Help diagnose a strange rod knocking noise. 5.7 Vortec content.

I have been noticing this for that last few years, but now it more pronounced. When I first start my van it sounds exactly like a loose rod bearing, this only happens when it first starts. After about a mile or so it is gone completely. Oil pressure is 70 cold and 50-60 warm going down the road 25 at idle when hot. Oil pressure has not changed at all in the last 50,000 miles.

In my experience it acts the opposite of a loose rod bearing, they are usually quiet with cold oil and loud with hot oil.

What could it be, flex plate, power steering pump, timing chain, rod bearing?

2001 GMC Savana 5.7/ 4L60E, 185,000 miles, runs great no noise at all when warm and revs smooth.

Piston slap.

I also have 15-50 oil in it from pulling a 6000# trailer in 90 degree heat. Let me get some 10-30 in it and see what happens. It it is piston slap should I just continue to ignore it?

My lemons car did this. It was a very slightly bent valve.

dean1484 wrote: Piston slap.

Second.

Usually happens after getting too hot and the piston shrinks a few thousandths. When they warm up the piston expands just enough to stop making noise. My 5.7l DD does the same, for that reason.

I've also had 2 Ford 460's that I put a combined total of about 75k on them, before selling them still running, that did the same.

I'd go with piston slap also. My 99 LS does it also the first minute after start up but only when it's below 60 outside.

Best to let it warm up and quiet down before you put any load on it. Even after quieting down you likely still have excessive clearance that's slowly chewing stuff up.

Put me on the list of guys who own a chevy with the aforementioned piston slap. I have been told they build them looser to improve power and fuel economy and that causes this, but I can't back that up with any factual data.

Stampie wrote: I'd go with piston slap also. My 99 LS does it also the first minute after start up but only when it's below 60 outside.

Yep. I had a 99 G3500 that had it too.

It's a Chevy. Ignore it. When it's truly worn out, the cold crank rattle will drown out the piston slap.

Even then, I'd ignore it. It will rattle, while cold, for a long time, before anything actually fails.

Sounds like a plan, do nothing and hope for the best. This truck in nice enough to warrant another engine, its just a pain to change in a van. GoodWrench 5.7s go an sale for less than $1500, if it ever dies it may get one. I wonder if the GM employee discount works for engines, my dad retired from GM.

Probably piston slap, they all do that.

gearheadmb wrote: Put me on the list of guys who own a chevy with the aforementioned piston slap. I have been told they build them looser to improve power and fuel economy and that causes this, but I can't back that up with any factual data.

I heard that sometime in the mid 90s they quit doing A-B bore/piston matching and just started slapping stuff together.

does it have hydraulic lifters ?

+1 for piston slap. Nothing quiets it but new rings.

Crackers wrote:

dean1484 wrote: Piston slap.

Second. Usually happens after getting too hot and the piston shrinks a few thousandths. When they warm up the piston expands just enough to stop making noise. My 5.7l DD does the same, for that reason. I've also had 2 Ford 460's that I put a combined total of about 75k on them, before selling them still running, that did the same.

third, fourth, and fifth. In about 1992-93, GM started using very short piston skirts to combat friction. My 96 LT1 developed piston slap when cold at about 20k. It now has 149k.

Good news. GM knows it happens. Bad news GM knows it happens.

They updated the oil filter so to reduce the amount of noise you hear on cold starts. Buy adding a valve that stops the oil for draining back into the pan. If you have an oil cooler the gasket inside the oil cooler may have failed. Allowing oil to drain into the pan.

Does this pertain to a 5.7, the Van chassis didn't get the 5.3 until maybe 03?

Updated and corrected my response

My high mileage 99 4.8 LS motor started doing this several years ago-I switched from Mobil 1 to Shell Rotella and the noise stopped. About 80,000 miles ago.

Not to try to start another "what oil" thread. But, what oil are you using? Just an idea.

I have a friend with a 5.7 Vortec Chevy Express 1 ton van that has 380,000 miles - fyi - it will run forever.

akylekoz wrote: Does this pertain to a 5.7, the Van chassis didn't get the 5.3 until maybe 03?

Yup. It pertains to pretty much every GM V8 and 90-degree V6 back to about 1992 and even earlier in the TPI engines. TPI, TBI, LT1, LSx, 4.3, etc. It was a GM decision on the larger engines to use shorter piston skirts to help reduce friction. Shorter piston, more piston rocking, more noise.

It has gotten to be less of an issue in the last decade or so; hypereutectic castings have gotten better at being thermally stable (not changing size with heat) so they can run tighter piston-bore tolerances. Much of the piston slap issues are when its cold and the piston is smaller. More stable alloy means that the bore can be smaller (or piston larger) without running into binding at operating temps.

TIGMOTORSPORTS wrote: My high mileage 99 4.8 LS motor started doing this several years ago-I switched from Mobil 1 to Shell Rotella and the noise stopped. About 80,000 miles ago.

Interested in this. The 5.4L in my F150 has some piston slap and I'm running Mobil 1. I bought it at 98k and it has 107k now. I mostly did the Mobil 1 because the underside of the hood was plastered with Mobil 1 stickers with mileages written on them (and Advance was having one of those $32 for a 5-quart jug and Bosch oil filter sales). I don't know that the previous owner used Mobil 1 (he maybe just had some stickers), but maybe I'll try something else next time and see if it changes.

The switch to Rotella could have helped simply because its thicker too.

"How cute, you have piston slap that goes away after a few minutes" said every Subaru 2.5 owner ever.

Interesting.

I changed my oil yesterday, dumped out the 15-40 Delo and replaced it with random 10-30 and about a half of a quart of mystery oil. This morning when I started it the noise was gone. 40 degrees out the coldest morning yet, it should have been worse.

Pleased yet puzzled as to why this happened. Also filled the power steering reservoir, it wasn't empty but below the full cold mark.

MMO probably loosened up a gunked up ring adding some stability would be my guess.

Turn the radio up so you can't hear it, which also has the extra benefit of putting a little more load on the alternator, which works the engine a little harder, which warms it up faster..