First Project Truck!! 1998 Z71

So, I found out today that a cracked flexplate can make a similar noise to a rod knock. Anyone have any experience with this? I want to rule out any possibilities before I rebuild my other engine.

JakeZ71 said:

So, I found out today that a cracked flexplate can make a similar noise to a rod knock. Anyone have any experience with this? I want to rule out any possibilities before I rebuild my other engine.

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I've seen happen on couple of these trucks in the past

Start pulling spark plug wires one at a time. If the noise lessens or goes away with one particular cylinder, you have your answer. If not, flexplate is a possibility. You should be able to hear it very clearly with the inspection cover off.

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Nice looking truck for $600. Congrats on the purchase and hope you have it on the road soon.

96-1500 said:

Not sure what your plans are for the donor motor, but I'd suggest at the bare minimum doing the rear main seal and intake gaskets while it's out of the truck. Those are two extremely common problems on Vortecs.

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I agree, Valve cover gaskets as well if it's out.

JakeZ71 said:

So, I found out today that a cracked flexplate can make a similar noise to a rod knock. Anyone have any experience with this? I want to rule out any possibilities before I rebuild my other engine.

Click to expand...

Yeap, I have done plenty of them. Easiest way to check, pull the inspection cover and look around the crank bolts for cracks. You might have to take the torque convertor bolts out and slide the convertor back a bit for a clearer view.

JakeZ71 said:

So, I found out today that a cracked flexplate can make a similar noise to a rod knock. Anyone have any experience with this? I want to rule out any possibilities before I rebuild my other engine.

Click to expand...

Yup... just replaced one for a customer actually..... his had a bad cracking noise at start up and shut down and while running.... take the cover plate off the front of the trans and with a flash light look up at the center area of the flex plate where it bolts to the motor.. usually you can see the cracks pretty easy...

This is what my customers looked like.. cracked all the way around.. had about a 1/4" of movement within its self....


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magimerlin said:

Yup... just replaced one for a customer actually..... his had a bad cracking noise at start up and shut down and while running.... take the cover plate off the front of the trans and with a flash light look up at the center area of the flex plate where it bolts to the motor.. usually you can see the cracks pretty easy...

This is what my customers looked like.. cracked all the way around.. had about a 1/4" of movement within its self....

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sent from what use to be a great country...

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oh damn!

Oh wow.. Magimerlin, that's insane!

So, I did a bunch of investigating on the truck today. I pulled the front off the bell housing, and checked the flexplate; it appears to be in good shape. I also checked the out the cylinder that seemed to be misbehaving. The spark plug looked new, and after running a compression check on the cylinder the reading came out close to 150 psi, meaning the rings and valves check out.

TL;DR: The motor is toast.

On the plus side, we also did a compression check on the new engine. Every cylinder was 150 psi or more, meaning the new engine is good! So, moving forward all work completed will be on the new engine.

Thank you to all the have replied.

So, update. I had a couple of friends who've done more work on vehicles than I come over a few nights ago, to look that the truck. After crawling around under it, they diagnosed that it was the flex plate making the noise after all. Apparently you can feel the flex plate rubbing the bell housing while it's running if you crawl under the truck, something I wdidnt want to do while by myself. Also, as smooth as the enign runs, there was no apparent way it could be something internal. So, it looks like I'm just going to be into this thing for a flex plate!

In light of this, my friends said I could use their garage to complete the repair, so after exams are done, I'm going to put temporary tags on it and drive it over there. In the mean time, I'm going to address the ignition and brake issues it seems to have, as well as fix some unfortunate graffiti scratches in the hood.

In short, I'm excited that this thing won't be as expensive to fix as I thought!

Sounds like a plan...

sent from what use to be a great country...

So, unfortunately life has a way of derailing the best laid plans. Despite the best of intentions, my new truck has been doomed to the ignoble position if a yard ordernment; a position it will continue to hold until it starts raining, so work can let up on the farm and allow me time to tinker. We're still operating on the theory that it's a bad flexplate, which all the symptoms seem to point to.