Rod Knock when cold - Ever had this happen?

[Yamariv]

My L6 250 seems to have developed a rod knock on startup when cold, once warmed up though (2-3mins) it goes away and the engine purrs like a kitten and sounds and drives great. I was running 5w30 oil in the winter and switched to a 10W30 hoping this would help but it hasn't. I just took it on a 350 mile road trip to pick up my future 350 and it didn't miss a beat.

Anyone ran into this before, it's definitely a knock. Can knocks go away like that after warm up? Should I jump to a much thicker oil like a 20W50. I was planning on driving my L6 250 for a year or so while I rebuild my 350 so I hope to extend it's life until its ready.

Cheers

[A1971Blazer]

I had a Cutlass once that had a rod knock on startup....it went away as soon as it got oil pressure....eventually the rod bearing hammered out and spun, locking up the engine.
I put in a crank kit and one new rod and it was still on the road years after I sold it.
Take it easy on it and it might last until you get the 350 ready to go in...

[ghackett1]

Originally Posted by Yamariv

My L6 250 seems to have developed a rod knock on startup when cold, once warmed up though (2-3mins) it goes away and the engine purrs like a kitten and sounds and drives great. I was running 5w30 oil in the winter and switched to a 10W30 hoping this would help but it hasn't. I just took it on a 350 mile road trip to pick up my future 350 and it didn't miss a beat.

Anyone ran into this before, it's definitely a knock. Can knocks go away like that after warm up? Should I jump to a much thicker oil like a 20W50. I was planning on driving my L6 250 for a year or so while I rebuild my 350 so I hope to extend it's life until its ready.

Cheers

Not good, RIP L6250. It’ll probably last a little while — add some of that thick LUCAS it may help.

[Steeveedee]

Do you have a gauge or a light for oil pressure? If it's a gauge, what is the oil pressure? Knocking for a few minutes every start-up is going to hammer that shell out soon.

[Yamariv]

Originally Posted by Steeveedee

Do you have a gauge or a light for oil pressure? If it's a gauge, what is the oil pressure? Knocking for a few minutes every start-up is going to hammer that shell out soon.

I don't have a gauge right now unfortunately, I was just about to swap my cluster from the 3 dummy light to a 7 cluster.

[Accelo]

I suspect it's piston slap not a rod knock. The piston warms up and it goes away.
Typically the colder it is and the longer it has sat the worse it is.

My 2000 Silverado has been doing that since it was new and it has 175K on it now.
Cheers

[Yamariv]

Originally Posted by Accelo

I suspect it's piston slap not a rod knock. The piston warms up and it goes away.
Typically the colder it is and the longer it has sat the worse it is.

My 2000 Silverado has been doing that since it was new and it has 175K on it now.
Cheers

You might be on to something!! Here's a bit of what I found for piston slap, sounds like what I have. Biggest thing is that it goes away once warm.

The engine has good power for what it is and doesn't burn oil.

"-Piston slap is a light tapping noise that is more prevalent when the engine is cold, while rod knock is a heavier, metallic sound that gets worse as the engine warms up.

-Piston slap is caused by the piston slapping against the cylinder wall, while rod knock is caused by the connecting rod knocking against the crankshaft.

-Piston slap is more common in engines with aluminum pistons or cylinder blocks, while rod knock can occur in any type of engine.

-Piston slap is typically not as loud as rod knock.

-Piston slap usually goes away when the engine warms up, while the rod knock gets worse.

[RustyPile]

How did you determine it's actually a rod knock?? Do you have experience with detecting rod knocks, or did an experienced mechanic tell you this? Lots of "stuff" inside an engine can make knocking sounds.. Unless we can, in person, hear the noise, we're only guessing at the cause.

First off, let me say this.. I'm not a fan of multi-viscosity oils for these old engines.. I use only single weight oils with lots of ZDDP (zinc) additive. Valvoline VR-1 has sufficient zinc as part of the formula.. Lucas Oil products has a ZDDP additive for "non-zinc" oils.

I'm with Accelo on this one.. In the early stages of a rod knock, the knock will go away with oil pressure, within a matter of seconds after start up.. Whereas with a piston slap, it will take longer, a matter of several minutes.. You probably have a piston that has developed a crack.. It slaps until the piston builds some heat and expands. Then the crack closes and the noise stops..

Under any circumstances, your engine is running on borrowed time.. It's anybody's guess as to how long it will run until it has a cast-iron hemorrhage.

[72SB]

rod knock typically does not go away when engine is warm. Piston slap does typically

Get OP gauge and see what it is at start up. My stock OP gauge does take a few seconds to read as it is a mechanical vs sender unit so takes a little time for the oil to actually get up the tube to gauge

also sometimes WP can grumble when cold then be fine a few minutes later as things warm up.

[truckster]

I'm with the piston slap crowd. I have piston slap on my 2002 Yukon. The key to making the engine live is to let it warm up before you put a load on it.

[Yamariv]

Originally Posted by truckster

I'm with the piston slap crowd. I have piston slap on my 2002 Yukon. The key to making the engine live is to let it warm up before you put a load on it.

Just watched a few videos on Youtube of Vortec piston slap. Same sound as mine has exactly! Well, it's on borrowed time but I'm glad it's probably only piston slap. I'd like to run the old girl for a little while longer so I'm not rushed to rebuild my 350!

Besides the slap, she runs strong and purrs like a kitten.

[Yamariv]

Thanks for all the help guys!! I was thinking it was a matter of weeks before she blew up..wasn't even thinking of piston slap!! I guess it's kinda good news lol

[Sheepdip]

Originally Posted by Accelo

I suspect it's piston slap not a rod knock. The piston warms up and it goes away.
Typically the colder it is and the longer it has sat the worse it is.

My 2000 Silverado has been doing that since it was new and it has 175K on it now.
Cheers

My buddy bought a brand new 98 Chevy 4x4 and it had piston slap that went away after warming up. He took it back to the dealer and the service manager said keep driving it and bring it in just before warranty expires.
He did and they dropped a new engine in. It was installed only a short time and soon develop the same issue. The original never died nor did the replacement.

[chevmn]

I bought a new 92 K1500 pickup, and had a knock on startup, and when I had the truck in for a warranty issue (unrelated), I mentioned to the service mgr. He immediately asked what filter I ran, and at the time I used Fram filters. He suggested I get away from those, and at the time suggested I run A/C Delco, or at least a good filter with anti drainback feature, and likely the filter I was using did not. Keep in mind the K1500 of those years had filters on the side, so in my case it was always having to fill the filter on startup. I have not used Fram filters since, but I do not always run a name brand either, but in that case the truck never had a knock on startup after. Not trying to get into one of those arguments on what is the best filter, but he was obviously right on that one.

[jabborabbo]

My experience with cold engine knocking found here:

http://67-72chevytrucks.com/vboard/s...bborabbo+knock

[bcfishon]

Loose piston wrist pin

[Yamariv]

Originally Posted by bcfishon

Loose piston wrist pin

I hope not!! lol

[geezer#99]

Any chance you could post a video of the knocking?

[Yamariv]

Originally Posted by geezer#99

Any chance you could post a video of the knocking?

How do I post a video on this site? I assume I have to do a Youtube video and attach the link?

[jabborabbo]

Originally Posted by Yamariv

How do I post a video on this site? I assume I have to do a Youtube video and attach the link?

That’s what I did in the referenced thread.

[Getter-Done]

I had a 350 sbc in a 74 k10.

It had piston slap.

It use a quart of oil every 1000 miles.

I kept Lucas oil treatment and 20/50 oil in it.

I drove it for 2 years.
This was back when oil was cheaper, and this was not my DD.

[Micmac]

Originally Posted by Accelo

I suspect it's piston slap not a rod knock. The piston warms up and it goes away.
Typically the colder it is and the longer it has sat the worse it is.

My 2000 Silverado has been doing that since it was new and it has 175K on it now.
Cheers

Same here with a '03 Sierra I actually just sold a couple of months ago with 200K on it.

[A1971Blazer]

most of these piston slap comparisons relate to later model Gen 3 engines….the LS based engine was prone to some piston slap when cold…..not so much for the 250 inline 6
Rod knock is hard hammering…I can easily tell but I suppose someone unfamiliar could be uncertain?

[fatbass]

Pull the plug wires one at a time, piston slap will change on a dead cylinder.

[jumpsoffrock]

Originally Posted by Yamariv

How do I post a video on this site? I assume I have to do a Youtube video and attach the link?

Yes, you will have to host the video elsewhere and link us to it.