02 Silverado, Wierd Knocking Sound

[ylikebeef]

2002 Silverado, 4.3L V6

Lately, ever since the cold weather started here in Dallas, my truck has been making this weird knocking sound when i start it. Honestly, it sounds like a pushrod went (happened to me before with my Camaro). But here is the weirdest thing, the noise goes away after a little while.

Sometimes it won't make the sound at all, then sometimes it makes the sound for a few seconds, then sometimes it wont go away for minutes. Another weird thing is, not only does it make a knocking noise, but I can hear in my exhaust, it sounds like a piston is misfiring or something because you can hear what sounds like a backfire from the exhaust at the same time.

Thanks in advance for any suggestions.

[J-Ri]

Sounds like a lifter for an exhaust valve is bleeding down. These engines use hydraulic lifters that use oil pressure. After sitting for a while, the pressure can bleed out, causing clicking in the valvetrain. After the engine has been running, it fills back up and the noise is gone until it sits again. If you're using a heavier oil, it can make it worse. I would use the thinnest oil your owners manual says you can use (probably 5w30). You may also want to try a crankcase cleaner and/or some Lucas oil stabilizer.

[ylikebeef]

Two questions:

What would I be looking at cost to repair this?

and

What is the possible damage that could be sustained if I do not get this repaired ASAP?

[J-Ri]

Call local shops for an estimate. Price will vary greatly in different areas.

There is little risk of damage if you don't fix it. You will have slightly elevated valvetrain wear, but the fix for that is the same fix you are looking at now. If I were being charged for the work, I would let it go until the pushrod and rocker arm need to be replaced as well. If you decide to wait on the repair, just let it idle until the clicking goes away.

[Tdragone]

This is a known issue of the larger engines.. I know my 02 4.8L v8 is part of the "Noise Abatement" Class action lawsuit. Lots of people have this issue; few actually got GM to resolve it free. The lawsuit is for those who it wasn't fixed free for. I've had it since my truck was 3 weeks old...

It doesn't hurt anything; but is kinda annoying. It was reduced slightly when I switched to Mobil 1 full synthetic.

[01silverado03z400]

Its sounds like piston slap. All of the big 3 have the same problem. Not causing any damage just annoying.

[usmcnova22]

GM has done away with piston skirts in cost saving effort, therefore, if piston-to-bore specifications are even a little out, piston will wobble in bore causing even more excessive wear and eventual engine failure. Piston slap is a known problem by GM however, what the fix is you have to deal with GM.

On a side note....I did go to my local GM dealer and they told me all about the problem and that they know about it. When I asked what they could do about they told me to call GM directly. I can see where this is going......

[ylikebeef]

I've come to a simple conclusion. If I have to yank the engine out to do any repairs, might as well convert it to a V8. I've already found quite a few tutorials online, and it seems like it's a possibility. Looks like this is what I will be doing.

[AaronIROCZ]

Have somebody else start your truck when it does this the most, you lay under the truck and listen to your cat. converter. My truck 5.3l did the same thing sounded like i spun a rod bearing, I was about to start yanking the engine when I crawled under it and listened to the knocking and it was coming from my passenger side catalytic converter. Then the next day the knocking turned into a rattle at any temp, rpms ect. Then the cat became plugged and needed replacement. Now needless to say I replaced the cats and now all is quite.

[j cAT]

Originally Posted by 01silverado03z400

Its sounds like piston slap. All of the big 3 have the same problem. Not causing any damage just annoying.

i believe the piston slap/knock when engine is cold under load is only v8engines. as mentioned synthetic 5-30w usually reduces this noise. i have 2000 5.3l. also if carbon is excessive in combustion chamber this noise will be much louder and last longer. i can understand that, if cat's are carboned up and plugged the noise would be loud.............

[mbumgua]

Actually the knock is in the 4.3's as well as v8's. I changed out my brother's 2002 4.3 for a 5.3 last July. His 4.3 with 75,000 miles had bad piston slap. Anybody want a 4.3 with 75,000 miles?