I need hive guidance - keep it or move on

This is gonna be long, so bear with me. I'm in a quandary and need some opinions. I own a 2001 Suburban, 166k, in our family since new. Spectacular condition and insane maintenance over its lifetime. This is the truck you would dream to find on the used market.

KBB shows it as 4100-5300 for private sale (fair to excellent) or 2400-3900 for trade-in value. Not huge, but not throw away.

It has an engine problem (5.3 Vortec) and I can't pinpoint the source. Started as an engine code for misfire on #4, then became a rough idle and kept just throwing P0304. Swapped coil, wire, plug and fuel injector and no change. Compression check showed #4 lower than the rest (110 vs 145 for all others).

My time was limited, so I took it to a pro for another opinion. He did a leakdown which was fine, and found a chafed wire on the coil harness. Replaced the harness and no change.

I then had to take it on a road trip of 300 miles. Started with a rough idle. By the end of the trip I had a loud engine knock, but the rough idle and misfire code vanished. While driving around the city it was loud - like hear it across the parking lot knock-knock-knock. Drove it back home and no change. Still a loud tap at idle that changes with rpm. Not as loud now but there is still no doubt there is a bad sound. Other than the sound, the driveability is perfect at all speeds and conditions. Turn up the radio and you have no idea anything is wrong.

Have a discussion with the pro again. He sethoscopes to confirm it's not a tensioner or something else. Can't pinpoint it but it sounds louder more bottom end than through the top end. He thinks maybe a compression ring scraping. He drops the oil pan - main and rod bearings are fine (look new actually), and there is still crosshatch in all of the cylinders. He said the bottom end looked amazingly clean. This would leave a failing cam? Wrist pin? I'm running out of ideas and things to test. I can't figure out the failure mechanism that went from rough idle to bad noise.

So now my dilemma. I don't have a lot of spare time and work slowly so this would be a lengthy teardown for me. We only drive the truck 5k/year, but it is mainly to support my wife's classic car insurance booths at trade shows and car shows, so it is important that it get us where we need to go on time for those limited miles per year. Our season down here starts in October, so my time to act is short.

I don't want to buy a new one for $70k for as little as we use it. I can afford to spend $15-20k for a newer used one, but spending that much just generally galls me. I struggle with spending a couple grand on engine work (with no clear problem to fix or assurance that this won't be the beginning of a string) based on the value of the vehicle. In a world with unlimited time and without a business need for it, I would just drive it until it tells me what is wrong, probably in spectacular fashion, or do the Copart plunge and buy a cheap destruction title truck and do a basic engine swap and hope for the best.

If I could pinpoint the problem, at least I would have a repair path, but at this point it would be exploratory surgery until I find the problem. Or I just do an engine change, but there are risks and/or $$$ involved there, too. Help me think through this. WWGRMD?

I'd pull the cam. You started with a rough idle and lowish compression and now have a noise in the block area, that doesn't appear to be main bearing related. Could be caused by a intake lobe that is wiped. It would probably have glitery oil in the filter as evidence. Did mechanic not find anything in pan when he removed it?

You know there is a problem with #4 but you have tried everything that is try able without a teardown. In all reality once you pull that motor to check a wrist pin wouldn't it just be easier to swap In a known good but used 5.3 and move on?

I agree with itsarebuild. A known good 5.3 seems like the most sure option compared to chasing flat came lobes or lifters.

Put a dial indicator on each valve rocker and measure the lift. It'll take time, but arguably less time than pulling the cam.

The timing chain/gears may be shot. They don't last forever. Might be able to check these by rotating the crank and looking a the lag between crank movement and valve rocker movement.

Many years ago, the 5.0 in my E150 developed a weird knock. I thought it was the water pump. Turned out to be the power steering pump (diagnosed/replaced by the next owner).

Pull it and swap it. Then tear into it to figure it out and tell us what it was.

Have you pulled the valve cover on #4 to see what I going on? This would be the next logical step. Look at valve lift and timing chain lash.

I would also consider an oil analysis. That may be able to tell you what is going on.

Lastly I would look for carbon build up in #4. Not common but with it not being used all that much who knows and it could have hung a valve a little than come loose and I bouncing around in there.

Beyond that a crate engine from gm with a warranty may not be a bad idea in this case if the truck really is that nice.

Stop everything. I bought a 77 Chevy C10 when I was 16. It was making a sweet knocking noise, so we got it cheap thinking we would have to rebuild it. Luckily my dad pulled the valve cover off before anything else happened. BROKEN VALVE SPRING. Fixed it for free. Drove it like a boss for almost 2 years before the dumbasses at the alignment shop decided to park it in the pit below the rack in a violent manner, nearly hitting one of their coworkers.

Keep looking at it. No need to replace yet. Find the problem and fix it. Sounds like it's a pretty damn nice truck with the engine it has NOW and worth more than what you would get otherwise.

Pull the valve cover. My guess is a valve spring or bad lifter. I've seen both of these on 5.3's. Unfortunately you gotta pull the head to do lifter on an ls. So you should just spend the extra 2-3 hours and pull the balancer and front cover and cam and put a Comp 212/218 in it...Cheap way to 30+hp.

Find a good, low-mileage 5.3L for $800 and swap it.

My guess regarding your problem is piston slap. Very common with GM V8s starting about 1994. In an effort to reduce friction, GM uses very short piston skirts. My LT1 started piston slap at 20k and it now has 140k. But excessive piston rocking could cause low compression (poor ring seal) without showing up as cylinder wear.

Wrist pins are another thing that could be failing, but again, not a common thing unless there were some kind of oiling failure.

The chances that a cam lobe has failed are almost impossible unless you had a lifter roller seize which is very unlikely. Possible, but not where I would look first.

But you'll spend a lot of money chasing it down when you can just swap for $800. Then start diagnosing the old one. Maybe its a bent pushrod and therefore a $10 fix and you can sell it for $600. Maybe it needs a rebuild and you can sell it as a core for $400.

Or maybe it needs a rebuild and you have a Miata that could benefit from a 5.3L?

doc_speeder wrote: Pull the valve cover. My guess is a valve spring or bad lifter. I've seen both of these on 5.3's. Unfortunately you gotta pull the head to do lifter on an ls.

Can I diagnose a bad lifter by just removing the valve cover and not pulling the head?

curtis73 wrote: Or maybe it needs a rebuild and you have a Miata that could benefit from a 5.3L?

Paging hobiercr....

Dude, Have you removed the serpentine belt and started it yet?? I had a NASTY knocking sound in SWMBO S10, that I chased everywhere, alt, PS pump, AC compressor, WP. I mean EVERYWHERE. Tensioner LOOKED fine, when I replaced the damn thing, it went away. Well mostly, we had driven it so long that it actually elongated the positioning hole in the block so it doesn't sit quite right anymore. I still get some low RPM knock out of it.

And when I say it had a knock, I mean you couldn't stand next to it running and talk knock. It wasn't a good sound. It also started right around the 150k mark.

EDIT: Just remembered the low compression part. I would still drop the belt and try it. If the knock is still there it's time to pull the valve cover.

Not quite. Sprayed the belts and put the stethoscope to both main and AC tensioners. I'd be more tempted to chase this if it didn't start with the misfire. Only plausible scenario there is carbon buildup the broke loose and healed itself just as a tensioner died.

Some of the earlier 5.3Ls had issues with lifters collapsing or soft cams. I'd be pulling the valve covers. Even if the engine that's in it turns out to be utter garbage, 5.3Ls are so cheap and easy to find, it seems like it would be money ahead just to put another in.

There is that... Shop around a bit and I'm sure you can find a shop to drop in a crate motor for 4-5k. While painful, it's still a heck of a lot cheaper than the alternative of buying another one. Edit: Rock auto says they'll send me a long block with a 3year unlimited mile warranty for >$2300. Now you just have to add gaskets, water pump, and a few parts from your old engine.

Spinout007 wrote:

NickD wrote: Some of the earlier 5.3Ls had issues with lifters collapsing or soft cams. I'd be pulling the valve covers. Even if the engine that's in it turns out to be utter garbage, 5.3Ls are so cheap and easy to find, it seems like it would be money ahead just to put another in.

There is that... Shop around a bit and I'm sure you can find a shop to drop in a crate motor for 4-5k. While painful, it's still a heck of a lot cheaper than the alternative of buying another one.

These motors are good for 250K without any major maintenance, barring any unusual failures (Like the OP's). I would have no qualms about grabbing one from a junkyard and pressing it into service. I mean, guys snag junkyard ones, throw on a turbo and make 650hp all day without issue. Using one as designed would certainly be no problem.

Find a better pro? Sorry, but with the symptom of low cylinder pressure staring him in the face, I would have expected a definitive diagnostic.

The first thing I thought of was a dead cam lobe, and I don't see where he checked that one out.

Do another compression test, maybe it healed itself or you had bad readings the first time. Then remove the belt and check if the noise disappear. Hopefully its the tensioner, or ps pump. Maybe remove the belt first.

Any chance the ECU has gone nuts? Had a 95 9C1 that would do all sorts of weird crap. Turns out the brain box went bad.