My 1968 327 SB boiled over twice this weekend, both times after shutting down following a 45 min cruise at highway speeds. It was a hot afternoon (90+ deg) but it was low 80s in the morning when it boiled the first time. I have a THM 400 with a 3.73 rear, turning about 3200 at around 72 mph. Radiator appears to be original, and has been painted. I have a flex-fan so the fan is not the problem(and NOT the topic of this thread--thanyouverymuch). Car doesn't run particularly hot in town. There is some yellow-ish sludge on the radiator cap seal, but the inside (what I can see of it) looks pretty clean. I'm considering the following:

1) Clean and flush the radiator/block
2) New hi-flo water pump & new thermostat
3) Oil cooler
and last & most expensive
4) new aluminum radiator

I don't think there's anything wrong with the water pump, but if those with experience think that a new pump is called for, I'll do it.

I would love to avoid buying and installing a new radiator, but I also want to avoid spending a whole day cleaning/flushing/filling only to find I need a new radiator anyway.

An oil cooler seems like a simple and prudent add-on, but I don't know anything about how it might affect the engine.

Any insight or advice would be appreciated, thanks


Chris

 

If the temps are OK while running, there's two causes of boilover after shutdown.

1) bad radiator cap (releases at too low a pressure)

2) overfilled radiator or expansion tank depending on what config you have. Neither is designed to be filled to the top, most cars run best at or below the 'fill' line.

Check #2 first, it costs nothing.

 

Have you done any work on the cooling system recently which would have required you to top up the fluid? If so, you may have air trapped in the system.

 

Was the radiator painted recently? If so, the paint can be too thick and is impeding the flow of cooling airflow.

Check the lower radiator hose...is it missing the spring inside it? That spring is designed to keep the hose from collapsing under vacuum which generally occurs at highway speeds. The hose could be rusted away or was never there.

Are the seals in place around the radiator and the air deflector in place under the front end? They are critical to keeping the temperature in check on a C3.

Plus what the others have already mentioned.

 

In addition the above mentioned cap check,

If you're not keeping the puke tank (white overflow bottle) sufficiently filled, the cooling system can draw air in upon cooling. This would cause the system to "burp" next time it gets hot enough.

If you keep too much fluid in the tank, then it will run over due to natural expansion.

 

How hot did it actually get? I'd bet its a bad cap.

 

Sorry I left this info out but to clarify, the temps were creeping up the whole time I was on the highway, and the guage was pegged a time or two once I got off and into traffic. Didn't boil until I shut it down though.

No expansion tank on my car.

 

If the car didn't start puking until you stopped the car, it may simply have been too much coolant in the system and it's finding its own normal level. Did you recently add coolant? If not, you need to identify why the temps were creeping up.

After you shut the engine off the temperature continues to rise for a period of time since the water pump is no longer circulating coolant and no more air is coming through the radiator.

 

That's not good. If your temp gets anywhere near pegged pull over stop and let it cool down. Check the basics like the thermostat, fanbelt, coolent leak.

 

In this case, you should have an RC15 radiator cap rated at 15 PSI. Rad caps are so inexpensive, I'd replace just to eliminate that as a possibility.

If the car ever had an expansion tank and doesn't now, that could easily cause this. (I'm figuring this isn't the case though).

I think Mike Ward was dead on with this early on in the thread. My only difference in perspective is do #1 no matter what.

 

New thermostat and rad cap will be purchased this week. I don't know if it was over-filled before, but it is now. I didn't see the "fill line" mark on the outside of the tank (it was covered with paint) until I looked closely this afternoon, and I filled it brim-full yesterday. Another use for my handy siphon pump! Spring is in place in the lower rad hose. The fins have been painted, but they are clean & straight, shroud and baffles are in good shape.

That said, I spoke with a couple guys at local shops today, and after describing the symptoms they both said the radiator is almost certainly bad, either clogged up or corroded.

I was looking around a little online, and the cheapest direct-fit radiator I found was $500 bucks. So I got that going for me. Which is nice.

 

Have you priced out re-coring at a local rad shop?

 

No puke (coolant recovery) tanks on any Corvette prior to '73. Many people mistake the aluminum expansion tanks on pre-73 cars for coolant recovery types. They're not.

 

Don't get a generic radiator, they rarely work. Get one that's built specifically for your year and engine combo.

 

If you have a brass radiator your best bet is to have a new core installed in your present tanks. A good radiator cooling shop can handle the job...You know it fits, so have it repaired.....cost is about $200-250.00...leave the stock pump on, unless it leaks...

 

I will look around some more; one of the guys yesterday told me it would be $400 for a re-core. Of course, that was after I told him it was for a Corvette...