c322348

If you fill at the thermostat housing, the radiator cap should prevent coolant from entering the (fender mounted) expansion tank. The boiling temperature of the coolant increases with pressure. If your cap is venting pressure out of the radiator, your coolant will boil at too low a temperature.

You might want to try a new radiator cap with the correct pressure rating.

Usafstingray

I've been down this whole road...but with the L76. The design margin on the cooling system wasn't much, so minor problems are big problems with mid year vettes. The previous mods created this overheating problem and it is now compensated for by removing the thermostat. Dump the aftermarket flex fan. It isn't moving enough air, especially at idle. The shroud looks wrong to me, but I'm not a big block guy...yet. The stock shroud setup with any filler panels/pieces ensures you pull as much air through as possible. Burping...never had a problem with this. As long as the pump is full of coolant, it will push coolant through and air out. If you really want this fixed, you need to get back to an all stock setup and get away from aftermarket mods. Don't try high flow pumps either. That's another mod that is unlikely to compensate correctly for the other non-standard mods. Your overheating isn't bad enough to suggest that you have any blockage issues or a bad pump. Use the IR gun. The gage in the car is probably off...most are with the aftermarket sender units. In my opinion, the best aftermarket upgrade you can make after everything else is back to stock is a good aluminum radiator and an electric fan to keep things cooler at idle on hot summer days.

tuxnharley

That's a good point, and well worth checking out - but, if the problem is with the cap wouldn't that have been the case before the thermostat was installed also?

c322348

Your flow rate through the radiator may be better without the thermostat, so the cap may not have mattered. Probably didn't need a thermostat to get heat built up to use the heater in Florida.

Cooling is a balance. In some applications, without the thermostat to restrict flow, coolant flowing too quickly through the radiator will not have enough time to exchange all the heat through the radiator fins.

It sounds like you have a combination of issues though, not just a single issue. GM spent a lot of time engineering the system and oftentimes changes are made by consumers. Flex fans are a good example. They are touted as being low drag to increase horsepower. Maybe important in a competition application, but probably no detectable difference on the street. Someone bought into the advertising though, or bought the flex fan rather than replacing a defective fan clutch.

I found some cooling FAQs on this forum in case you haven't already found them:

http://forums.corvetteforum.com/1558302870-post3.html

babbah

If you put 70% antifreeze and 30% water, I would suggest that you run a 50/50 mix instead.
Antifreeze can't dissipate the heat as well as water.

If you put 70% water and 30% antifreeze your system, you are more better off for maximum heat dissipation of your coolant in your system.

The higher the concentration of antifreeze in your system, the hotter it will run.

I had like 75% antifreeze in my system and it ran very hot. Reduced the antifreeze amount to 30% with 70% water and the system worked perfectly and the motor ran much much cooler.

Good luck!

play2win

Thanks for all the info! I picked up an IR gun to get some true readings.....I'm having fun pointing it at my beer to see how cold it is, and the temp of my dog, but I would really like to explore its intended use. Where should I be pointing it (intake man, t-stat housing, rad hose, radiator in/out??) And most importantly, will these numbers be able to diagnose anything besides the obvious which is high temp?

Usafstingray

You should check a number of locations on the heads, the manifold near the sender unit, and the block. Also check the radiator at various locations from top to bottom. The heads will generally run a bit hotter than the block and manifold from what I've noticed. The radiator will be hot on the top and cooler on the bottom. Nothing should be hotter than about 195 from my experience if you run a 180 t-stat. Checking a lot of spots should find hot spots which might suggest coolant flow issues. Good luck.

JohnZ

Shoot the upper radiator hose just above the thermostat housing, and the lower radiator hose between the radiator outlet and the water pump inlet; the upper hose reading should be compared against the temp gauge reading so you know what it's really telling you, and you should see a 20*-30* coolant temp drop between the thermostat housing outlet (radiator inlet) and lower radiator hose (radiator outlet).

play2win

Ok gentlemen, here are the readings I have been getting off IR gun:

Upper radiator: 180 - 200
Lower radiator: 160 - 180
Upper rad hose: 205 - 215
Intake manifold: 210 - 240 (it gets hotter as I work my way towards firewall), and these numbers are 230 - 260 when I shut car off

Can these numbers help me diagnose anything? Water pump? Combo of other things?

Temp gauge in car is pretty accurate to manifold when running.....also, how can you keep engine temp from shooting up when car is shut down? That is when car pukes out antifreeze (after I shut engine off), I can hear water boiling in expansion tank.....yes, I know it doesn't belong on big block

Wayne1964

I find electrical fan of helps . 50 /50 Mix Maybe check heater core ? just a thought.
Maybe thermostat needs a small hole drilled to let pressure out.

JohnZ

You should be able to get each of those temp readings down to an actual single degree; ranges of 20 degrees each don't tell us anything in terms of diagnostics. The closer you get the I.R. gun to the target, the more accurate the readings are.

Coolant temp will always spike after shutdown due to "heat-soak", as the engine is still transferring heat to the coolant, but no heat is being transferred from the coolant to the air, as the coolant isn't circulating through the radiator, which isn't getting any airflow anyway. In a normal cooling system, there's room for this coolant expansion without "puking" through the overflow hose, but I still haven't figured out what's connected to what in yours:

The 5/8" heater hose from the bottom nipple on the heater core - what's it connected to as it goes forward to the intake manifold?

The 3/4" heater hose from the upper nipple on the heater core - what's it connected to as it goes forward to the water pump inlet?

What are the fittings on your expansion tank connected to?

Mr D.

Why is that expansion tank still on your car?

KC John

Not saying this isn't important, but what does the expansion tank hurt? I understand that it isn't come with one, but it doesn't seem like adding one would cause any problems. Can someone explain?

DucatiDon

It could also be something as simple as a loose fitting or leaking hose/gasket.
I had similar issues on my small block until I noticed that the water pump bolts were loose, and the PO hadnt used any gaskets...so the water pump was likely sucking air and defeating my closed system.
WIth proper gaskets and bolt torque, the problem disappeared.

Westlotorn

It does sound like the system is not holding pressure.
The radiator cap and coolant tank cap could be releasing pressure and letting your system boil.
The caps are sold with a pound rating. Each pound gives you 3 degree's extra protection against boiling. So a 15 pound cap will give you 45 degree's more boil protection.
You can test this with a cheap radiator cap from the parts store and see if the boiling stops. If it is not boiling running you may be good and just need to fix your caps or search for leaks as indicated.

Flex Fans cool best at idle, they cool less with any RPM added. A change to a quality Thermal Fan Clutch and a good fan blade will help. A good Thermal fan clutch can spin the fan blade at 90% engine speed when engaged, that pulls a ton of cooling air through the radiator when needed. When not needed the fan spins only 10-15% the speed of your engine so it is quieter and takes less power to turn. That is why all Pre Electric fan cars ran a Thermal Fan clutch.
If you still build pressure in your system after good Radiator caps are in place you may have a leaking head gasket as was noted earlier putting exhaust and combustion pressure into your coolant. You can mask one of these failures by pulling the thermostat, I hope your previous owner did not do that to you.
A pressure test of the system is a good idea before replacing a lot of parts.

mikem350

I have a BB with aftermarket fan too. Your FAN is one problem, its blades can hardly move any air!!! Look at pictures in Jegs/Summit for an RV fan with BIG BLADES like fly swatters, and space it so its half in the shroud..like 2 inch spacer. When running the fan should blow like a hurricane, even at idle. I live in FL and have done 2 BB cars this way with no problems.

Use the surge tank like a modern car's overflow tank..leave the cap off!!!
Make sure the rad cap is for a newer car with the overflow tank

play2win

Wouldn't there be a leak somewhere if system wasn't able to hold pressure?
I have a flex fan, and it is the opposite.....it overheats at idle, and holds a good temp when cruising

KC John

I think this points to the fan not doing it's job at idle. Once the car is going down the road you really don't need the fan because the radiator will get plenty of air flow. I'd change out that flex fan for an original style and I bet that cures your problem.

c322348

There probably is, from the radiator cap into the expansion tank.

I bought a flex fan from Summit that had big fat blades that moves a ton of air at idle for my hot rod that suffered from overheating. That (and a tight fan shroud) did the trick for me. Those pointy bladed Flex-A-Lite fans are worthless for cooling.

I second the thought that you might look for signs of a blown head gasket. If a blown gasket didn't result in a cooling problem, the overheating situation might have caused one to blow.

Westlotorn

I agree, at idle you create the least amount of heat normally in an engine and the RPM is low, the fan is your only source of cooling at idle and it sounds like your flex fan is just not pulling any air.
A flex fan is at its best at idle so you need to improve your system.
A Thermal Fan clutch with a 6 or 7 blade fan will pull the most air for you. As several have already mentioned your fan shroud may need improvement.