So I'm driving my 1990 Burb (350, mostly stock) up the steep hill to my house after work. Suddenly the (cold) engine starts knocking up a storm. My first thought was I had a bearing failing unusually quickly, but home was only a quarter mile away, so I continued on. When I got there I parked and the knock quit completely -- maybe a bit of a lope left. So I started moving again and it came back. So what I've got here is a severe knock only under load. I can sit in park and slowly rev it up to 3000 rpm and it sounds fine, but when I put it in D and start moving the knock is there. if I hit the gas kinda hard while in P it will knock a little while the load is coming up. When I drive up the big hill it knocks like crazy and power is clearly diminished. We're talking 'knock' like someone's hitting the block with a hammer. It's not a 'rattle' or a 'ping' or a gnarly exhaust leak. (I've had plenty of those.)

I've had a few hot rods and rebuilt a few 350's but I've never thrown a rod or spun a bearing, so I'm not 100% certain what that should sound like. I had a guy tell me he had a cracked flex plate do something like this. I've also never had a lifter go flat, so I don't know what that sounds like either. Plus, it seems like bad bearings or lifters would keep knocking at idle. Anybody ever had this?

Thanks for all input.

 

Welcome to the forum.

Might check the flex plate on the torque convertor. Could be loose and banging around as soon as you put it in gear.

 

no good things come of that... I think you've got something going wrong in the motor if it's effecting power... plus, the whole hammer to the block... I'd guess you've got a rod going...


:chug:

 

Thanks, guys. You're not gonna believe this but we were all wrong. I kept driving the burb since work is less than a mile away, thinking if it blew up at least I wouldn't have far to walk. After a few days of that the exhaust output got more and more dramatic until i'd get home amidst billowing clouds of steam that smelled like antifreeze. Yep. It was the noisiest fried head gasket I'd ever seen. Luckily I found exactly the right engine on craigslist for $250, has abt 100k miles on it. That'll run long enough to build the original up the way I really want it.

Anyhow, thank for the opinions. Mystery solved.