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LS7 bubbling coolant reservoir + positive block test, but no smoke or overheating
Hey everyone,
My name is Andrea. I’m from Europe and recently bought a C6 Z06 with an LS7. The car already had head work done by the previous owner, and before shipping one of the heads (passenger side) was repaired/reworked by BMS.
I’m trying to diagnose an issue and would really appreciate input from people experienced with LS7 failures, head gasket issues, cracked heads, etc.
Current symptoms:
Engine runs strong
No smoke from exhaust
No misfires
No overheating
Oil pressure is stable and normal
No metal/glitter in oil
No crankcase pressure/blow-by symptoms
Heater works normally
Coolant system builds pressure
Upper hoses get very hard after driving
Coolant reservoir bubbles from cold start
Coolant slowly pushes out of reservoir
CO2/block test is definitely positive
Additional info:
When I first received the car, oil had a sandy/light brown color, clearly with some coolant contamination
I changed the oil and coolant
Since then oil does NOT look like milkshake and oil level has not noticeably increased
I had to add around 1.5L coolant total over time
Cooling system pressure test did not reveal any obvious external leak
At this point I’m trying to understand what is statistically most likely on LS7s with these symptoms:
head gasket sealing issue?
cracked/repaired head?
sleeve/block issue?
something else?
I’m especially interested in hearing from people who had:
Sounds like not a great repair job before you bought it? and hopeful just head gasket....Bubbles in water usually mean air from combustion chambers getting in coolant passages...Also when moving temps should be 192 to 200 ish on the DIC...
Try to find someone with experience with those motors...
Welcome to the forums. Beautiful car.
With the heads being recently worked by a reputable company, it's fairly safe to say it's just a head gasket sealing issue.
Sounds like not a great repair job before you bought it? and hopeful just head gasket....Bubbles in water usually mean air from combustion chambers getting in coolant passages...Also when moving temps should be 192 to 200 ish on the DIC...
Try to find someone with experience with those motors...
Good luck hope it turns out good for you...
Yeah, not the cleanest repair history for sure. The seller told me one head was replaced/rebuilt before I got the car, but when I drained the oil upon receiving it the color was sandy/milky — meaning coolant had already been mixing with the oil while the car was being driven to the shipping point. Mileage checked out, so it happened en route.
What's interesting is that temps stayed perfectly normal the whole time — even on a warm day — never climbed out of the normal range. So whatever is going on, the cooling system is still doing its job keeping temps in check, just losing coolant somewhere in the process.
CO2 block test came back positive, and there's constant bubbling in the reservoir — so you're right, combustion gases are definitely finding their way into the coolant passages. The car runs well otherwise, no smoke, no misfires.
Good news is I found a shop with real LS engine experience. Planning to get it in there soon for a proper teardown and diagnosis. Will report back once we know what we're dealing with — hoping it's just the driver side head gasket and nothing deeper.
Welcome to the forums. Beautiful car.
With the heads being recently worked by a reputable company, it's fairly safe to say it's just a head gasket sealing issue.
Thank you! Yeah that's exactly what I'm hoping for — fingers crossed it's just a gasket sealing issue on the untouched driver side head and nothing more sinister underneath.
Got an LS specialist lined up. Will update the thread once we're into it!
Hopefully, it is just a bad head gasket. But there is the possibility of a cracked sleeve/block as well.
To see what the problem is, it will require removing the heads.
Whatever it is, it sounds like it is a very slow coolant loss.
Yeah, hoping for the best but preparing for the worst mentally — light blood only please!
You're right, whatever it is it's definitely a slow leak. No dramatic symptoms, just quietly losing coolant over time with bubbling in the reservoir and a positive CO2 test. Heads have to come off either way to know for sure.
On that note — has anyone here dealt with a cracked sleeve or block repair on an LS7? Curious what the real-world options are — Darton sleeves, full block replacement, or something else? And what kind of money and time are we realistically talking if it goes that direction?
LS7 bubbling coolant reservoir + positive block test, but no smoke or overheating
