I used to be active here 15-20 years ago (StealthZ06). I'm back because my 2023 C8 has been grounded at a local DFW dealership for two weeks with a coolant loss/overheating issue, and the service department is feeding me a narrative that goes against what I know of this car.

Breakdown & Diagnostics:
Cruising at 70 mph, got a sudden "Engine Overheating Idle Engine" warning on the HUD. Looked down and it was at 261 degrees.
Coolant is actively pooling in the engine valley (the "V").
The service manager performed some testing:

1. Coolant pressure testing - a drop in overnight pressure was observed each time they hooked it up, however they cannot find where the coolant is going (no apparent leaks, or sources)
2. Compression and leakdown - I was told these came back okay, but he didn't share any info beyond this
3. Borescope - camera showed that two cylinders are "cleaner than expected." I'm no mechanic, but from what I know in engine diagnostics, a "cleaner than expected" piston head is the textbook definition of steam-cleaning from a coolant leak. They also observed some pooling in the valley between cylinders 4&5, but not as much as expected.

Dealer’s Story:
Despite the steam-cleaned cylinders, the service manager explicitly told me they are "looking at a porosity leak on the intake manifold itself, not the engine."

My thoughts/sanity check:
The LT2 intake manifold is 100% molded composite plastic. Plastic cannot have aluminum "casting porosity."
The LT2 manifold is completely dry. Zero coolant routes through it; it only processes air.

If coolant is pooling in the valley and washing out two combustion chambers, it physically has to be a casting defect in the aluminum cylinder heads or the engine block itself.

There is an active GM TAC case, but the shop sat on the car for a week before touching it, and GM Executive Care just quoted me a 4-business-day callback window. I'm convinced the dealer is just throwing out nonsense to buy time because they are overloaded and terrified of a full warranty engine swap.

My Questions:
Has anyone dealt with a confirmed casting porosity issue on the LT2 block or heads?
How heavily does corporate TAC override a local dealer's diagnosis once the raw telemetry and borescope photos are uploaded?

I’m hoping GM Engineering audits the TAC file next week and rejects this "plastic manifold" patch-job. Any advice on pushing this through corporate would be appreciated.

 

I'm confused by the statement, coolant pooling between cylinders 4 and 5.
As those two cylinders are not directly across from or next to each other, 5 is 3rd back on driver side, 4 is 2nd back on passenger side.
And, then the statement that they are saying they can't find a leak, how can that be if coolant is pooling in the valley?

 

Spot on, and that's exactly why I'm losing sleep over this narrative. The service manager (actually the service director) told me verbatim they found pooling "between 4 and 5" in the valley, while simultaneously claiming they can't find an obvious leak source via standard pressure testing.

To your point, since 4 and 5 are on entirely opposite banks across the valley, a fluid path spanning that area heavily suggests it's weeping directly out of the block's central structural casting under thermal load, rather than a top-end peripheral component. They seem to be using the "normal" static leakdown test to ignore the physical pooling right in front of their eyes (although I'm not even confident it was normal, the service director couldn't elaborate on the results when asked other than normal).

 

Well, there is no coolant in that area of the block or heads, no coolant passage that I can see from any photos or diagrams, so no area of the block to have it be weeping from the casting.
Unless there is some sort of coolant flow to the intake that is used to warm it up, maybe a hose leak, but can't find that.

 

Thanks for clarifying that, and seriously, thank you for taking the time to respond. I think I see what you mean looking at the block layouts now. If the valley floor itself is completely dry of passages, then would the fluid be migrating there from an elevated pressurized source under load?

Given that the tech also found two steam cleaned cylinders, is it likely a structural crack or porosity defect right along the interior deck/head mating surface

Something has to be allowing pressurized coolant to breach the combustion chambers and simultaneously weep outward into the valley. Regardless, I just can't see a the dealer's "plastic intake manifold" theory being viable.

 

The only way coolant can get into the cylinder is from a cracked head, unless it is leaking into the intake port.
My 23 has never been to the dealer for a problem, but I "had" a new Chevrolet EV last year and I was not impressed with the knowledge base of 2 dealers.

 

Which cylinders were "steam cleaned" because if they are adjacent what about a head gasket leaking between cylinders.

 

I appreciate the insight, and that's exactly what I'm thinking, the intake manifold logic just doesn't hold up. If it were truly an intake side issue, I don't think there would be visible contamination in the combustion chambers. I know dealers can be hit or miss, but I'm genuinely surprised that are pushing the intake theory considering this is coming from one of the largest Chevrolet dealers in the world

 

It was implied that it was cylinders 4 & 5, but I need to get clarification on that (I know for sure he said there was pooling in the valley between 4 & 5, I wrote it down). I thought once they found evidence of the steam cleaning and the coolant in the valley, they would realize that this is likely to be some sort of major structural issue. This is kind of difficult for me, I know a little about engines, but I'm no mechanic and I feel like I'm consistently a day behind. I'll talk to the service director, he'll relay information and I'll ask a few questions, but often it's not until later when I research what he told me that I realize things don't seem to add up (to me at least).

 

My problem is the opposite side cylinders. Not a blown head gasket between cylinders and a coolant port. I'd think a crack in one head shouldn't effect 2 cylinders on opposite banks. What's common to both washed cylinders? The intake might make sense? And maybe I just don't know enough, but that's my take. Hope they get you sorted out quickly.

 

Very sorry for your issue. I'm afraid I can't be of any help. Since I'm in the DFW area can you tell me which dealer is looking at the car? From what I gathered, Reliable in Grapevine and Huffines in Plano are the go to Chevy shops for warranty work/repairs.

 

As far as the cylinders getting washed down, maybe a head gasket, that would allow water to get into two adjacent cylinders, but not 4 and 5, opposite sides of the engine.
If coolant was getting into the intake somehow, then it could be cylinders on opposite sides getting washed down.
Were it me, and coolant was getting onto the valley plate, I think pulling the intake would be first.
But, since it is at the dealer, I'm sure there is a diagnostic procedure for the tech to go through, if he can follow diredctions.

 

One thing I forgot to mention is that the pressure test results

Cold test: Lost 4 PSI.
Warm test: Pressure drop actually accelerated and lost even more.
Compression/Leakdown: He claimed they were "normal" but wouldn't elaborate when I asked for the exact numbers.

To me if it were a rubber intake gasket or a plastic seam, I would think it would usually seal up better as it warms up and expands. In this case the leak path is expanding with thermal growth, which feels more like a hairline crack or porosity issue somewhere in the aluminum casting.

 

I'm not sure if we're allowed to name dealers, but it's at one of the ones you mentioned.

 

One thing I forgot to mention is that the pressure test results

Cold test: Lost 4 PSI.
Warm test: Pressure drop actually accelerated and lost even more.
Compression/Leakdown: He claimed they were "normal" but wouldn't elaborate when I asked for the exact numbers.

To me if it were a rubber intake gasket or a plastic seam, I would think it would usually seal up better as it warms up and expands. In this case the leak path is expanding with thermal growth, which feels more like a hairline crack or porosity issue somewhere in the aluminum casting.

 

Actually it's Classic that's located in Grapevine NOT Reliable. I have been a Classic customer for over 20 years with my Corvettes and they have always done a fantastic job on both warranty and non-warranty work.

 

I second this. I don't have personal experience with the service department but Classic in Grapevine is one of the top Corvette dealers on the country. My family members with corvettes take all their cars there. Might be worth a shot for another opinion. (If you're allowed to move it to another dealership. No idea how that works)