2002 6.0L Yukon Denali Backfiring thru intake under load...need suggestions

[huston2326]

OK...so here is the backstory
Driving on highway @75mph one evening and all is fine. Suddenly heard a loud noise and truck shuttered for a second. Thought i blew a tire or something. Pulled off the highway and got out to inspect. As soon as I opened the door there was the obvious very loud exhaust leak @ driver side front. Broke out the flashlight and saw that the exhaust pipe had broken and separated at the flange that connects to the exhaust manifold. Now back in the day I owned an 87 Fox body GT with clogged cats and it couldn't drive past 30mph. This 6.0L still drove normally and seemed to have all its power and decided it would just blow the pipe apart instead of being held back...lol

Soooo...I limped it home sounding like a drag engine w/open headers.
After weighing my options on replacing the cats and O2 sensors I decided aftermarket cats were mixed results and OEM were out of the question due to costs. No emissions inspections here
All exhaust pipework was in good shape..no rust, so I removed the pipes and cats to hollow them out and re-weld and re-install. I also installed new upstream O2 sensors. All was good at restart...I was even ok with the new sound of hollowed out cats. In park or neutral engine revving is smooth and normal. Under load the slightest aggressive acceleration causes what I feel is an internal backfire popping noise..possibly @ intake. If you baby it and even piss off Ms. Daisy its fine. The only code coming up is for catalyst efficiency reported by the downstream O2 sensors I believe, because they are not there anymore. I am ok with the engine light on for that and did not try the anti fouler method.

As my truck is well deserving of some TLC I decided to go ahead and change the plugs, wires and coil-packs and see if any of that was involved. No Change as a result.
I have visually inspected the intake manifold for cracks...nothing obvious. There is no valve-train noise out of the normal. No obvious bad Vacuum lines, hissing air sounds etc.

Any insights are much appreciated

 

[B-train]

I remember back in the day, plastic intakes would melt internally from EGR issues. I wonder if the increased back pressure and heat caused some internal damage to the intake or valve train.

How did the plugs look when removed? Have you tried a compression test? How's the fuel pressure or has the pump and fuel pressure regulator ever been replaced? Have you ever cleaned the throttle body (aka: brush it's teeth with some carb cleaner and and an old toothbrush).

 

[rockola1971]

In my experience these type of exhaust separations and engine popping almost always lead back to an ignition problem. Bad coil(s), Spark plug wire(s), cracked Spark Plug(s), Timing Chain jumped time or wiring harness problem to associated igniton system components. Sounds like you replaced all but the timing chain.
I would run a scanner to verify no active misfires and if any check affected cylinder ignition system components and wiring harness. Wouldnt hurt to verify fuel pressure, especially off idle. Check your fuel trims and see if one side is off from the other substantially.

 

[OR VietVet]

Anyone that knows me, knows I am going to suggest a compression test and every time I have a chance, like when you did the plugs, I will do a compression test. Then dig deeper if needed.

 

[mb1500]

Without the cats you’re probably running too lean and misfiring during that specific condition. Check misfire data, fuel trims, etc

Also, were all of your exhaust manifold bolts in good shape?

 

[MassHoe04]

Sticky valves can also do that. Exhaust valves hanging open when the fuel is ignited can make a pretty good bang in the header!
I would suspect that kind of thing would show up with a cylinder reading low on the compression test...

 

[huston2326]

I remember back in the day, plastic intakes would melt internally from EGR issues. I wonder if the increased back pressure and heat caused some internal damage to the intake or valve train.

How did the plugs look when removed? Have you tried a compression test? How's the fuel pressure or has the pump and fuel pressure regulator ever been replaced? Have you ever cleaned the throttle body (aka: brush it's teeth with some carb cleaner and and an old toothbrush).

Too Funny...are you trying to get me started on my RANT against so much plastic these days? You may be on to something tho i will agree about the excessive back pressure and heat. The plugs were Bosch platinums and all seemed to have equal burning, only 7 already had chucked 1. Compression test incoming along with a fuel pressure test. Fuel pump replaced 2 years ago, not sure on the regulator. I have cleaned the MAF sensor before but never tackled the throttle body.

 

[huston2326]

In my experience these type of exhaust separations and engine popping almost always lead back to an ignition problem. Bad coil(s), Spark plug wire(s), cracked Spark Plug(s), Timing Chain jumped time or wiring harness problem to associated igniton system components. Sounds like you replaced all but the timing chain.
I would run a scanner to verify no active misfires and if any check affected cylinder ignition system components and wiring harness. Wouldnt hurt to verify fuel pressure, especially off idle. Check your fuel trims and see if one side is off from the other substantially.

You are correct! I have not done anything with timing components. Are you suggesting the extreme back pressure may have caused a chain jump? Odd thing is the popping/backfiring noise I hear does seem to be more so towards the drivers side. If the timing had jumped wouldn't I have the issue among all cylinders seeing as how this is a single cam? Scanner wise...I have an el cheapo code scanner not one of those super complicated ones and no codes present at moment. Even the CEL is off currently. Harness and wiring all appear to be unmolested. I will be testing fuel pressure from the port on the rail, as far as the trims go...thats where the Fancy Dancy scanner comes in?

 

[huston2326]

Anyone that knows me, knows I am going to suggest a compression test and every time I have a chance, like when you did the plugs, I will do a compression test. Then dig deeper if needed.

10-4 Compression test incoming.

 

[huston2326]

Without the cats you’re probably running too lean and misfiring during that specific condition. Check misfire data, fuel trims, etc

Also, were all of your exhaust manifold bolts in good shape?

OK, so I will need either a new fancier scanner or a shop to to check misfire data and fuel trims.
Oddly enough your manifold bolt suggestion has possibly yielded something. When I did the plugs, wires, coil-packs I must have had blinders on see pics for what was found.
The first 2 pics are driver side. The 2 far end bolts appear to have missing heads. The 3rd and 4th pics are passenger side. 3rd Pic appears to show manifold mounting flange separation between the 3rd and 4th bolts from the right, or is that how they are? 4th Pic shows another missing bolt head. Funny thing is I bought this truck with 36000 miles and to my knowledge these manifolds have never been removed.