69mako

The vehicle, 1994 Corvette coupe, stock LT-1 motor

The issue, lower recovery tank over flowing-empty upper recovery tank

This issue is a bit puzzling to me. I took the Vette off jack stands this weekend and fired her up. She runs great. I had a “low coolant” light on the dash. I took a look around-No leaking hoses, no leaking water pump, no coolant mixed with the oil

I topped of the upper coolant tank and the light went out, as the motor warmed up all temps came up to normal.

I drove the Vette for about 45 minutes-normal operating temps on water and oil, I then let it sit. I noticed that coolant was coming out the top of the lower recovery tank-from the valve in the cap. It was overflowing-strange.

I let everything cool down and took a look around the motor. I found that the coolant I had added to the upper tank was gone-tank looked empty; again I inspected the motor, no leaking hoses, no leaking water pump, no leaking radiator, no coolant mixed with the oil and no dreaded leaking heater core. The coolant I added had recirculated through the motor but ended up over filling the lower recovery tank.

Is there a check valve in the upper recovery tank that has stuck and is not allowing the coolant to refill the tank and is pushing the overflow into the lower recovery tank?

Thanks, Larry

Nathan Plemons

Coolant flows into and out of the recovery tank via the same hose, there is no check valve, at least there certainly should not be.

It sounds like you just have a great big air bubble in your coolant system. The LT1 engine can be a little tricky to get bled out properly.

69mako

It was fine and burped when I set it on jack stands in Dec, I'll burp it again and see what happens. I'll break out the Helms book and see what lines should flow what to where.

93Rubie

I was going to say, are the lines routed correctly? They are labeled top, bottom, etc...

Also, is your pressure cap good? That is what allows coolant into the lower tank as it warms up and back into the system as it cools down.

Kmcoldcars

The pressure from the expansion of the coolant as it heats forces the coolant into the lower tank and is overflowing it. As the coolant cools down and contracts the vacuum created in the cooling system should pull the water from the lower tank into the cooling system. If it does not then you may not have any vacuum, which would most likely be a bad cap, or could be a big water bubble in the system.

ddahlgren

A while back I think someone tracked their similar problem down to a cracked upper tank that would not allow vacuum to pull the coolant back but did not visibly leak. A search may help as it was only weeks ago from what I remember.
Dave

summerst

Now I understand why my mechanic was so insistent about replacing the upper tank on my '91. It looked fine to me but I did it anyway (along with a new cap) and the "running hot" issues went away. This was after we tried in vain to cool the car down including replacing the temperature sensor and thermostat. That was back in 2005 but I remember it as though it was yesterday as the car had real trouble getting through emissions here in VA because it was running so hot.

Old Ben

Also check the hose from to the overflow tank. Even a very small crack or puncture can result in the vacuum sucking air through it and not the water from the overflow tank.

69mako

The surge tank cap was my first thought. I’ll replace that and put in a new thermostat (its due) and see what happens. I’ll also check the tank for pin leaks.

I read the posts on the cracked surge tank when I researched my problem.

XR_Strider_GuY

Yeap I have one of these tanks on the way. Same symptoms as everyone else, lower tank filling up but everything else is ok. I will report once installed.

corvetero

Same problem to me, even after replacing radiator cap. Anyway, I've check the rubber seal of my new cap and I noticed is smaller compares with the old one.

69mako

I'll check that out when my new surge cap arrives today. I'll file a report on this problem and its solution when I'm finished.

caddyboy84

The stat may be hanging up. Whenever I had an issue like this, it was a bad stat. Take it out, put on the stove in boiling water and see if it's opening.

69mako

If I'm going through the trouble of pulling it out-I'm just going to drop in a new one...which I have ready

caddyboy84

But.....If it don't open, you'll know that was it

HUPCEJ4x4x454

Worst case scenerio (which is what has happened to me)... a very small head gasket leak between a cylinder and a water jacket passage. After running for a while (30 mins to an hour), the cylinder pressure would slowly pressurize the cooling system, eventually displacing much of the coolant into the overflow tank, along with the "Low Coolant" light coming on. Other than that, there were no other warning signs (no white smoke, milky oil, overheating, etc.)

Hope this is not the same problem with yours. Good luck!

69mako

I’m leaning towards a bad surge tank cap. Mine is OEM-18 years old and the gasket fell off in my hand last night. I have the new OEM replacement and will be installing and testing the cooling system Saturday morning.

The new cap looks to be constructed with a better gasket.

desertmike1

X2

This happened to me....

69mako

What test procedure(s) did you utilize to discover it was a leaking head gasket?

caddyboy84

There is chemical test that is done to see if there is exshaust emmissions in the antifreeze.