I just bought a 2000 Blazer with a 4.3L, I test drove it and it ran great. Bought it and brought it home. Started it up the next morning and the engine was missing and there was a knock coming from the engine. The check engine light never came on. After a few minutes the sound went away and the engine was running fantastic again. Everytime it sits for more then and hour or so the pattern repeats. I'm pretty sure its a lifter issue unless someone else has any other ideas.
I guess my question is what is the best additive to help get this lifter unstuck. I have heard people say seafoam, lucas, marvelous mystery oil, and even engine flush.
I plan on replacing the lifters and rebuilding the heads eventully( probably a few months from now) but I would like this issue to go away until then.
If a video of the issure would help I can upload one. Thanks in advance for your responses and input.

 

A video would be nice for hearing the pattern and what it sounds like. Keep in mind as engines get miles on them things get worn down. The sound you could be hearing is the pistons expanding as they heat up do to the engine being slightly worn. This isn't a major problem, most engines do this. As for the rough Idle I would be leaning more to a small vacuum leak that is compensated when the engine is heated up.

 

OK, cool. I will upload a video when I get home from work tonight. Thanks for your response.

 

Good chance, you're right... it is lifter(s). If it taps while idling and doesnt get louder as you accelerate. If it is almost quiet idling no load and gets louder at accelerate.. then it's probably piston slap. Especially if you have an oil change and it doesnt knock immediately AND your oil pressure is still good, Which is no doubt what Thor was referring to.
Bearing (Rod) knock often gets worse as the engine warms.

Lifters: They probably arent worn, they are probably gunked up. They get gunked up for a couple reasons, the main one being using highly advertised, or else really cheap, oil then not changing it often enough for the way the car is driven.
If they were worn then seafoam or an engine flush would make that worse. But That would tell you for sure, wouldn't it.

Mine is doing the same thing in the morning in cold weather, my oil pressure is fine and it doesnt use any oil. I waited a little long to change the oil and now I expect the Mobil 1 I am using now will take about 500 miles to make it go away.
I forget.. because I only drive high mile cars, only use Mobil1 and havent had this problem for a long time. The LAST time I bought a car with highly advertised oil in it.
*UPDATE: Nope.. what I have is piston slap.
Read up.. lots of opinion Summary no sweat

And here's what swartlkk, our resident guru, says:
Which explains why it got worse once I changed oil (5w30). Sorta thought the feel of the old oil was a lot like slick honey. So am I gonna rebuild.. NOPE! I drove a Tbird 3.8 for 75,000 miles with a lot worse knock, but from damaged bearings, than this one and it still ran fine up till a guy wrecked into it. I just ran it with Mobil 1 20w50.

So now on this one.. I'm going to just run normal weight Mobil 1 and a bottle of Lucas Oil Additive. - provides extra 'cling' which will solve what it can.. otherwise wont worry about it. The Lucas wont hurt anything, I'm dang sure of that.
Now exactly why is it you want to rebuild the lifters and heads?

 

Here is a link to the video, it's a little long, I tried to get it from the onset when first started until it went away completey but my phone ran out of memory. It is a little windy sorry about that. About 1 minute after the video ends the tapping gets progressively quieter until its completely gone.

2000 blazer tapping - YouTube

I wanted to rebuild them so I can extend the life of the motor, I really like the truck and dont want to get the engine to the point were a rebuild isnt cost effective compared to the value of the truck.

My work blocks youtube so I'm not sure if it has finished processing on the site or not. I just uploaded it a few minutes ago.

 

That's lifters as near as I can tell. Not piston slap. Which is a 'deeper' knock.
Like I said crap oil and not changing it right.
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Look .. you can do what you want and I'm sure any shop would gladly agree with you.

But I would bet that an engine flush to clean them up then running Mobil 1 to keep them that way will do you fine.
I know what I'm talking about because my cars NEVER do that.. even at 300,000. And I dont exactly change that oil religiously.
My net oil costs are less than the guy who runs down and buys Pennzoil every 5000 miles guarandamntee you.