UPDATE....
It's been several weeks of trouble free driving until last Thursday. The car had not been driven for several days. (the only common link to this story) My wife started to drive down the street and observed the temp gauges approaching over 240 and rising. She immediately pulled the car over and called me. I arrived and noticed that alot of coolant had entered the overflow tank and actually "overflowed" into the street. I had a 7mm wrench with me and slowly opened the bleed valve near the water neck. alot of air and steam flowed out. As soon as I saw coolant flowing out of the bleed valve, I noticed that the entire contents of the overflow tank got "sucked" back up into the surge tank. I started the car and the temp was at 195 to 200 where it remained. I was able to drive the car home with no problems. When I arrived at home, I elevated the front wheels and let the car cool down to about 100 degrees. I checked the surge tank and it was full. I squeezed and held pressure on the radiator hose and opened the bleed valve and a little more air escaped. I shut the valve, topped off the overflow tank and drove the car the remainder of the day for about 50 miles in heavy traffic with no problems. I continued to drive the car for 5 days in a row with no problems. The average temp was between 195 and 200. Note: both temp gauges were reading the same, so I was confident that all the air had been purged. The car sat for 2 days. I went to start it and warm it up, and the same symptoms were present; The analog gauge displayed a quickly rising temp, 240, vs the digital gauge which remain under 160 degrees. It was clear that I had air in the system again. I raised the front wheels and let the car cool down. I opened the bleed valve slowly and alot of air escaped drawing the entire contents of the surge tank into the engine. I went thrun the purge proceedures once again and topped the overflow tank and went for a trouble free drive. Both gauges in sync and average temp was between 195 to 200. No leaks are visible, no white smoke out of the tail pipe, no water in the oil, no coolant leaks under the car etc...etc...etc...the only common thread is if the the car sits for a day or two, air gets in the cooling system. If it is driven everyday, I have no problems with it. I am at a loss. I have worked on cars for many years with the majority being chevy small block engines and have never seen this before, but this is my first experience owning an LT1. It just doesn't make sense to me. I appreciate any ideas or directions to the nearest cliff where this car will take it's final drive! Thanks! lol UPDATE....
It's been several weeks of trouble free driving until last Thursday. The car had not been driven for several days. (the only common link to this story) My wife started to drive down the street and observed the temp gauges approaching over 240 and rising. She immediately pulled the car over and called me. I arrived and noticed that alot of coolant had entered the overflow tank and actually "overflowed" into the street. I had a 7mm wrench with me and slowly opened the bleed valve near the water neck. alot of air and steam flowed out. As soon as I saw coolant flowing out of the bleed valve, I noticed that the entire contents of the overflow tank got "sucked" back up into the surge tank. I started the car and the temp was at 195 to 200 where it remained. I was able to drive the car home with no problems. When I arrived at hom, I elevated the front wheels and let the car cool down to about 100 degrees. I checked the surge tank and it was full. I squeezed and held pressure on the radiator hose and opened the bleed valve and a little more air escaped. I shut the valve, topped off the overflow tank and drove the car the remainder of the day for about 50 miles in heavy traffic with no problems. I continued to drive the car for 5 days in a row with no problems. The average temp was between 195 and 200. Note: both temp gauges were reading the same, so I was confident that all the air had been purged. The car sat for 2 days. I went to start it and warm it up, and the same symptoms were present; The analog gauge displayed a quickly rising temp, 240, vs the digital gauge which remain under 160 degrees. It was clear that I had air in the system again. I raised the front wheels and let the car cool down. I opened the bleed valve slowly and alot of air escaped drawing the entire contents of the surge tank into the engine. I went thrun the purge proceedures once again and topped the overflow tank and went for a trouble free drive. Both gauges in sync and average temp was between 195 to 200. No leaks are visible, no white smoke out of the tail pipe, no water in the oil, no coolant leaks under the car etc...etc...etc...the only common thread is if the the car sits for a day or two, air gets in the cooling system. If it is driven everyday, I have no problems with it. I am at a loss. I have worked on cars for many years with the majority being chevy small block engines and have never seen this before, but this is my first experinence owning and LT1. It just doesn't make sense to me. I appreciate any ideas or directions to the nearest cliff where this car will take it's final drive! Thanks! lol

 

That is really weird.
My only explanation would be, that there is a very small pinhole leak sucking in air when the car cools down.

Maybe the car does not cool down completely over night. But sitting 2 days lets the engine get fully cool down to ambient temp.

Water expands when heated, does the opposite when cooling down.
So in a working system the density of water is the lowest at around 40F, so it would suck down more coolant from the overflow tank.

Now, for doing this the system has to be COMPLETELY airtight. If there is a leak anywhere, air will get sucked in instead of coolant.

 

Cap ????
Blown headgasket???

 

Tellavision's problems are almost the exact mirror symptons I recently experienced. Due to the recent bout of very cold and nasty weather, for the most part I have kept my car in the garage. About three weeks ago, the weather was nice enough to get it out for an hour or two and the car reacted the very same. I went through the purge procedure and the symptons went away.

Another bout of cold weather came and the car set again for more than a week. Once out of the garage, the problem came right back. To make a very long story shorter, the problem was a bad head gasket. No oil in the water (and no water in the oil, thank goodness) but I ended up resurfacing both heads since I had the engine apart. It appears the "leak" was a very fine passage in the gasket between cylinders 6 & 8 right near one of the temp sensors.

One of the things I did notice was when it was overheating, I could let off the gas and the head temp would begin to drop almost immediately. Once I begin to apply power again, the temp would begin to shoot up on the temp sensor on the head. The water pump temp sensor would see some of the same temp changes but would lag behind the head sensor.

I hope you don't have to tear down your engine but your problem could be the same as mine. As a note, my engine just turned over to 174K miles without a previous problem of any kind but a Optispark and water pump. Good luck.

 

Thanks for the response! I guess it's time for the leakdown test...headgasket sounds suspect....

 

Buy Felpro teflon coated head gaskets, they last many times longer than non coated gaskets. C4 engine non coated head gaskets don't last very long. Check you heads for flatness too!

 

I also had a problem last summer with my Coolant recovery tank filling up to the top!

Head gaskets where leaking.

A second clue was my anolog gauge would climb in temp much faster then my digital.

Both problems went away when I did my top end, and I eleminated a slight dead spot in acceleration (roughness) at around 2200 RPM



notice: were the combustion was entering the water passage, this would push the coolant out of the head.



This one shows the leaking gasket.

Hopefully you don't have the same problem...

 

Are those fel pro 1003 gaskets? I just got a set for mine and they look the same.

 

Thank you for the info...I'm going to conduct a combustion leak down test this week and I will report the results. I'm from the old school non computer era, so to repair this engine will be a new experience for me....

 

Head gasket leak is the prime suspect. As far as the "computer era", a head gasket is still a head gasket.

 

Even with computers controlling the engine, head gaskets have never been computerized!

 

I was simply making reference to the era of my experience base. Even I know what an EMC controls. I guess I should have made reference to something else......thanks!

 

A faster, easier, and highly accurate way of testing the coolant portion of your head gaskets (since that is what you are suspecting in this case) is with a Block Test Kit (also called a Combustion Leak Tester). They use blue test liquid that will turn yellow if there are any traces of combustion gases in the cooling system.

The leak-down test is ok but it relies on you being able to hear with your ear or with a mechanic's stethoscope where the air is leaking from (intake, exhaust pipes, dipstick, radiator etc) by filling one cylinder at a time with compressed air. This means you have to remove all the spark plugs, rotate the engine by hand to TDC on compression stroke 8 different times, fill each cylinder one at a time with air, go around the engine 8 times trying to hear the leak, etc. This takes way more time than you need for this and requires an air compressor.

By the time you do all that you could have just used a liquid Block Test kit aka Combustion Leak Tester and known within 5 minutes if the coolant portion of your head gaskets are blown or not. And all you'll need to do is remove the radiator cap and suck out a few inches of coolant from the radiator with a turkey baster.

Your choice.

You can find these for purchase or rental at some parts stores.

Autozone has this one that works great:

The Block Tester is $25

OEM Part# 27145



The Test Dye is $8

OEM Part# 25739



The kit has printed instructions on the case.

Carquest sells the Combustion Leak Tester in this video for $54.95 and it includes a 16 oz bottle of test liquid.




This kit features double test chambers which is even more accurate.

Both are super easy to use and inexpensive.