GM 5.7 Engine Knock...

[Storm 007]

I have a 1996 Yukon 2 door sport, with the 5.7 liter Vortec Small block, it has about 140,000 miles. During cold start-up, there's a noticable "knock", untill it warms up after about 2 to 4 mins, then dissapears. is this common ? i have heard many different stories from local friends. Is it easyly fixable, or serious, or somthing common that i shouldn't worry to much about. I have changed the oil every 5000 Kilometers, and use only Mobil 1 synthetic, with a premium oil filter.
Anybody with any info on this ? ...(a little concerned)

[31rx7]

I have a similar issue with my 97, and have had it since 75k or so (I now have 116k). From what I could gather, there is a TSB concerning some engines being built with the undersized bearings on rear main and so as the engines wear, you get a knock on startup. My mechanic has the same issue with his Suburban.

[herefordman]

I have a 97 305 Vortec that has done this for a long time now, don't know what it is, but it sounds a bit like a lifter tapping, it goes away once warmed up and my truck has 200,000 miles on it, and its probably made this noise for at least half of that.Make sure you don't deviate from the oil type eg:5W30 or 10W30 or whatever the manual states, as a thicker cold oil will make this sound louder too.

[herefordman]

Just noticed something else, check out the "Piston Slap" thread in the Yukon forum, and specifically the GM Merlin post, this sounds a lot like your problem.
Cheers

[GMMerlin]

Click here
http://www.automotiveforums.com/vbul...d.php?t=150872

[70camaro]

My 97 does it too... If it only happens when its cold, try this: Look to see if you have an electric plug (regular house type w/ground) towards the front of the left frame rail (near radiator). If you do, this is a heater for your engine. Plug it in on those cold nights and when you start it in the morning - TA DAA - no knocking! PLUS your engine is already warm and ready to go!!! Try it and let me know if it works for you too....

[mdunlap13]

The knock is lack of oil. Once the pump charges and circulates the knock stops, correct? You could get a stronger pump theoretically making the knock go away quicker, or use heavier oil coating the parts longer.

[GMMerlin]

Originally Posted by mdunlap13

The knock is lack of oil. Once the pump charges and circulates the knock stops, correct? You could get a stronger pump theoretically making the knock go away quicker, or use heavier oil coating the parts longer.

The cold start knock on GM engines is caused by a carbon build up on the piston...when the engine is cold, the carbon is hard and causes the knocking noise..as the cylinder temp rises, the carbon deposit softens and the knock goes away.
Using a heavier oil will cause lubercation problems due to the fact that it will take longer for the oil to be drawn into the pump and pressurize the engine.