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Discussion Starter #21
Not sure if thus question was asked.
Is there a coolant drain plug in the engine block that is accessible?

About air in the system....
Is there a low-point valve to drain the radiator like many vehicles have? If so, I wonder if you attach a hose there, raise the other end of the hose 6 feet high on a ladder with a funnel and add coolant from up there. This would allow coolant to fill from the bottom up to move the air up and out the top. There has to be an easy method that we are all missing here because when on the Kappa assembly line, they couldn't afford to fuss with air in the system.
I'm not aware of another drain other than the petcock on the radiator. The correct GM way to fill the system is under a vacuum similar to the tool posted above by @lorennerol. As I understand it though, this is also not always a 100% method to eliminate air pockets. Due to the coolant air pocket issues, GM released a TSB to specifically address by installing 2 check valves.
 

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My ex's Sky had random overheating for 4 years before she met me. 245F is when she'd pull over and shut down, and it happened often. At least she was smart enough to do that! I resolved the problem in a week. She was pissed at her dumb-A prior boyfriends who changed the thermostat several times, then changed the RADIATOR, and the last one said she "just needed a new engine". Morons...

Anyhow, I drained and refilled using the secondary methond mentioned in the service manual. Detach and hold the overflow tank 14" over it's normal position while filling. You'll need to unclip the electrical connector as the line isn't long enough to allow this.

The preferred method of filling is with a vacufill kit like this one:
https://www.amazon.com/WIN-MAX-Engine-Cooling-System-Universal/dp/B0787JQSFH

I've cured this problem on several Sky's since. I'm not familiar with the compressor & clamp method but given it's eay to just hold up the bottle, I'd stick with that at the least. Also, I have always seen fit to add the check valves. You don't need the two tiny brass ones. You can get larger aluminum commodity ones and put them on the line that goes into the top of the bottle from the Y that joins the lines to the head and radiator. Takes longer for liquid to flood the lines to that point, but then it's just as effective.
 

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Discussion Starter #23
I went yesterday for really my first cruise since doing the flush. A total of about 100 miles. Outside was sunny and in the upper 70’s low 80’s through out the drive. A mixture of highway and back roads at varying speeds. I watched the temp the entire time, and temps liked to be between 199 and 205. So I’m pleased with those results.

Why I’m concerned, is I made a sharpie mark on the reservoir from doing the coolant bleeding and this was my cold starting point for the coolant. After the drive I was now about a 1/4 inch below that. After I let the engine cool I opened the cap to add back to my line and there was a vacuum on the coolant. There is still no signs of any leaking from the WP at all so I’m still concerned it’s a leaking head gasket. I haven’t drained the oil yet to see for coolant in the oil (there is no signs of oil in the coolant when I flushed either), but I’m going to pull the plugs today to see any signs of burning coolant on the pistons or plugs. The exhaust still has a slow drip at extended idle so the prognosis isn’t looking good. I will take it into a shop for a compression and pressure test to confirm.

I’ve done head gaskets before so I’m not too overwhelmed with the task at hand. But before I start buying parts and get elbows deep, are there any other thoughts on what this could be?
 

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Discussion Starter #24 (Edited)
I pulled the plugs and they have carbon build up. I’m not sure if that’s just the direct injection normal byproduct or not, this is my first DI engine. I have no idea how old the plugs are either, but they were the ACDelco correct ones. Since it was on my list anyway I change them out with the NGK equivalents. The piston heads are also fairly carbonized. I didn’t see any signs of scaling or any other signs of coolant being burnt besides the carbonized plugs.
114049

After I swapped the plugs, I buttoned everything back up and ran the engine. No misfires, smooth idle, but the exhaust is definitely condensating more than what I would expect from normal cat operation and it starts to drip after about 10 minutes or so. No white smoke or even vapor, so if it’s a leak, it must be a small one. I’m guessing if it’s a small coolant leak, then it has been present for some time as the exhaust is discolored where it has been dripping.
114048

I never noticed the exhaust drip prior to the coolant flush, as I hadn’t ever let the car idle for more than a few minutes and we’ve only put on about 2k miles since purchasing. I’m going to check after the car has been driven a bit and see if it still drips when warm or only after cold starts.
 

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Changing out the plugs judging from your photos of the old ones was a good thing here.
The WP not leaking is a good thing too.
Changing your oil out might tell you something.
Color, texture, condition maybe if there is a head gasket leak?
The carbon build up is going to happen on DI motor.
IF you do have to pull the head to replace the gasket in the future now might be the time
for a de-carbonization of the valves too?
My exhaust tail pipe has a smaller stain then yours does, minor condensation.
No smoke or vapor ever. Cold mornings during high idle before kick down- tiny amount.
Twice a year I give the pipes, muffler, tips, seams & weld joints a good cleaning.
Wire brush, emery cloth to see if anything has gotton worse.
So far I not seen anything different over the years of usage on mine.
Keep checking the overflow tank level before and after your runs.
No high temp readings is a good thing in our cars.

LAC
 

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Discussion Starter #26
Changing out the plugs judging from your photos of the old ones was a good thing here.
The WP not leaking is a good thing too.
Changing your oil out might tell you something.
Color, texture, condition maybe if there is a head gasket leak?
The carbon build up is going to happen on DI motor.
IF you do have to pull the head to replace the gasket in the future now might be the time
for a de-carbonization of the valves too?
My exhaust tail pipe has a smaller stain then yours does, minor condensation.
No smoke or vapor ever. Cold mornings during high idle before kick down- tiny amount.
Twice a year I give the pipes, muffler, tips, seams & weld joints a good cleaning.
Wire brush, emery cloth to see if anything has gotton worse.
So far I not seen anything different over the years of usage on mine.
Keep checking the overflow tank level before and after your runs.
No high temp readings is a good thing in our cars.

LAC
I did some research online, not necessarily specific to the Kappa platform, and I was able to fine quite a few articles about pretty similar symptoms amongst GM vehicles of the era. Since I don't have any overtly obvious signs of a blown head gasket like overheating, misfires, milky oil, or white smoke and the WP has zero signs of any coolant loss. It's possible there is a very small coolant weep or the condensation from the exhaust could just be normal catalytic converter vapor, and the coolant loss is simply something that could be checked off as "other". It looks like the GM coolant reservoir caps can cause these unexplained coolant loss issues also. The o-rings and spring cap can and do fail overtime and result in poor PSI management and loss of fluids. Since , it's a low hanging fruit item, I went and picked up a replacement Dorman reservoir cap for $10. First observation is the replacement cap fit much more snug and I even needed to lubricate it with a little water just to get it to seat fully. Maybe it's just that easy, we'll see. It will probably be a bit before the next time I take the Sky out to confirm though.
 
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