Hi Guys,

I have a 1970 LS5 convertible that doesn't like to keep her coolant. Every third or fourth time I stop the car all the antifreeze pours out of the overflow. Completely purges the system. I have replaced the radiator, water pump, hoses, radiator cap and thermostat (while we were in there we did a new radiator core support as well). Drove the car to work, it was a little warm on the way here and when I shut it off all of the antifreeze came out of the over flow again. I am at a loss of what it could be and thought perhaps one of you had a like experience with a 427 or 454 and might could shed some light. Anything is appreciated.

Pete

 

What temps are you getting while it's running? Sounds like it's running too hot.
When you say it completely purges the system are you talking about it coming out the overflow line that's in the passenger wheel well ? how much is completely purges?
When you are filling the system do you run the car with the rad-cap off (on the expansion tank) and see it flowing hot anti-freeze and top it up then or do you try to fill it closed?
A/C? Mechanical fan?
M

 

it usually runs between 190 and 210. I fill it up with the radiator cap off, wait for the thermostat to open and then fill it half way up the expansion tank. Yes, the coolant comes out of the overflow line in the passenger wheel well, usually almost all of the coolant. No AC, mechanical fan and totally stock. I dont get it.

 

That's a lot of pressure.
I'd be temped to verify the temp gauge reading with another gauge or you could try a themal gun on the thermostat housing. You are running the short bypass hose from the intake to the water-pump as well
Since it's relatively cheap and easy I'd put in a new stant rad-cap.
(Next time you're out and it pukes, check the temp gauge real quick as well)
M

 

Will do. Thank you for your feedback my friend.

 

what color exhaust do you have?

 

Its fine. I do not think its a head gasket. Car runs great, no smell, no smoke.

 

Head gasket would normally make it puke every time after about 5 minutes, usually not waiting until it's turned off. I really think it's running hot

BTW, did you put the spring in the bottom rad hose? For whatever reason I think the new ones don't come with it and you need to re-use the old one...
M

 

Good question. Yup, i bought the expensive GM repro hoses and it has the spring in there.

 

Two things will cause coolant to escape out of the enclosed cooling system through the overflow tube.
1) Heat - As the coolant heats up so does the internal cooling system pressure until it over comes the 15 PSI radiator cap.
2) Radiator Cap - A radiator cap that can't maintain a system pressure of 15 PSI.

In your case I would say the coolant is reaching a temperature point that is overcoming the radiator cap. There are a number of reasons for this from engine timing to faulty cooling system hardware or incorrectly installed cooling system hardware. The first line of defense is knowing what's going on and when, what's the actual engine temp when this happens? A good IR Gun is your friend here and you should carry one in the car to verify this. A good rule of thumb is here is there should be roughly a 20 degree split between the inlet and outlet of the radiator if it's doing its job.

Given the parts you have already replaced I would focus on the following;
1) Timing, read LARS paper on this subject.
2) Airflow, is fan and clutch correct for the car and working.
3) Make sure the cooling and air flow (seals, air dam etc.) system is 100% OEM so you have a good baseline to troubleshoot from.

The Chevy BB runs on the hotter side of life, My last 427 390hp engine (1965 Vette) always ran on the hotter side and I felt it was a stoplight away from overheating but it never did puke out of the overflow tube.

 

Attached is an article written by John Hinckley that is a good read.

 

Sums it all up.....only thing it doesn't say is to actually increase the spark advance which will lower EGT's.....it just mention ported vs. full manifold vacuum.

To the OP.....I urge you to get Lars papers and set your timing per them.....for efficiency and performance.....
I have had some cars at the house.....(like the bonehead who ended up with my Dad's 32' Ford..) who have no concept of ignition timing and just move it around or have their buddy move it around until it "sounds right"......this is bad. My Dad's old 32' with my hot 350 I built 20 years ago was here in 2015 for overheating......first thing I did was get the timing light out and the owner started asking a bunch of questions......the base timing was like 4 degrees and the hose was on ported vacuum....for a total of 25 degrees total....car ran at 230 degrees and ran like **** to boot.....I advanced it 11 degrees and the temp went down to 190.....no lie. I told him if he ever moved the distributor again.....don't bring it back to me.
On a C3 the problem is compounded because of lack of underhood airflow......so high exhaust temps make everything else hot.
Check your timing and get back with us

Jebby

 

Attached is LARS technical paper on the subject of timing.

 

You replaced a lot of stuff, but how does the fan thermal clutch check out? Might be on its way out causing the fan to not work properly.

 

As I'm reading this thread one thing comes to mind in spite of what others said. Head gasket, what else in the cooling system is going to produce the pressure to push coolant out at 210 degrees. Well maybe this. Did you start having this problem after the water pump replacement? I'm just wondering if you got a water pump for a serpentine belt setup? I believe they run in the reverse direction.

 

It is a good read. However, in his discussion regarding the function of the thermostat, he states that the ts does not not set the maximum temperature of the engine. I think he means the the engine will go into a borderline overheating condition (ts fully open) because the ambient air temp has increased to a point where it can't absorb enough heat being transferred from the radiator to the air. Should the air temp go up another few degrees, there is a good probability the engine temp will shoot up and overheat. That said. the ts does control/maintain the engines normal operating temperature if the rad capacity is large enough to reject the excess engine heat which he does go on to say. But all this goes back to the design conditions for cooling systems 40 or more years ago e.g. will it need to prevent overheating when it's 110*F outside? Will the temperature be maintained at 220*F with a 190* ts when its 110*F outside? Modern cars have better designed cooling management technology and the temps are pretty much maintained at rated ts temp within a couple degrees + or - as the cooling systems are much more efficient. C3's also can easily handle cooling for stock and increased horsepower outputs just like todays cars. This is easily achieved by installing a larger capacity aluminium radiator. Stock clutch fan is adequate as the article correctly points out.

 

another is intake manifold at the crossover.
I have dealt with an intake that only allowed cyl pressure into cooling system but did not suck coolant to make smoke.

 

In 1970 did a Corvette with a BB installed fresh off the assembly line puke coolant out the overflow every time you shut the car down? If not than why does it happen in 2022? Was 110F outside temp different in 1970 than 110F outside temp in 2022? Did a 180F TS open different in 1970 than it does in 2022?

 

I think the thing that gets folks off-track (or maybe I'm off-track) is that that 220°F engine will attain 220° no matter what thermostat rating is installed. The only thing a thermostat does is restrict coolant flow up to and to maintain its design temperature. The t-stat determines the speed from start-up at which the choke will disengage (through heat transfer to the choke controls) and maintaining the efficiency of the combustion process once up to temp. Our engines (generally speaking) are not as efficient at 160°F as they are at 200°F, so you don't want a low temp thermostat in there staying open as coolant temps drop below optimum operating temperatures – plus it won't keep your engine from attaining the temp that reflects the system's heat transfer abilities anyway.

Whether the normal operating temp is maintained depends entirely on the efficiency of heat transfer to the atmosphere. Is coolant cycling through the radiator at the proper speed to allow transfer? Are the radiator passages fully open to transfer the coolant temp to the metal tubes? Can air pass through the radiator fins such that the tubes' heat will be "carried away"?

 

There used to be test strips that determine if there are exhaust gases in the coolant.
They are like pool test strips for alkalinity/chlorine levels in the water.
For a car the strips test for gases from the cylinders.
good luck