5.7 Vortec backfire/shudder

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bartonz

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This will be long, trouble some bastard... any thoughts will be greatly appreciated.

Info:
96 GMC Yukon with the vortec pushing 290,*** miles. Many parts have been replaced along with engine rebuild. I have a backfire and shudder problem that arose over summer. On cold start up there is usually an intermittent backfire that won't stop until driven accompanied by a blinking SES. The idle is very rough on start up and the motor is sluggish once in gear and going, I've found that babying it up to ~35 MPH the engine smooths out and it is like the problem never exists. Once the backfiring episode is over there is an idle fluctuation in gear at a stop from 650 to 500 rpms and once it dips there is a shudder. I have noticed this shudder a hand full of times while 65+ MPH but goes away with some gas. I have had nothing but problems with this vehicle from day one. I have done a tune up since the rebuild with AC Delco plugs gapped accordingly, Taylor 8mm wires, Delco rotor, Accel cap (didn't have Delco in stock), MPFI spider update, timing retard set within spec of +/- 2 degrees. Everything under the sun to get misses and BS over with. All was well until the summer.

What I have tried since:
Half bottle of MMO in crankcase for possible sticky valve. Little in gas tank. This quieted up some small ticking and seemed to help the backfire as well but it is still there on start.
Cleaned MAF, IAC, EGR, checked for vacuum leaks didn't catch any.
Really stumped. I plan on changing the cap and rotor again this time all AC Delco and maybe the coil. Heard accel's caps can cause crossfire which could be my whole stinking problem. Again thanks for any insight.
 

AppocAlexx

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Dang, you have done a lot already and you said you're still planning on some that I would suggest but some things to check would also be compression on each cylinder, fuel pressure from the pump. With the things that you have done already you never know you may need to up the fuel pressure with an after market pump or even replace or clean the spider, could have gotten a bad one if its aftermarket. Also during your tune up did you do fuel filter?
 
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bartonz

bartonz

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Yes I've replaced the fuel filter with a Napa or wix one. That filter was getting clogged I had a bog high speeds. But the fuel pump I had never touched as of yet nor tested the fuel psi, if I recall it's something along the lines of 60-66? Compression was tested good awhile back. Like I've said this dude has got me. I've surpassed my knowledge and skill on this one.
 

AppocAlexx

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Most vehicles are anywhere from 35-50 min. This being a truck and bigger engine I would lean toward the 50 min for pressure at the lines. Im sure you can find the exact specs somewhere online. You may have to check pressure at different intervals of engine under a load with the kinds of issues you are saying in order to rule out fuel pressure. May take some extra tools as well to test.
 
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bartonz

bartonz

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Any excessive exhaust backpressure?
Excessive like crimped pipes? The exhaust consists of shorty headers, pipe welded in place of old cat locations, and a 3 chamber muffler
 
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bartonz

bartonz

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I'll figure it out soon one day and when I do I'll update for anyone in the future with similar problems.
 

TigerEyz3

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Could be as simple as having some moisture under the cap - strange things happen when moisture and electricity get together.
Could be one or more of the following related to the distributor:
  • loose hold-down
  • worn distributor gear (especially given the mileage) - I'd replace the distributor with a metal one rather than a plastic one.

Are you getting any codes?
 

east302

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I'd start with the distributor cap, see if it is cracked or if there are carbon tracks, but go with a Delco.

Double check the wire routing and condition as well.

You're sure that you have the new injectors in the right intake ports?

A worn distributor gear could impact it as well.

Have you checked fuel trim levels to see if it's running rich or lean? Are the oxygen sensors in place? A scanner could also tell you which cylinder is misfiring.



Sent from my iPhone using Tapatalk
 
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bartonz

bartonz

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New cap, rotor, and coil on the way. I will double check wire routing and everything. I'll go from there.
I don't have access to a scanner when this blinking SES light is on. Only on the first start up of the day really. If it was something super detrimental the code would remain illuminated or at least I figure.
 

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