in relation to engine speed, load, and throttle position. Go too far forward, you get a motor that will fire while the piston is still on the way up, aqueezing the mixture. Do that, and the engine will spin backwards, like what yours is doing. Just to be sure, this was happening before the new distributor was installed right? Who did the install? Or more importantly, was it done right? Fuel tank vacume could end up causing the distributor Vac advance to activate. I'm not sure if your truck has Vac advance on the distributor, and it would be a stretch, but with all the vacume hoses on these newer trucks I can't rule it out. Better to perform a simple test, and rule out the possibilities, and take care of the cheap easy stuff first. There are people here who can attest to plugged gas caps causing issues. Sometimes several small things can add up to a big one. The throttle position sensor can be replaced cheaply enough, that if you have done all the other cheaper tests, that it wouldn't be too much of a burden to replace it, even if it wasn't the main issue, as they do wear out anyway, and since it is probably original, wouldn't last much longer anyway. However, it should have thrown a code. If you unplug the battery cables, the ECM codes may be lost, or it may be dropping them due to an internal issue. Once set, they are supposed to stay there for retrieval later. Try unplugging the EST connector from the distributor, as well as the knock sensor, and see if it will start. It may refuse to start due to a no spark condition set because the EST is unplugged. If so, plug it back in, and retard the distributor manually, un til it stops kicking back, and it should start. Usually turning the distributor clockwise retards the timing. If equiped with Vacume advance unplug it and cap the hose, and try it before changing the distributor location. You may want to mark the spot it was at so you can replace it, once things are better. I would simply remove the distributor, set the engine to TDC and reinstall it, with the rotor facing number 1 cylinder. But then I know all the tricks to this. If you haven't done it before it will be tough to do. If you like I can give you an easy step by step on it. How do ya like me now?
    Brian L.




    Brian Larson CST. KC9DAK.
    Janesville WI.
    87 GMC G20 6.2 diesel.

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