5.3 Intermittent Knock

[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.