If you ran distilled water through the system, and did not drain the block after, then you are running more than 50% water. Not sure what climate you're in, but you may want to test it with a hydrometer.

 

Thanks for the tip. I will add 100 percent for the coolant lost I have some left. Right now I just want to know why its boiling over when I stop. I dont think its because of my coolant mix. It cant be that off that much because I only had a LITTLE of the 50/50 left.

 

If your coolant fans are NOT running,, when you slow down or STOP and the air flow thru the radiator no longer exist,, its going to over heat. As soon as you move forward, you reintroduce air flow and it cools back down. See if BOTH the fans are running and running at the correct speeds.

AHH,,,,, I see that you checked the fans and they work.
Make sure that the system pressurizes to the correct pressure and see what happens.

BC

BC

 

Ok it runs at about 208-220 range on a long 30+ minute drive mostly in traffic. Never going any higher. Is that long enough to rule out a head gasket? I could have went longer but after it staying at 220 that long I started bringing it on home.
Now after that run I turned it off no more boiling over.....WHEW!!!! after replacing the surge tank with a new one that doesn’t leak. (the first one I ordered was a defect and had an obvious leak)

HOWEVER when I turn on the A/C…. within about 2 minutes it starts slowly rising and then pause around 240ish…. Then go on to 248 where I shut it down. It was going up like this when I was sitting still. I got out of the car and felt the fans blowing both of them. They did appear to be on the LOW speed. With HPTuners I can turn them on high or low speed and this seemed like the low speed. They will definitely work on high speed because I activated high speed with HPTuners.

Not sure if high is required at that point or is lower normal. Tonight when it cools off im bringing it in the garage and really brushing the radiator fins. I cleaned the obvious already and the radiator looks pretty clean but now I am about to really carefully get in between the fins.

Any other suggestions are appreciated!

I found this so I guess the low speed was normal:
FROM THE SERVICE MANUAL

C5 Cooling Fans Operation

The low speed cooling fan is commanded on when the coolant temperature reaches 108°C (226°F). It is turned off if the coolant temperature lowers to 104°C (219°F). The high speed cooling fan is commanded on when the coolant temperature reaches 113°C (235°F). It is turned off if the coolant temperature lowers to 108°C (226°F). When the A/C is on and the coolant temperature reaches 85°C (185°F), the low speed cooling fan will be turned on at vehicle speeds less than 56 kPh (35 mph).

 

Do you still have your air dam in place? It aids with the cooling.

 

Nope it hasnt ever been there. of course I havent had my AC working either until now...lol. Ill look for one when I can find a part number. Ill TRY anything. I have damn near replaced the entire cooling system already. Anyone on here have one let me know. Or hell a picture of one. When I take the top shroud off I can see right to the ground there is NOTHING between my condensor and the radiator on the bottom and no air dam. But its doing it even sitting still too.

 

It was just a thought. Some people say that it doesn't make a difference and I say that it does. The reason that I believe it does is because my old '99 lost a part of the Air Dam and my temps went up 15 degrees on the highway. Unfortunately, I don't have any hard data to back it.

 

You dont need it for me... im ordering one tomorrow!! If it helps which from what I have read its almost mandetory.... Ill get it. I found them for like 95.00 shipped for all three parts and the hardware. I actually have 1 part of it the driver side.

I also found just the main center part for 25.00. If it will fix the problem I would buy a big mouth piece for 200.00.

 

This is what HVAC guys use to clean condensors. Should work on radiators to. You spray it on from the clean side and it turns to foam expanding and pushing the dirt out of the fins as it goes. We have a fleet of over 100 CNC machines at work and I'm always amazed at how little dirt it takes to cause heat related failures. http://www.hydrobalance.com/CoilCleaners.htm

As for ruling out the head gasket because it doesn't overheat on the highway, not so. I had a 95 Neon that had a leaky head gasket, what Neon didn't? The dealership tried the usual dye test and new thermostat. I drove the car nearly 300 kms home and it didn't overheat until the off ramp to my neighbourhood. The test that found the fault for me was warming the car up from cold and watching for bubbles in the coolant at the rad cap. Within a couple of minutes they were there. Another tell tale sign was a tick, similar to a rocker tick that appeared as the car warmed up instead of going away as it got warmer.

Now I should state that even though it was overheating per the temp gauge it did not boil over.

 

The 408, new build?

Spray it lately?

Any power lost?

 

I have that direct port kit up there on it. No noids have even been hooked up.
The build is not new. It was when I brought it from TSP. I have never sprayed it.

All the power appears to still be there. Seems a lot stronger but I have only done two 35-80ish bursts here since I got it back. from finally getting it unchoked last week. I had stock C5 mufflers and 2.5" exhaust. Now its 3" with 3" DMH low profiles.
I have the lingenfelter 002 timing control I dont have the micro edge yet and its because OTHER stuff keeps happening....lol.

 

I know this may sound like one of them Blonde questions but because of that one reply I am wonderring if that truely is my problem. I have a powerduct and vararam going over the UPPER radiator shroud...the long bar, the only other part is the shroud that holds the fans. Nothing underneath, nothing on the front side. Just two pieces. I have been reading and reading and I se that I am missing three pieces that ram the air into where the radiator is. I am ALSO missing I think its called the lower part? Its the size of the radiator and looking at a pic of a vararam installed I can see it. ITs the part that some people cut with some air intake systems.
I dont have that either. It had a huge hole in it so I tossed it when we put the engine in. Now I have made many long drives and I think thats why it always ran a little hotter than any of my other 4 LS1s have ever run. It has never done like it has here as far as boiling over (surge tank fixed this) but always 220-235ish on the highway. Now it appears it only gets hotter when i turn the AC on. I am about to replace these parts and ill let you guys know how that works out. I really appreciate ALL the replies period. I would have never known those parts were suppose to be there if it wasnt for you guys.

 

http://www.gmpartseast.com/diagrams/...-1y0103001.png

Sounds like you may have found the problem. Look at the picture. Is #4 what you're referring too? If so, that piece is CRITICAL for directing airflow to/through the radiator. It's called a "Radiator Lower Support". It is even more critical than the lower spring-loaded air deflector.
Unfortunately, this is what happens when trying to help someone diagnose an ongoing problem WITHOUT (a.) pictures, and (b.) an adequate description of ALL the parts involved (or removed) and/or previous work that's been done.

 

I just needed confirmation that it would help because for a LONG time I never had either pieces. The air dam that scoops the air nor the radiator lower support.

 

I would humbly suggest that you do a complete "burping" of your cooling system. There are many threads with detailed instructions on this forum so do a quick search. You have not mentioned in any of your responses this procedure. If you do have a air in the system you may have circulation and pressure on the tank which would explain why you do not over heat when driving. If the heads were not getting sufficient cooling due to air in the system it might explain why the temps go up when you shut down as the heat does not migrate with the coolant. Have you performed any "burping" of the system? Just a thought. Good luck!

 

Did you burp the system to insure all the air is out?

 

You are not alone, 98 coupe, I replace the overflow reservior two mohths ago, and serviced per the instructions, all was well. Went on two long trips with no problems. Now, I have the same overflow problem after shut down. Called another vette guy, he thinks it might be a blown head gasket. Just guessing for now, we have not tested anything yet. My area temp is around 90+ for the summer. I did hear it rumbling after I raised the hood.

 

I don't see it mentioned but pressurise the system see if it's holding then start narrowing it down. You may have a leak or small pinhole air bubble getting in/out. You can rent a tester from vatozone for free.

 

A new radiator cap is a cheap fix... if your cap has been comprimised...

 

For mine I did a new radiator cap actually ended up with three different brands, new thermostat, new water pump, and my first overflow tank had a bad spot where the top and bottom were mended together. Ordered another one and THAT stopped my boiling over issue. My car on hot days never really go over 185 a 2.5 hr ride Friday and 181 was the highest it went.

Another part of MY cooling issue was that I didnt have the lower radiator shroud. Never knew one was suppose to be there. It was never there when I got the car. I also didnt have the air dam. After having all of the components added, my car runs really cool.

The boil over was from a bad surge tank though.