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Thread: 5.3 misfires, can't find the problem!

  1. #11
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    A noid light is a small diode style bulb that plugs into the injector connector. You plug it in, crank the motor over to see if it flashes. This tells you if the pcm is firing the injector.

    You can also do this with a good DVOM. My fluke has the capability.
    99' Silverado RCSB Z71: 5.7L Iron LS1, 11.2:1, Custom EPS Cam, Ported 862's,Ported TB, E-Bay Cold Air, E-Bay Long Tubes, Dynomax X-Pipe Exhaust, Electric Fans, Innovative LC-1, Self Built 4L60E, Circle D Pro Single Disc, 4:10 ValueTrac


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  2. #12
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    Seafoam can break the ring lands between the rings if poured in too fast. People have been known to bend rods when using seafoam.

  3. #13
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    If they are. 706 heads, the tend to crack more than any other head
    PERFORMANCE-TUNING-FABRICATION
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  4. #14
    Join Date
    Jan 2012
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    Sorry for the slow response, I got a noid to confirm spark and fuel, a compression gauge to do a compression test, and a service manual. I will post results, tonight if work goes smoothly. Will also confirm casting number on heads and look for crack if visible. Thanks for the help. -Andrew
    Last edited by andrew04; 01-25-2012 at 03:04 PM.

  5. #15
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    862 Heads!, I gather this is a good thing?

  6. #16
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    it is. The ones GM has a bulletin out on are all 706 castings. That being said not all 706's are bad, just ones manufactured by Castech.
    2006 Silverado
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  7. #17
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    Quote Originally Posted by AFSP1211 View Post
    check the coils by removing the boot at the coil and see if it arcs. If you get a good arc, it's not the coils. Maybe post a sound clip of if from under the hood.
    You do not want to fire a coil without a load on it. A good way to check out a coil to is to swap it with another. To kill a coil but not damage anything you can put a piece of heater hose in between the coil and the plug and touch a grounded test light to it to kill spark at that coil, or just unplug it at the primary side. You could maybe do that and see if the misfire gets worse. If it does, it could be fuel related or valve or head problem.
    Last edited by screaminchicken; 01-27-2012 at 11:07 PM.
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  8. #18
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    good news! all 8 cylanders were 120-150 psi when I compression tested them. Will troubleshoot fuel and spark tomorrow. At this point I'm thinking a computer tune to remove emissions devices and optimize a/f ratio would be the best plan of action.

  9. #19
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    Good to know that it is external and not an internal problem. I would diagnose the miss before I messed with the tune. It is always better to tune a known good running engine. That way the tune does not get blamed for the miss or make it worse.
    99' Silverado RCSB Z71: 5.7L Iron LS1, 11.2:1, Custom EPS Cam, Ported 862's,Ported TB, E-Bay Cold Air, E-Bay Long Tubes, Dynomax X-Pipe Exhaust, Electric Fans, Innovative LC-1, Self Built 4L60E, Circle D Pro Single Disc, 4:10 ValueTrac


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  10. #20
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    Quote Originally Posted by andrew04 View Post
    good news! all 8 cylanders were 120-150 psi when I compression tested them. Will troubleshoot fuel and spark tomorrow. At this point I'm thinking a computer tune to remove emissions devices and optimize a/f ratio would be the best plan of action.
    While it's good info to know, I'm surprised you aren't doing the easy things first? Changing out a couple of coils is by far easier than doing a compression check ... JMHO. I agree with country_3030 in waiting until you have a good running engine to worry about the tune. Also, previously you stated you were getting 60psi at 2k rpm. I don't see an issue with this, as long as it isn't up there at idle. This is especially true with fresh/full oil. If you are running higher viscosity than recommended you'll see higher readings as well. I have 122k on my truck. It maintains around 55-60psi running down the road at highway speeds. From my experience, the LS engines seem to maintain great oil pressure even with some higher mileage on them. A LOT better than the Gen I/II engines of yore.
    - Paul J.
    2006 Silverado LT3 L33 Z71 - Mods: K&N Air Filter; Taylor Wires
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