I have a stock LS2, low miles with a suspected cracked piston. The engine still runs very well, but the compression test on #7 is considerably lower reading (70psig) than the other cylinders. I will pull the head next month to verify.


I have another low mile LS2 engine that has been taken apart.


Questions:
1) Will there be obvious signs that the piston is bad? I have seen missing ring lands and even destroyed pistons on other engines; those are obvious. What should I expect to see in this situation?


2) Can I simply swap a good piston/rod assy from the spare engine into the engine still in the vette?


3) Are the rod and piston assemblies well balanced from one engine to the next?


4) Should I use new rings and rod bearings, or could I use the ones that come off the engine currently in the vette?


5) Is there sufficient space to drop the oil pan (around the frame) without raising the engine?


6) Are there any other things I should be concerned or pay special attention while I am disassembling and replacing the piston/rod assy?


Thanks in advance!

 

Hello,

Do a leak down test on that cylinder before you pull it apart. A leak down test may indicate that you have a burnt or damaged valve and only need to have the heads repaired.

Douglas in Green Bay

 

Is your car boosted? If not, kind of hard to have a cracked piston or damaged ring lands.
Do as Douglas has suggested with the leak down test, and listen for where the air is leaking from. Could also have a broken valve spring, as LS's are know for this issue. At a stretch, a cam lobe could be wiped out given lower compression. Seen this issue here and there, but usually higher mileage cars.

 

with KB9GKC's comments, and added what I know to your post in RED

 

Tear it down in a day and find out what you have, heads can be off in 3 hours, then you know what you have.


Until the its guessing

 

I know for the LS3 Vette, the engine doesn't have to be lifted to drop the oil pan; may be a little different in the LS2 but probably not. However, there is some loosening of suspension parts.

 

I have thought that it may be a valve issue. I should be able to see that when I pull the driver's head. I will be removing the head before opening the bottom-end.

Thank you for the response on each of the questions. Much appreciated!




I am only planning to pull the driver's head, so I am hopeful to be able to do it in a few hours and know exactly where I stand.


That is good news. And I agree that the LS2 would be similar to the LS3 as far as effort to remove the oil pan.

 

Car ran for almost it's entire life (~25k miles) with the APS TT system.


The car still runs very well, just has a CEL illuminated for the reduced compression in #7 cylinder.


I guess I could spin the engine and confirm both intake and exhaust lobes are moving the valves on #7 approximately the same as the other cylinders before I remove the drivers cylinder head. I will look into this.

 

Perform a leakdown test to see if its piston/rings, intake valve, or exhaust valve before tearing it down so you know where to look. Just tearing into a engine blindly is not the way to properly diagnose your issue. Once you verify that you don't have a worn/broken valve train I would remove the rockers on that cylinder and perform the leakdown test.

 

Get an inspection scope and put it down the #7 spark plug hole and inspect top of piston.

 

A bore scope is a useful tool. You can get ones for a smart phone pretty cheap. And definitely a leak down test. There are several places (rings, head gasket, valves, cam, springs) that can result in lost compression. Start with the easy stuff first, that's usually the cause.

 

I'm not sure how the computer could know about low compression on number 7. There is no compression sensor connected to the computer. It would be good to let the forum know what exact code is being shown...might help people here with a diagnosis.

 

Unfortunately, I am overseas so I am not able to provide the exact code. IIRC, cylinder misfire #7.


I hope that helps.

 

Could be a misfire code. Computer has no idea what the compression is.. but a constant misfire could set some codes off.

 

He stated this in his OP



So somehow he knows the actual reading. If so head/s comes off.

 

Sounds like a plan.


So many things it could be from this part of the game.

 

Yep, I ran a compression test and all cylinders are normal with only #7 considerably lower at ~70 psig.


Best case scenario would be minor damage causing a valve not to seal (doubtful). Worst case scenario is that there is a clear indicator that the #7 piston is cracked / ring land lifted. Either way, the driver's head has to come off.


Hopefully with the spare parts I will have readily available, I will be able to get it fixed and up and running on the same day. I remain hopeful!

 

it was a cyl 7 misfire code.

Cyl 7 compression was sub-70 psig, 1-6,8 had 150+.

wet plug and no intake or exhaust thumping noise.

oil smells like fuel.

my bet is a damaged piston.

 

Update:
Remember this is a boosted LS2 engine with ~20K original miles (all boosted), and about 1-2K miles driven (since Oct 2014) once it started throwing #7 misfire codes.


My son (TravisSchoech) came down and prompted the teardown of the LS2.


With some telephone guidance from my brother (Got Juice?) and skilled labor from my son, we spent about 10hrs going through the engine. We were able to pull the L head to verify that there were no cylinder head issues. There was a very slight vertical line down the cylinder bore. The piston was visually intact from the topside (just had a dark spot at about the 10 o'clock position, and rocked considerably more in the bore vs 1, 3, and 5 pistons.


We dropped the cradle to allow access to the engine oil pan. Popped #7 piston out of the block and found that the 2nd compression ring was broken in four pieces and the oil control ring was partially pushed out of its proper location. Once the piston was out of the bore "chunks" started coming out of the piston.


Cleaned everything nicely, installed new piston, rings, and gaskets and reinstalled all. Got it running about 10:30PM on the 29th and have been driving since. It runs well (although it ran well even when hurt), but no longer throws codes or has that deep misfire in the exhaust.


I am hopeful that it remains healthy and happy....

 

Thanks for the update, and glad it worked out for you, not as bad as it could have been!