Loud Engine Noise... Maybe Serious Issue?

[davidzog]

I would suggest AFM lifter spring(s) partially collapsing. That’s what I found on mine.

 

[Kwing]

So after quite some time poking, proding, scoping, pulling valve covers, and replacing a headlight bulb in between I have determined.............. Absolutely nothing.

Transmission mount is bad, but not the source (rigged the scope to the frame and drove it, no movement with the noise). Looked in the cylinders through the plug holes and no disruptions in the layer of carbon that would indicate valve impacts. Torque converter is solid as a rock. Pulled the valve covers and it's like jewlery in there. No busted springs of any kind. Cranked it over with the covers off and no indication of sticking valves.

This is not going well.

So I took a couple videos for you to see/hear it first hand. One with the hood up (sound happens at about 9 seconds, just as i went to climb in the truck) and one while driving (sound happens at 7 or 8 seconds). This was the first time I heard it with the hood open and it almost sounds like a backfire. If you agree, i need two things: what would cause a backfire on a computer controlled 5.3 and why is the computer oblivious to it?

If you don't think it's a backfire, keep the suggestions coming please.

 

[07 black beauty]

Check or have someone check your fuel pressure. When i was having a mild backfire noise from engine area my fuel pump was going out.
Just my 2 cents.

 

[Kwing]

Check or have someone check your fuel pressure. When i was having a mild backfire noise from engine area my fuel pump was going out.
Just my 2 cents.

There's an interesting thought. I'll check it just because it's a 194k miles if nothing else.

I will add this, for what it's worth: consistency of the noise is either improving or I just didn't notice before, but it seems pretty reliable that the sound comes as RPMs drop.

Both times in the videos did that. Hood open popped when i took my foot off the gas to climb in and the driving video popped right after a shift.

Obviously not always the case, such as the cases i mentioned in the OP, but it's about the closest thing to consistency it's shown.

Anyway, just food for thought. Where can I find specs and tolerance for the fuel pressure so I can throw my gauge on there?

 

[07 black beauty]

sorry for the long lapse in response. I do not know rated pressure, I was laid up from injury at work.
Tahoe was taken to shop and pressure was read.

 

[Kwing]

So I know everyone is brimming with anticipation to know the final answer....

Ok back to reality now. So here's the deal, last week i was trucking home from work and as i accelerated to merge onto the freeway the truck fell flat on it's face. Went from american V8 to lawnmower power. MIL started flashing. Pulled it over and hooked up the code reader to find multiple missfire detected on #7.

What I concluded was a backfire that i've just been living with stopped when cylinder 7 stopped firing. So i went about checking the coil pack, the plug and wire, the injector... all was well. So i popped off the driver's side valve cover and cranked it and i'll be darned, #7 lifters are colapsed.

So it WAS the valves slapping on the piston. So much for my scope being helpful. Shoulda stuck with my gut.

Oh well, my new crate motor was delivered friday and that will be my journey this coming weekend.

The truck just cleared 200k, so it doesn't make a ton of sense to do a top end rebuild for $1000 when a reman long block is only $3000.

Anyway, that's the story and thanks again for all the help. Hopefully someone down the line will get some help from our discussion too.

 

[swathdiver]

So I know everyone is brimming with anticipation to know the final answer....

Ok back to reality now. So here's the deal, last week i was trucking home from work and as i accelerated to merge onto the freeway the truck fell flat on it's face. Went from american V8 to lawnmower power. MIL started flashing. Pulled it over and hooked up the code reader to find multiple missfire detected on #7.

What I concluded was a backfire that i've just been living with stopped when cylinder 7 stopped firing. So i went about checking the coil pack, the plug and wire, the injector... all was well. So i popped off the driver's side valve cover and cranked it and i'll be darned, #7 lifters are colapsed.

So it WAS the valves slapping on the piston. So much for my scope being helpful. Shoulda stuck with my gut.

Oh well, my new crate motor was delivered friday and that will be my journey this coming weekend.

The truck just cleared 200k, so it doesn't make a ton of sense to do a top end rebuild for $1000 when a reman long block is only $3000.

Anyway, that's the story and thanks again for all the help. Hopefully someone down the line will get some help from our discussion too.

Sorry I missed this post Kevin. Are you going to keep AFM in the new motor or delete it? Or did you source an L92 or L9H 6.2 to drop in there?

 

[thompsoj22]

So I know everyone is brimming with anticipation to know the final answer....

Ok back to reality now. So here's the deal, last week i was trucking home from work and as i accelerated to merge onto the freeway the truck fell flat on it's face. Went from american V8 to lawnmower power. MIL started flashing. Pulled it over and hooked up the code reader to find multiple missfire detected on #7.

What I concluded was a backfire that i've just been living with stopped when cylinder 7 stopped firing. So i went about checking the coil pack, the plug and wire, the injector... all was well. So i popped off the driver's side valve cover and cranked it and i'll be darned, #7 lifters are colapsed.

So it WAS the valves slapping on the piston. So much for my scope being helpful. Shoulda stuck with my gut.

Oh well, my new crate motor was delivered friday and that will be my journey this coming weekend.

The truck just cleared 200k, so it doesn't make a ton of sense to do a top end rebuild for $1000 when a reman long block is only $3000.

Anyway, that's the story and thanks again for all the help. Hopefully someone down the line will get some help from our discussion too.

At least you found it and got home! The crate motor for $3k is a great choice and should get you another 200k trouble free miles. Let us know how the install go's and pic's of new engine, I just love new engines!!!! I think it is OCD, obsessive combustion disorder!

 

[Kraig]

The lifters collapsing doesn’t mean the valves were touching the pistons, so don’t beat yourself (or the scope) up for not seeing anything.
The noise you were hearing was likely the cam smacking the bottom of the lifter on up through the valve train.

 

[Kwing]

Swathdiver, despite the AFM being the root cause of this evil, I decided to keep it. It lasted until 200,000 miles before the 1st cylinder saw the lifters collaps. If the truck hsd seen better maintinance from the previous owner(s) it probsbly would have gone even longer. On the new motor, sould one collaps again unexpectedly, I saw a neat trick to tap it free. All you have to remove is the intake/valley pan. Then you can apparently reflash the ECU to disable AFM. In my case the engine was a safe bet EOL anyway. We'll see how it goes...

Thompsoj22, i'll have plenty of pics, part of my own OCD. I'll post 'em

Kraig, that is a possibility. There is visible deformation at the tips of the pushrods. But i tell ya what, that noise was awful loud for a rotating cam lobe. Regardless of a smacking lobe or piston though, it was the AFM in cylinder 7 and it goes away with my pretty new long block... and coil packs, and plugs, wires, thermostat, tensioners, idler, belt, and because the whole front clip will be off i got new headlights to boot...

And just coincidental timing i'll be doing the drivers side door lock and either a window switch or motor on the drivers side door (still trouble shooting that, but schematics seem hard to come by).

It's gonna be a very long weekend.................