It's driving me nuts trying to figure out what it is.

When I start the blazer- a full 2 to 3 seconds later I hear a knocking sound. 30 seconds later it goes away. With the skid plates off and under the truck I still cannot figure out whether it's comming from the bottom or the side wall of the engine. It's not as deep or loud as a rod knock.

My mechanic said he didn't hear it when ileft it overnight for him to listen to in the morning- he must of blew me off because it is loud.

does anyone have any suggestions- the 2 to 3 full seconds after start is throwing me off.

Thanks-

 

I'm sure that all who viewed this message feel as I do:that I would have to hear it to make even any kind of diagnosis. Good luck!

 

I have heard of a loose flex plate causing a noise similar to that.
But yea, gonna be hard to diagnose without hearing it!

 

I figured it would be heard to do over the internet. With it starting 2 or 3 seconds after start i was hoping that may give an idea.

what's the flex plate?

Thanks

 

Hey! After replying to your post,I have thought about it & all possibilities-I would go where kristoph30 went! A flexplate is the equvilant of a flywheel-used on auto trans units to smoothly transfer power,but also resiliant enough to prevent damage from starter over-engauging. This is also helpful when a wheel is slipping and suddenly gets traction-enough breakage can occur there!The flex plate will break instead! Many manufactuers offer replacements ecomically-Its the job that is costly at retail,as the trans must come out! Do not have this job done at retail! Good luck!

 

The flex plate the flywheel at the back of the engine when equipped with an automatic transmission. It is made of sheet metal and it is bolted to the crankshaft in the middle and the torque converter bolts to it closer to the outer edge where the ring gear for the starter is welded on.

The name "Flex Plate" comes from the fact that it intentionally flexes as the torque converted expands and contracts for changing RPMs and fluid pressures.

It NOT designed to break for any purpose. It can fail if it is damaged, poorly designed or poorly manufactured like any other part.

 

Thank you for getting back to me guys.

I'll will absolutely look into this.

Would it make the sound only when it's cold?

Thanks again- forums are awesome

 

A cracked flex plate typically will not go away.

A sticky lifter typically ticks right away and may go away after a bit. The ticking would be coming from the valve cover area and shouldn't really sound like it's coming from the bottom end.

If there is any way that you can get a video of the truck while it's making this noise uploaded to youtube/photobucket/picasa/etc, it would sure help in the diagnosis.

 

I don't have anything to record with but may be able to use my friends iPhone when he gets back in town.

It's not in the valve train.

Is piston slap immediately at start up?

thanks SwartlKK

 

Piston slap doesn't typically make a lot of noise at idle, but can last a minute or two at higher RPMs. If it does make a lot of noise at idle, it would get worse with higher RPMs (at least that is what it has done in my experience with a few vehicles).