I have a 96 LT4. I recently replaced the thermostat, and topped off the coolant. After running the car, I heard boiling from under the hood, opened it and saw coolant boiling over in the surge tank.

After letting it cool over night, I took it for another drive and same thing. Coolant level never lowers in the surge tank, and boils within just a few minutes of starting. The temp gauge never gets above 220, and I have tried burping the system, but only get coolant to flow out of the bleeder before thermostat opens.

Any suggestions? I cannot think of what could be the problem. Thanks!

 

Keep burping and check your cap and any crud on the seal. If you did not pull the knocks you left a bunch of crap in there.

 

Why did you replace the t-stat?

The symptoms provided suggest that your new t-stat is not opening.

The LT1 engine requires a special t-stat. Was the correct t-stat installed?

 

Thanks. I replaced the old t-stat is it was frozen shut; radiator hoses never got hot, and never got coolant through them.

I did replace it with the correct thermostat, and now I get coolant and pressure through the upper and lower hoses, but never get coolant from the expansion tank back into the main system.

Currently I'm hoping it's an air pocket somewhere in the system and that I could burp it out.

 

Sounds like the original problem might still be in there. When all is cold, remove the surge tank cap and rev the engine. You should see the coolant drop in level about a inch or so. If it does not the water pump is not pumping properly. Coolant flows thru the heater core surge tank even with stat closed.

Adding coolant and bleeding the air out is a simple thing. If things are not working right then the problem is somewhere else. Is the stat facing in the right direction?

 

The expansion tank is just an extension of the radiator. If coolant is not entering the block, the t-stat is your problem.

The LT1 is self-bleeding (some owners refuse to accept this as fact).

 

It sounds like you are describing a possible blown head gasket. Don't get alarmed yet, I could be wrong. I hope I am wrong.

But it seems to me that just "a few minutes" of running is not long enough for the engine to fully heat up and push coolant beyond the surge tank into the expansion tank. A blown head gasket could allow combustion gases to leak into the cooling system right after firing and pressurize it enough (in just a few minutes) to boil over into the expansion tank.

How well does the car run? Have you checked the spark plugs recently?

 

Not likely IF this is true;



The first thing that I would do is replace the "radiator" (surge tank) cap.

 

I tend to agree with starting with replacing the cap with the proper AC Delco cap

But I'm curious how you know it's boiling in the surge tank. The only way to see that would be to remove the cap with the system hot, not a good idea

is the boiling in the plastic tank in the nose?

 

Thanks everyone for the help! I've tried everything I could think of and most everything suggested, still without eliminating the problem.

I pulled the thermostat and tested it; it works perfectly. I put on a brand new Delco radiator cap and cleaned and checked all the seals. I filled the whole system up with coolant, starting by pouring it in through the thermostat housing and then through the surge tank. Revved the engine with the cap off of the surge tank, and watched the coolant dropped about an inch and then came back up, so water pump seems to be working.

Fired it up, and it worked perfectly up until 205 degrees, when the coolant started boiling in the recovery tank in the nose. Opened up the bleeder valves thinking it was air, and air (no coolant) came out of the bleeders for the next 10 minutes. During this time both radiator hoses felt full and under pressure, and I never read more than 215 on the temp gauge. I eventually shut it down but never did get any coolant to flow from the bleeder. After another 10 minutes the boiling settled down, but both radiator hoses are totally empty and can be squeezed flat. The coolant also never lowered in the recovery tank, only got added to throughout the process.

Why and how could there be so much air in the system? Why doesn't the coolant exit the recovery tank? And why would it still be boiling? Thanks again for all your help!

 

I'd take the thermostat out and see what happens

 

Let's go back to square 1.
Did you have this problem after you replaced the stat as indicated in post 1? Or were you replacing the stat to fix this current problem?

In post 10 you start talking about the recovery tank where you had not before. Is this just a mistake from when you were saying "surge" tank. Or is this new additional information?

If coolant is really boiling in the recovery tank, then you are pushing super heated steam and water into the tank.

 

Mike, air is entering the closed system. The reason the cap is rated at 15# is to raise the boiling point of coolant about 50 degrees. With outside air coming in, the pressure increase provided by the cap is negated, and the coolant will boil at 212.

It is possible that there is a rupture in the head gasket which is allowing exhaust gases into the cooling system, out the exhaust, or in the oil.

Exhaust gases in the cooling system will force super heated coolant out of the system into the overflow bottle. If no increase of coolant is evident in the overflow bottle, no breach has occurred into the cooling system. Next, could be that coolant is exiting the tail pipe. Smoke/steam might be obvious. If coolant is in the oil, the oil will be milky in color.

Even though the rad cap is new, check for gasket deformity. The downward pressure required to install the cap can tear/crumple the gasket. It is advisable to apply a light coating of grease to the gasket before cap installation.

The engine/cooling system can be pressurized to determine any leakage from hoses, fittings, etc. To pressurize the block, you must remove the t-stat, or exit hose.

A test can be performed to detect exhaust gases in the coolant.

 

Can't do that w/an LT1.

 

 

Is the hose to your recovery tank bad? It will suck air if it is. Or any of your other hoses. You can get the tester for free with a deposit at the part store to rule out the head gasket.

 

Just to throw this out there, which may or may not help.

Bleeding the LT1/4 cooling system is always a topic of discussion. The one thing, and reading post 10 brought this to my mind, that I found most successful in burping air out was to lift the front end of the car, and run it to operating temp with the surge car off, adding coolant as the level dropped.

Raising the front end puts the radiator as the highest point, where the air will go and get pushed out via the surge tank. I did this when I replaced the radiator and it worked well.

Assuming you have the correct cap and t-stat, it sounds like you have air trapped in the system. There could be more to it than that, but I believe in eliminating the simple, cheap stuff first.

 

Tom and select others have lots of mechanical experience; starting simply is always best, incluidng the thought process of the prior problem.

You could pull the plugs and check them for evidence of the prior O'heat damaging either a head gasket or the head itself. The aftermarket fixit books are not worth much but often offer color pics of how to read plugs.

Hopefully, the oil shows no indication of coolant entering it.

Certain chain stores rent testing gear, free, for T-stats, cooling system etc. but I "go there' after the simple steps.

 

You don't want the radiator at the highest point. The cap needs to be at the highest point....which it already is on an LT1 car, with the car sitting on level ground.

 

Well, you could be right. However, when I did it this way after the radiator install it seemed to work a lot quicker and better than when I did it with the car flat after the upper hose blow out. But maybe I just got lucky as well.