Lost Two Gen 3 LS Engines in my 1986 C10 2WD

[shortboxin]

Let me start by stating that both engines were at least 160k miles when installed.

Recently swapped my truck to a 2004 4.8L which I idled in my driveway 5+ times for extended periods before actually taking it out for its maiden voyage which was only 3 blocks. Ran great when parked, but when I went to start it days later, it turned over really easy like it had no compression. Suspected a starter initially, but quickly dispelled that by seeing the belt rotate. Then checked for broken timing chain by recording video through oil fill hole in passenger valve cover and saw the rockers moving. Started a compression test on cylinder #2 (easiest access) and got 0PSI. At this point, I resolved that the timing chain had slipped to a point where the valves where opened on the compression stroke.

Video of attempted start

Installed a 2002 5.3 this time using the fuel rail, injectors, and throttle body from the 2004. Similar scenario with startups before taking it for a run. Drove this one about 3 miles which ran great! Parked in in the driveway and next time I went to start it, exact same sound and same compression result on cylinder #2. Decided to check cylinder #3 as well and I had 130PSI, even with 0PSI on cylinder #2. Now I'm puzzled. Pulled off the passenger valve cover and turned it over. Found that the cylinder #2 intake valve was very slow to close back up, all others on that side acted normal. At this time, the noise that I was initially hearing during startup was no longer occurring, so I assume that the valve was making contact with the piston for a while, but no longer was making contact.

Still assuming the timing had slipped, I pulled this engine and removed the timing cover. Timing marks spot on, so either the timing slipped momentarily, slapped the valve and then slipped back or something caused two engines in the same truck to slap cylinder #2 intake valves.

Anyone have any ideas on the cause of this?

 

[Ricko1966]

What are you using for intake? If you starvev1 cylinder for fuel it can damage the piston to the point of no compression. Same intake both engines?

 

[shortboxin]

Thanks for the feedback, but at this point, I know the intake valve on cylinder #2 is slow to close after likely striking the piston, so I believe all gases are blowing out the intake valve. I did run a scope in the first engine on cylinder #2 and didn't see any holes blown through the piston. I'm not saying I could see 100% of the surface, but it wasn't catastrophically obvious.

Stock intakes both engines, never removed the factory intakes on either. Stock tunes.

 

[Zelph]

Where is the 4.8 engine? Can you take the head off?

 

[Ricko1966]

Just so everyone knows,they don't necessarily blow a hole in the top they often times melt and deform the top ring land. Or detonate and break rings or ring lands When I have some thing with no compression in 1 hole a blow compressed air in it and see where it leaks out.Through the oil fill cap,suspect a piston or rings,through the intake or exhaust,suspect a valve. Through the radiator cap expect a blown head gasket or a crack.

 

[shortboxin]

Where is the 4.8 engine? Can you take the head off?

Actually, that went back to the wrecking yard. I still have the 5.3 in my garage though.

 

[shortboxin]

Just so everyone knows,they don't necessarily blow a hole in the top they often times melt and deform the top ring land.

No disrespect intended here as I honestly don't know, but would an engine in that condition run great when shut off, yet act completely different upon attempt to start up days later? Wouldn't the damage occur while running?

 

[Ricko1966]

No disrespect intended here as I honestly don't know, but would an engine in that condition run great when shut off, yet act completely different upon attempt to start up days later? Wouldn't the damage occur while running?

How would it run great with one bent valve and no compression when you last ran it and suddenly not? Okay I just went and reread your post. You need to run compression on the rest of the engine. I didn't realize you didnt run compression on the whole engine.. See what the other cylinders say.

 

[Bloodhound1981]

Small block swap it.














Sorry, had to.

 

[TotalyHucked]

All we're gonna be able to do from here is spit ball. You're gonna need to pull the motor back out and at the very least pull the intake and heads off to see what happened. A hung injector could've bent a rod and/or valve if it filled the chamber up or they could've wiped a cam lobe each time. What kind of oil were you using? Timing can't slip and then go back, it doesn't work like that. On these LS motors, the chain essentially would have to break to get it to skip time, I've seen some seriously loose chains come out of a motor that was still running perfectly fine.

At 160k miles, I would've at the bare minimum pulled the heads off to inspect things before putting it in the truck.

 

[shortboxin]

How would it run great with one bent valve and no compression when you last ran it and suddenly not? Okay I just went and reread your post. You need to run compression on the rest of the engine. I didn't realize you didnt run compression on the whole engine.. See what the other cylinders say.

I'm not planning on fixing this engine, I just don't want a threepeat. Wondering if anyone has heard of timing slipping and then slipping back or if there are a number of things that might cause valve to piston contact.

 

[shortboxin]

Small block swap it.

I like factory injection. Can't stand carbs.

 

[shortboxin]

All we're gonna be able to do from here is spit ball. You're gonna need to pull the motor back out and at the very least pull the intake and heads off to see what happened. A hung injector could've bent a rod and/or valve if it filled the chamber up or they could've wiped a cam lobe each time. What kind of oil were you using? Timing can't slip and then go back, it doesn't work like that. On these LS motors, the chain essentially would have to break to get it to skip time, I've seen some seriously loose chains come out of a motor that was still running perfectly fine.

At 160k miles, I would've at the bare minimum pulled the heads off to inspect things before putting it in the truck.

Spitball is what I'm looking for. The injector theory sounds probable because I used the same fuel rail and injectors on both motors. Also using an aftermarket PCM harness which had numerous wiring issues that I discovered. I went through the whole harness looking for shorts and opens and had to change like 8 mistakes. If it's cylinder washing, the harness could still have something I missed. Thanks for the feedback.

 

[Grit dog]

I'm not planning on fixing this engine, I just don't want a threepeat. Wondering if anyone has heard of timing slipping and then slipping back or if there are a number of things that might cause valve to piston contact.

Then I’m gonna go with coincidence and say get another warranty junkyard motor before they say it’s yours forever.
On the upside, gotta be pretty quick swapping them now being the 3rd time in a row coming up!

 

[Ricko1966]

You need to run compression on all 8,maybe you havev7 dead cylinders and 1 good cylinder. 1 dead cylinder it should still start,run and go down the road with a p0302 code. Jumped timing does not just affect one cylinder,leaning out like I mentioned earlier could. Which was why I asked about intake,which sorry I didn't make myself clear enough when I say intake I mean air and fuel. Somethings very strange it should still start and run even with 1 dead cylinder. I'd be checking fuel supply,spark and injector signal. Once it's running again you can figure out what's wiping out number 2 cylinder.