Stutters and backfires - clogged cats?

[Rocket Man]

If it was me, I would buy a new one. They’re only $128 on RA. You shouldn’t need anything else, the gaskets are reusable and everything else just transfers over. https://www.rockauto.com/en/catalog/gmc,2007,yukon,5.3l+v8,1433252,engine,intake+manifold,5536

 

[RockGlock86]

So waiting for a intake to get here, and still considering that this somehow could be electrical aswell. Checked all my grounds on the coils and for chassis ground read 11.4 on cyl 1 and 10.7 on 3/5/7, for PCM ground they all 4 read at 12.04 volts. Checked Ohms on the chassis ground at 80ish, and 45ish on the pcm ground. The numbers were fluctuating to fast to get a good read, I'm not sure if that supposed to happen or not. All with key on engine off. Unless this is somehow a bad ecm. I'm not sure how you would check that. I just got it from black bear So I assume its good but who knows. The problem seems to intermittent to be ecm but I do not know alot about such things. Anyone know the signs or symptoms of a cracked intake? I assume it would lead to a lean code, but with the way these things throw rando codes for unrelated issues its hard to tell some times. From what I can tell all of my wiring is correct and operating correctly aswell. I also have a VXDiag on the way so hopefully that will help narrow things down. @iamdub I heard you know about wiring and such.

 

[OR VietVet]

Why would a cracked intake keep it from turning over, I read that in a previous post about 5-6 back. Or did I miss something?

 

[RockGlock86]

@PNW VietVet I think I am dealing with two seperate issues at the same time, The ongoing backfire and lack of power and 351/3/5/7 faults, and now this weird hesitation to turn over that just started happening. I am unsure if one is causing the other, or just happening at the same time.

 

[OR VietVet]

The Z71 that I just bought started and ran great when I first saw it. I then bought it about a week later. Drove home fine. I went to start it 3 days ago and the dash lights came on and when I went to crank the engine, the dash went black and I cycled the key and it would not light the dash and would not crank over. I popped the hood, checked battery cables and the negative cable was very loose. I got out my trusty 5/16" wrench and took off and inspected both cables and they were clean and no corrosion. I installed the cables and put my battery charger on it for 3-4 hours at a slow rate. Has not failed to start since then. Basics first ruled the day.

 

[Rocket Man]

Why would a cracked intake keep it from turning over, I read that in a previous post about 5-6 back. Or did I miss something?

I was only suggesting a possible cracked intake for the misfires on one bank, nothing more. I wasn’t referring to the latest issue with not turning over.

 

[Rocket Man]

The Z71 that I just bought started and ran great when I first saw it. I then bought it about a week later. Drove home fine. I went to start it 3 days ago and the dash lights came on and when I went to crank the engine, the dash went black and I cycled the key and it would not light the dash and would not crank over. I popped the hood, checked battery cables and the negative cable was very loose. I got out my trusty 5/16" wrench and took off and inspected both cables and they were clean and no corrosion. I installed the cables and put my battery charger on it for 3-4 hours at a slow rate. Has not failed to start since then. Basics first ruled the day.

Yessir.

 

[thompsoj22]

slow to turn over? im guessing that your timing/ecm is fubar. as long as you are sure it isnt a mechanical obstruction or low voltage than the timing beingadvanced will create the slow to turn as the cyls misfire on the upstroke. are the cyls misfiring the ones that the new lifters went into? i didnt go back and read but did compression test have good results?

 

[exp500]

Do you see the tach drop or fluctuate? I'm surprised no-one asked about crank position sensor.
Another way to test for manifold crack is spray can test- Use a flammable Brake cleaner, may not see an internal crack. Also, your Ohms test to ground are high. Try directly grounding ecm with alligator clips and retest.

 

[RockGlock86]

Tac does drop and fluctuate with the idle, it is representative of the actual idle and matches my torque pro. Vam position sensor is a month old and also did a cam relearn on it aswell after replacement. I will try out the brake cleaner, just spray some on the Intake while it is running? And for ecm ground just clip to the case of the ecm and to a ground location?