Knocking On Cold Mornings

On mornings when it is cool outside (50 degrees or cooler) and I start my truck for the commute to work, my engine knocks for a few minutes until it finally warms up. When the engine is warm, the knocking is gone. This happens on most cooler mornings, and this morning it was louder than ever. What is causing this and what can I do? I know knocking is not a good thing. Thanks.



1998 GMC K1500 z71
125,000 miles

my 97 with 115,000 miles would sound like shit on cold mornings but I switched to synthetic oil and it sounds normal now. I don't know if it is actually knocking, I have read a few posts like yours on these trucks and I told them the same thing. If you already use synthetic oil then...?

Yes, I have been using synthetic oil (Mobil 15W-50) since I bought the truck a year ago. Could it be because I am using this heavy weighted oil?

That I don't know. On the oil cap on mine it recommends 5W-30 so thats what I use. It might be worth a try but maybe someone else knows.

They all do it, but i think 15w50 is a little much. remember the colder it is the harder it is for the pump to pump it.

It's called "piston slap". The pistons are smaller than the cylinder bores (obviously), but since they expand at a faster rate than the block, they have to be machined with extra clearance, meaning they are a little loose in there until the engine warms up and then they (the pistons) expand to fit properly.

This is a VERY common issue and is said not to cause any harm. Mine does this also. I take it a little easy on the engine until she warms up anyway, just to be on the safe side.

piston slap was only an issue on 99 and up GM engines.

Why are you using such a heavy oil? Use 10W-30 and see if the problem goes away.

piston slap was only an issue on 99 and up GM engines.

Dunno where you get your info, but I been working on cars (professionally and non-professionally) since 1970 and piston slap was around back then. Chevy engines, especially the big blocks like the 454, were the worst offenders.

My '98 Silverado with 5.7 Vortec also has a good case of piston slap. The colder it is, then the worse the "slap" will be, as the piston has shrunk at a faster rate than the block.

I'm sure you guys all know this, but the "slap" is caused by the piston skirt hitting the cylinder wall.........................

yes we know what it is caused by...Someone had a thread a while back about piston slap so I researched it on the internet and found a bunch of stuff about lawsuits having something to do with piston slap and there was nothing about pre 99 gm engines. Not saying I couldn't be wrong though.

I agree with trying a thinner oil. I went to 10w40 on my truck to stop it from smoking some, and it worked, except I hear something knocking when its cold. When it warms up, turns to a slight ticking. I'm definately goin back to 10w30 or 20w50 in a couple thousand

I have owned two Toyota Tacoma's and both had the same problem with piston slap that went away after a short warm up. This is a very common problem with Toyota's and it is said not be be a concern unless it last for over a couple of minutes. On a Toyota the piston is aluminum and shrinks alot when cold causing the problem. I put a little over a quart of Lucas oil treatment when I changed my oil and it cured the problem on both trucks. Not sure if it will work as well in a Chevy but its worth a try. They say not to drive it until the slaps stops, hard on the piston. Hope this helps. My mechanic said it wouldn't work but it did.

On a Toyota the piston is aluminum and shrinks alot when cold causing the problem.

FYI: All pistons are made of aluminum. If they were steel, the engine couldn't go over a couple thousand RMPs (maybe less) without blowing apart.

I use the 15W-50 because I saw that Mobil-1 made it in synthetic. I asked my mechanic what the purpose of such a heavy oil like that was and he said that gaskets and rubber parts inolder engines with lots of miles tend to shrink and crack and the heavy weight oil will condition the rubber parts better than thinner oil. And since I live in South Texas where its normally always very hot, he said that the heavier oil would be good. That's the reasoning behind it.

FYI: All pistons are made of aluminum. If they were steel, the engine couldn't go over a couple thousand RMPs (maybe less) without blowing apart.
Thanks for the info, makes me feel like a dummy. But the Lucas oil treatment did work for me on two different trucks so maybe it would work for someone else. Not sure why it works but my engines went from knocking (piston slap) for 30-60 second to quiet on all but the very coldest of mornings.