I just don't understand this. After sitting for 8 hours my 4.3l has a nasty knocking for 1 minute. If it sits for 2 hours not a peep. Worse on cold mornings. 1999 oil presser is 60psi and 40psi at hot idle. Run nice and smooth. No codes and good mpg. Drained oil and its clean except for a pee size ball of something on the drain plug magnet. 180,000 miles. Took it out on the hwy and figured It would go out with a bang but nope.
Does not sound like a lifter tick??????? WTF?????

 

Piston slap. Very common on 4.3L & 5.3L GM engines. Caused by a lack of oil to the lower cylinder wall on cold start up. The noise you hear is the piston skirt slapping against the lower cylinder wall when it first starts going up from BDC. My 4.3L has done it for ~150K miles, and a customer of mine has 386K on a 4.3L and it doesn't slap at all, never has.

 

It was also a common problem on GMs 2.8 and 3.1 engines.

 

ai captain.....so piston skirt is always hitting and the oil keeps it quiet? Is this caused by worn wrist pins? my 250,000 astro is quite

 

The skirt should not come in direct contact with the cylinder wall. There should be a "layer" of oil between the skirt and the wall to "cushion" it and keep it from scuffing, (slapping). When the piston reaches BDC, (bottom dead center) the skirts extend below the lower end of the cylinder wall. As the piston begins to travel upward, it wobbles slightly before it gets centered in the cylinder. If there's insufficient oil on the wall, the skirts will contact the wall making the noise.

After sitting 6 to 8 hours or more, the oil runs off the cylinder walls and usually takes a few seconds before it gets coated with oil when you start it. The cylinder walls rely totally on oil being splashed on them to lubricate them. The noise is most noticeable below ~40F. Back in the early 80's, Ford had trouble with piston slap on the 2.3L engine. Their "fix" was to install connecting rods that had small holes drilled in them with a passage going to the bearing. When the engine was running, pressurized oil was squirted on the lower walls... end of problem.

It's not a problem with wrist pins, although wrist pins do make a very similar sound with one exception.... wrist pins make the noise all the time, hot or cold.

 

My 1995 Z28 (LT1) did the same thing. Basically during the whole 115k I put on it until I sold it. Never any issues. I was told it was common.

 

is a result of out of tolerance bore/piston.Not sure about other manufactuers but GM lately seems to lack quality on engine manufacturing probaly due to all the out sourcing of manufactured parts.Back in the 60`s/70`s/up to near 1990 this never used to happen.One thing about the 96 and up 4.3`s GM produced are notorious for spinning rod or crank bearings.Once again this all depends on what plant the engine was manufactured.Either Romulus mi plant or Tanawanda New york plant which is where most of the faulty engines were produced.The cranks are different in both blocks and cant be interchanged with each other either.I loved the old school GM products back in the day but now it doesnt matter.

 

OK but what about the first 179,000 very quiet miles????

 

Well I`ll be honest I dont know maybe you got lucky,Maybe you finaly got to the point that there was enough wear and tear on the crank it finaly started knocking.It could depend on how hard you run your engine.Maybe an oil passage got clogged up and caused it.Hard to say.

 

I drove it last night for over and hour at 80++ MPH with no noise and 60 psi oil pressure. The guy door heard me start it and yelled "Good luck with that" Everything I've learned about cars in the past 40 years tells me engine knocking is bad??? Any know what year engine can I put in a 99?

Thanks.....Mike