I posted a few weeks ago concerning my 68 convertable running hot, and I took the advice but still running hot. I rebuilt a 66 289 and did .040 high comp pistons (not sure ratio) rv cam, Edelbrock performer intake and 600 carb, new water pump and 3 row radiator with shroud and 6 blade flex fan wiht longest spacer I could find, timing is at 6deg, and plugs look a little light but not running too lean. Fluid doesnt' bubble like a blown head gasket but will surge if reved up. It runs hot and will puke fluid. SOme suggestions were, new rad cap...tried 2!!, spring in lower hose...installed one and didn't work, 180 thermostat...didn't work, 160 thermostat....didn't work. Today I back flushed the system but it didnt' work. Here's the wierd part...starting from cold, let it idle and it will warm up to 160, hold all day, and not puke, but take it on the highway for a short 5 mile loop, 3/4 gallon will puke and temp will rise to 200! The only thing left I can think of is a head gasket on backwards. I wasVERYcareful in rebuilding but I'm at a loss . ANY SUGGESTIONS PLEASE does anyone think that my symptoms sound like a head gasket backwards?
Thanx

 

Try a heavier radiator cap. Appearantly it's not holding the psi while driving. Do you have a recovery tan(overflow tank?)
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I rigged up a bottle for an overflow tank, that's how I know it's about 3/4 gallon. I got the heaviest cap I could get...13 lbs I'm starting to really think it's the head gasket

 

is the radiotor new? if not, i would check the fins and make sure that air is being able to pass through it. I know on alum, rad you can see light passing through. I dont know if that works on the steel. I would put a fan in front of the raditor and use a tissue on the other side of the raditior to check if air is passing through.

 

the more you overbore a 289, the hotter they get. I've never dealt with a backwards head gasket issue, but before you pull her back apart, just for fun pull the thermostat, pull all the guts out of the housing (you will need to clip the 2 little arms that retain the spring, and slap the housing back in. Why not just pull the thermostat all the way out you ask, water flows too fast without at least the housing in there and the water dosent get time to cool in the radiator. Just my 2 cents worth.

 

First off, don't expect a lower temp t-stat to make the engine run cooler if you've got other cooling problems. It regulates minimum operating temp and warmup time, not maximum temp. I'd recommend a 190 high-flow t-stat (available at Summit Racing) for proper combustion efficiency and good flow. If the engine is running colder than 190, you're going to lose combustion efficiency. My 331 runs at 200-215 all day with a 190 high-flow t-stat.

Second, get rid of that flex fan and get yourself a clutch fan when you get a chance. I've run a solid steel fan (REALLY loud), a flex fan (better, but still loud), and am currently running a thermostaticclutch fan. The clutch fan is by far the best choice. It'll lock and cool just fine in traffic, and disengage at higher speeds where there's plenty of airflow for quiet running.

Now...why is that much coolant puking out at 200F? 200F is perfect operating temp for that engine. Are you running a proper 50/50 mix of coolant, or is it pure water? Water by itself boils at 212F, which is above your operating temp, but you can get hot spots in the block and heads that spike above that and will boil pure water in those points, which in turn can cause a spike in pressure and push coolant out the overflow.

Also, how are you getting 3/4 gallon in an overflow tank? Most overflow tanks I've seen are 1/2 gallon at best. Keep in mind that if you fill the radiator up to the top, you're going to push some fluid out just from heat expansion.

 

+1

 

Starfury is correct. You may also want to try water wetter. Back in the day I had overheating problems in SoCal and used to gut my thermostats. This just restricts was flow.
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200F is no problem on a small block. I wouldn't worry unless it goes significantly higher. My 302 will run 230F-240F at the end of the 1/4 mile or when being driven very hard. No problems. It also depends on where you're taking the temp. If your sensor is on the intake next to the head, you will record higher temps. I run mine on the t-stat housing.

You'll need to make sure you have a good fan shroud for cooling at idle.

Without a recovery system, you do not want to fill your radiator full. You will need some room for expansion.