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5.7 Vortec misfire when warm

TruckMeRunnin
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TruckMeRunnin Enthusiast

TruckMeRunnin

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So I'll make it short need help got my cam retard set perfect just did spark plugs truck runs fine one day then starts misfiring while driving the next checked cam retard while driving it's good so I don't think it's timing and it drive fine when I pulled out of my driveway but 10min after leaving started missing again my buddy thinks it's old plug wires what's your thoughts buying parts tomorrow need opinions fast

SierraDually Enthusiast

SierraDually

Posted
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What year is your truck? Are you getting a flashing check engine light during misfire or any codes stored?

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TruckMeRunnin Enthusiast

TruckMeRunnin

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97 GMC Sierra k1500 and no it's not throwing any codes or flashing cel. Funny thing is it ran just fine on the way home from work. I hooked up my scanner and ran live data on dash command it's missing on cyl 5 at idle and at low rpm when driving but when I was driving at higher speeds it was not missing at all on any cylinders then when I got down to 5mps in my driveway it was showing a miss on 5 again still not throwing a code and now it's not missing like crazy how it did on my way to work today. This is confusing lol 

Logan Lanfear Enthusiast

Logan Lanfear

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Plugs, wires, cap and rotor.  If that doesn't fix the problem make sure you're using high quality stuff.  I like to stick with AC delco for my ignition stuff.  Its not too expensive ordered online.  Ive heard decent stuff from Bosch as well.  Just don't cheap out on them and they'll last and work well. 

green 788 Enthusiast

green 788

Posted
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I ran my truck on 7 cylinders for a long time... that wasn't a good idea, but you couldn't tell it was running on 7 cylinders above 1500 rpms or so .  I had a bad head gasket.

 

If the engine idles fine when it's cold, then it starts idling rough after warm-up... shoot a blast of freon or freeze spray at the ignition module during rough idle, and see if it smooths back out.  If it does, that module is the problem.

 

Clogged cat converters caused my original problem with my 1996 5.7 engine.  First the head gasket blew on one side, then on the other.  If you can get the O2 sensors removed (the 2 before the converters) that will leave a hole in the exhaust there to see if the truck idles and runs better... if it does, clogged converters are the likely cause.  A GM tech who is a friend of mine say they use this technique at the dealership where he works to diagnose clogged converters.

 

You should also check for bubbles in your radiator, or antifreeze smell at the exhaust.  See if the engine oil is milky tan looking on the dipstick.  That's coolant in the oil, likely bad head gasket.  If a head gasket is seeping, it can cause the misfire problem, but I don't think the computer would log it as a specific cylinder misfire since gas and spark would still be present.

 

If you do find evidence of a bad head gasket, it may be caused by clogged converters.  They need to be dealt with in some way (or other :) )...  and then the head gasket repaired.   The K&W block sealer in the radiator apparently works well enough that many reputable mechanics say it's worth a try to fix a leaking head gasket, to avoid pulling the heads.   Blue Devil and Barrs Leak also make head gasket sealers that have decent reputations for working.   If the engine has more than 180k miles on it, installing new head gaskets and re-worked cylinder heads will probably cause the rings to fail (I've had this happen on a Pontiac engine, and was warned by my mechanic friend that new heads and gaskets on a motor with a lot of miles on it was not a good idea)...  so for an old engine with a minor leak in the head gasket, the sealer product in the radiator might be the best, and it'll certainly be the cheapest option.

 

None of this is to say you have a blown head gasket.  I just wanted to share a bit of what I've learned from my own recent experience.

 

Dan

 

 

SierraDually Enthusiast

SierraDually

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Posted

On that motor the fuel pressure regulator is under the plastic intake manifold plenum. If it fails and leaks it can leak into mostly one cylinder and cause what you describe. It won't occur at driving speeds. Just at idle. 

Not saying that's definitely your problem. But can easily be ruled out with a 5 min fuel pressure test which I would do before throwing parts at it. In my experience, spark related misfires usually throw a code, but not always.  

TruckMeRunnin Enthusiast

TruckMeRunnin

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On 11/30/2017 at 8:38 PM, SierraDually said:
On 12/1/2017 at 10:15 AM, Logan Lanfear said:

Plugs, wires, cap and rotor.  If that doesn't fix the problem make sure you're using high quality stuff.  I like to stick with AC delco for my ignition stuff.  Its not too expensive ordered online.  Ive heard decent stuff from Bosch as well.  Just don't cheap out on them and they'll last and work well. 

That's kinda the route I was thinking but I really am trying to narrow it down so hopefully I can just buy one part for now instead of multiple just throwing parts at it. I did further testing with my scanner while driving keep in mind there are still absolutely no codes being set off but the live data show's over 180 counts of misfire on #5 at idle zero while driving. And my culprit while driving it's #2 missing intermittently anywhere from 2 to 20 counts at a time? Now if I floor it I don't get any misses or stumbles does any of this help narrow down the most likely cause?

Logan Lanfear Enthusiast

Logan Lanfear

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Same thing was happening to mine, I was getting a misfire on cylinder 4 with no codes.  But if I ran it at higher RPM it would go away.  I replaced plugs and wires but that didnt do it.  So cap and rotor was my second choice, now it runs on all 8 like a dream.  

TruckMeRunnin Enthusiast

TruckMeRunnin

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So here are my thoughts this far. I'm getting under the hood tomorrow so I'll check plugs for carbon tracking or cracks and I'm thinking switching the 5 and 7 wire to see if the misfire at idle moves then I think that might narrow down wires maybe also will check pressure regulator for seepage. Is there any way to test and know for sure it's the cap and rotor if I don't find and issues with my wires and plugs? And can anyone explain why cylinder 5 doesn't miss while I'm driving and only at idle and why 2 does the exact opposite? Also this all started after I set my cam offset to 0 it was at +6 before I adjusted it and ran just fine... 

TruckMeRunnin Enthusiast

TruckMeRunnin

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Ok so I replaced cap rotor and wires the number 2 miss is gone and the truck is drivable again yay lol 5 is still missing at idle but it didn't seem to be an issue it still idle ok a little rough but not major and it doesn't miss at all when driving only at low low speeds and at idle ultimately I would still like to figure it out any thoughts given that I replaced the electrical

  • 2 years later...
Steven818 New Member

Steven818

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Posted
On 12/10/2017 at 1:12 AM, TruckMeRunnin said:

Ok so I replaced cap rotor and wires the number 2 miss is gone and the truck is drivable again yay lol 5 is still missing at idle but it didn't seem to be an issue it still idle ok a little rough but not major and it doesn't miss at all when driving only at low low speeds and at idle ultimately I would still like to figure it out any thoughts given that I replaced the electrical

Hello i just joined this forum. The first forum i have ever joined. Im just curious if this issue was resolved i have the same issue with my 98 Silverado.  Fuel systems new and ignition systems  new. And also replaced intake manifold gaskets and spider injector same missfire cly5

  • 1 year later...
AMPM64 New Member

AMPM64

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Posted

I'm new here but my 96 siera I bought 6 years ago I was told the timing chain was bad and I could have it for 500 so I bought it for reasons that have no bearing here the truck sat for 5 years I've replaced the chain and gears and nothing I thought maybe I was off or out 180 neither of these were the case the chain and gears I replaced were probably fine so someone said cap rotor...... When I pulled the cap the rotor was missing the center rounded portion of the conductor aha I thought obviously right but no so I ordered a cap cause I'm on SSI and hell yeah baby we got fire and actually runs great til she gets a little warm then she kicks and bucks and misfires like something just ain't right

 someone says o2 I have no reader I guess I could take it to O'reilys up the block any other ideas before I have to start throwing money I ain't got at her 

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