5.3 Intermittent Knock

[robertsjm711]

All,

I have a 2002 5.3 with an intermittent knock. I bought it last summer on Craigslist and have ran it for the past month or two with only two times with the knock. With the exception of these two instances, I drive it hard and easy, highway and in town. It seems to do great in town, but when I get on the highway and have constant RPMs it knocks, but not every highway drive. But the kicker is, before I get home it goes away, usually 10-15 minutes. When it does knock it is a constant volume and type, except speed with RPM and I can't get it to return.

Before I installed it I removed the pan to give it a once over and there was no sludge. It still had the oil in it, it was used and looked really good. Visually checked the pistons through the plug holes and they didn't look super carbon covered. I checked to oil pressure at that time and it was within GM spec.

It is mated to a Muncie 4 speed, and it is present moving or not and increases with RPM.

This is not a lifter tick, but a concerning knock.

Thoughts? Experiences?

[jessemthompson]

I have read that some 5.3s have an issue with the o-ring in the oil pump pick up tube that can cause intermittent drops in oil pressure. This could definitely cause knocking.

[robertsjm711]

I have read/seen that too in my searching, I did remove the pickup tube and look at it, didn't seem bad, still was soft and had resistance when replacing the tube. Guess I should have replaced it just because.

This morning I changed the oil and found minimal metal, not a shimmer but more of a dark grey sludge in the bottom of the pan and when I opened up the oil filter. Maybe enough to cover my pen magnet 2-3 times, and no chunks or flakes. This is my first change since installing the engine.

Also during my YouTubing it might be a sticking lifter slapping around. Some of those videos sounded just like mine, and it would explain why it went away. I did run a can of flush through before I changed the oil so maybe this will help if this is the issue.

I need to get an oil pressure gauge, it is on my list of things to do.

[mongocanfly]

A guy I work with has had stuck lifters in his....he said it sounds like the rods are fixing to fly out of it....he said it's easy to check if/when they're stuck...simply pull the valve cover and look for the loose rockers...

[DADS1971]

What is the outside temperature. My 2013 Silverado with a 6.2 was knocking when the weather was a bit cold. Took it to dealership and they showed me a bulletin saying that Chevy knows the engine knocks below 50 degrees. They said it was piston slap and it would not damage the engine. I checked and they were right. It only knocks when the temperature is below 50 degrees.

[robertsjm711]

It was over 70 both times.

[68c10airstream]

Lifter knock? install a quart of naphtha into a already full crankcase, drive carefully for a day or 2 and change oil. repeat as needed to remove varnish. This was a gm fix for 70's chevy chevette's (not corvettes) with stuck rings. I used this so many times for varnished up engines and stops almost all 3.0 v6 mitsubishi engines burning oil in dodges. A masda 4 cylinder was so bad that when i removed the oil filler cap on the valve cover when the engine was running that the cap flew out of my hand and dented the hood from all of the blowby. It smoked like a old locomotive!!. That was an old mazda with 40,000 miles, now at 250,00 miles and no burning oil, clean tailpipe 6 years later.It took 4 naphtha treatments.

Now onto ls piston knock cold. If it was there when new it will always be there until overhaul. If it started after some miles put on it's caused from carbon build up on the piston from the top ring to the top of the piston. The carbon build up causes the piston to drag and cock or tilt. I have removed plugs, sprayed carb cleaner into cylinders with the piston about 3/4 the way up with the valves closed. I take a blow gun and braze a 1/8" brake line onto it and bend the tip about 45 degrees and spray the cleaner and blow it around trying to get around the outer edges of the piston. Use white paper towel over the spark plug hole when blowing it out and when the paper towel is wet but no black carbon any more, move on to the next hole.

Subaru's are notorious for number 3 cylinder terrible piston knock. Killed cylnders one at a time to verify, remove plug and follow above directions. Had a friends subaru that knocked since 25,000 miles, at 140,000 miles i worked on number 3 and it took a complete can to get the paper towel to come clean. Sounded like it was going to fly apart before i cleaned it. Perfectly quiet when done.

I also clean injectors and de-carbon ls engines with a 50/50 blend of gas and naphtha through my home made injector cleaning unit. This works well also.

Remember that carbon build up causes what is called octane creap, meaning as the carbon builds up you need higher octane to get the same power and fuel economy. People usually get used to using 87 octane since new and the carbon build up is slow so their performance changes slowly and they don't realize it. I clean my injectors yearly, along with my mass air flow sensor.