Today my 1999 jimmy sle 4x4 had a loud knock when I started it. It's done it before in the past apparently but tonight it seem loud, little concerned. Anyone know what that could be?

 

I have owned several of these 4.3s and as they age it seems that they are prone to piston slap when they are started cold. It always seems more pronounced when mileage in excess of 150k. It sounds like a rod bearing noise(and it might be..) but I have run these engines to 300k with this noise. Keep fresh oil of the correct weight and let it warm up till it quiets and you may not have any issues,I haven't. Best of luck!

 

It's a rebuilt, very clean motor(oil is always clean). The engine was still warm when I started it. Maybe I'm being paranoid. It didn't do it today. But when it did it, it was only when I was holding the key over to start it then it when away when I released the key

 

Oh boy...if the noise is present when warm,that opens a whole new can of worms,especially involving a "rebuilt" motor...intermittant noise? That could be almost anything as well. One would need a capable wrench there when it is started to try and narrow this issue down. Paranoid? I think not,I don't know if I would go blasting up or down route 2 in winter with this uncertainty!

 

The knock was only present when the starter was engaged....is this correct?

Better check to see that a starter bolts are tight or perhaps one is missing.

 

By the symtoms you give sounds like something has come loose.If it continuesly does it on start up and last from 15 seconds to minute it could be piston slap or a engine knock.Before suspecting the worst though I would things like the starter ,engine mounts,clutch fan and maybe the exhaust pipe rattling against the underside.

 

Thanks for all the feedback. It hasent done it since. And it's only when the starter was engaged. But yeah, it hasent done it since. I'm thinking something is loose. I know the harmonic balancer has a wobble to it

 

If the balancer has a wobble to it, probably the rubber between the inner and outer ring has failed letting the outer ring move. You will need to replace the balancer.

Not hard to do, other than getting the fan blade off...will need the wrench for this....available auto part tools to rent.

 

Definitely replace that balancer that will cause major problems with your engine if not fixed soon.Never use a hammer to put them back on either.Also get a harmonic balancer pully remover to remove the old one.As for removing the fan use a torch and heat it.Its threaded on left handed.

 

I'm getting so many different opinions on the balancer. I took it in to the mechanic who rebuilt it and is a good friend and he said it was nothing to be worried about. Then I've had others say to replace it. I took his opinion cuz he is the one who put the thing together. What kind of damage will it do it it never gets fixed? It's been like that for a year now. That I know of anyways