Hey guys,

Hoping someone can shed some light on my situation. I am getting a loud knocking sounds from what sounds like the bottom end of my engine. This started when I first installed an '08 LY6 and 6l90e into my Cutlass. This only happens in very cold temperatures, generally below 15F and seems to be worse the colder it is. The noise will occur in all gears. (P,N,R,D etc.)

At first I thought it was engine know since when I'm in the vehicle it sounds like a ping or like someone is tapping on the headers with a wrench or something. Once everything warms up the noise goes away completely until the next time I start things up after it sits for a little while. Anyways, I ended up replacing the Engine (not due to the sound I'm referring to) for an LS3. This engine is spot less and and I have done the tuning and have scanned everything on this engine and am not getting any knock whatsoever and it runs great. For the first week or two I didn't hear the knocking noise at all so I figured it was something internal with the previous engine and was relieved until yesterday when we had another cold day. When I heard the sound I then investigated as best as I could in the driveway and determined the sound is in fact very prominent under the car, almost sounds like a rod knock. Another thing I noticed was that when I cut the engine while the noise was happening I still heard a few more pings after the ignition was cut which everything was rotating to a stop. No drivability issues have been noticed while the noise is present, everything seems to run and drive great. (Also use the recommended Mobil1 5w30 engine oil)

The transmission has low mileage, new/clean fluid and the level has been checked multiple times since the initial swap. Everything also operates properly and even get lockup with the Torque converter.

Everything that I have described is leading me to believe that this is a flexplate issue, or perhaps a Torque converter issue but I'm hoping someone can give their opinion before I drop more cash into the car. I don't mind spending the money regardless how tight things are right now, I just don't want to be spending time or money replacing parts that don't need to be.

I've never experienced a cracked flexplate so figured I would get some opinions.

 

Didn't you have to swap the flexplate to the new motor?
Thinking you would have seen if it was cracked

 

I did have to switch it over but it was one of the other guys in the shop that was helping out that did that ended up switching it over. Not sure how well of a look over he gave it before putting it on the new engine. Also the fact that is only when cold I assume it may be a small crack so perhaps it was missed?

It's definitely something rotational, and I can't imagine it would be engine related since it's the same issue with both engines. And I;m hoping it's not something inside the trans. When I look under the car it sounds like it coming from the engine oil pan or bellhousing area. And like I said, everything operates perfectly and I am not having any drivability issues. It's just a horrible sound that makes me cringe for a little while when starting in cold temperatures. That is mostly why I am leaning towards the flexplate.

Also the issue doesn't change if I command the TC to lock up as well.

 

Was the same oil pan setup used for both motors? Just thinking of some other parts that could be the same between the 2 setups. Could possibly be hitting windage tray.

 

Yes, the truck pan was used on the LS3 engine but the original LS3 windage tray was used. Also there was no markings or damage on the truck windage tray as well. All parts internally on the truck engine is in great shape. No excessive marking on the bearings or crank.

 

Did you have to shim your torque converter? It's pretty normal to have to do this. I am not sure what the clearance needs to be for the 6L80 from the mounting pads to flex plate, but it's easy enough (and free) to check. Perhaps your converter is too far forward out of your pump and it's causing some noise until it warms up. It's worth a shot. Maybe someone can chime in as to what the measurement needs to be. I've got an 4L80 in mine, and I had to shim it.

 

I did see a thread where this helped someones issues and it did cross my mind to try it, but this noise is the same noise that was happening with the original swap using the engine that was otiginally matched with the trans. Neither engine or transmission had been separated during the initial swap and I was and still am using the factory TC and flexplate.

 

Doesn't help but since it was brought up, with converter fully seated in tranny the should be between 1/8" and 3/16" between converter pads and flexplate.

 

Well I'm sorry that isn't of help. It was just a thought. This is a strange one.

 

No worries man, any help is appreciated. And it definitely is a strange situation. I seem to attract this sort or thing, it's never an easy solution.

For example... the reason I had to replace the LY6 I had in the car originally was because the block turned out to be so rusty that a hole formed in the block causing coolant to drain from it. It was a 2008 engine with 75k miles in it...

 

I seem to have similar luck myself. That's crazy that it rusted all the way through. And I thought my truck was bad. Damn.

I'll keep this in the back of my mind and if I think of something I'll pass it on. It could be a cracked flex plate as you were thinking. I had an 82 Z28 that all three holes had cracked on. Didn't know until I had a screwdriver through one of them to hold it while I was pulling the bolts out. I thought it was the screwdriver that cracked it at first... until I saw the other two holes. That was many years and many cars ago now.

I don't recall it clunking or making noises though... so that might be bittersweet information for you.

 

Have you figured anything out yet with this? Sounds like same issue I'm having. I had to get the engine rebuilt because the crank walked. New rebuilt engine and after about 500 miles the samething. Sounds like rod knock underneath. I used a stethoscope and can hear the knock from the tail of the transmission forward and loudest at bellhousing. I do the same from front of engne aand gets louder when I get to the bell housing. Mine is loud in engine bay at top til it warms up and gets a lot quiter. If I listen hard I can still hear it but that's cause I know it's there. It moves with rpm and gets louder. It doesn't sound like some rod knocks I hear on the street.
Im actually dropping the engine and trans to see if I can find anything. I'm afraid it is from the transmission and going to tear up this new engine also.
I have a 2011 camaro ls3\m6. Different setup but sounds like same noise.

 

Thanks for chiming in. Unfortunately I haven't been able to pull the trans yet to have a look but it's definitely coming from the bell housing area. I'm planning on ordering a new TC and flexplate soon and hoping that gets rid of the noise. If/when you find whats causing your PLEASE let me know.

Thanks in advance.

 

I will let you know. I been chasing this noise for quite a while so I'm not in a huge rush. The car just sits in the garage and takes up room and money. If you find anything out let me know also. I would suggest that you cut the filter open and check to see if any damage is happening in the engine. That's just me though.
​​​​​When I had the engine rebuilt everything was wiped out. Crank, pistons, cam and all the other goodies.

 

Might be worth checking your crank runout too. It should be around .003 iirc. The TCC can smoke a crank if it has too much line pressure.

 

I pulled the engine and trans last night. I haven't had time or energy look too much into but I tried to push and pull on the front of the crank and there is no movement. From what I been told there should be a little play. I'm not sure though how much when they are both connected though.

 

Normally it's checked at the thrust bearing, but this requires the engine to be removed typically. When it's measured, normally you just tap the crank one way or the other with a mallet, and use a feeler gauge to measure the distance from the thrust bearing cap to the counterweight. Then you tap it the other way and check the difference. Should be about .003", but not 0. This isn't really noticeable just by pulling on it by hand.

What I would do is tap the crank back with a softfaced mallet, attach a dial indicator to the oil pan or front cover, (so that the dial is forward of the damper) and then tap it back forward. You'll need a fairly large mallet to do this, or a block of wood and a hammer. Don't hit it too hard. Just enough to move it. It doesn't take much. There are a few ways you could set up the dial indicator.

I suppose you could pry on it too at the damper or the flexplate.

 

Here's a video showing it being done out of the car, but I'm sure it can be measured in the car. This is a SBC, but same idea. Only real difference is the thrust bearing is in the center on an LS, and not at the back.

 

Thanks chopperdoc. That's the info i need. Im going to pt pit on the engine stand and pull the oil pan and check. I'm trying to eliminate everything i can before have someone else look at it. That will be really helpful

 

I just realized you said you pulled the engine already. Yeah just pull the pan and check it normally then. And if it's bad, well atleast you're already in there...