Hi and looking for suggestions to solve an overheating problem with a 69 L36 427. Any and all ideas are welcome!

I bought a 69 L36 427 from a neighbor and the engine had been rebuilt by a professional shop but never started up.
So, I added Royal Purple breakin oil, installed a new DeWitts radiator, installed OEM shroud with extension on top, new clutch fan, set timing 4BTDC, set valves, ck'd new themostat @195, new rad cap and broke in per rec'd procedures.

Engine runs and sounds great but overheats in driveway @ idle after 15 min at OEM temp gauge 220 and starts burping overheated coolant out rad cap. Ambient @90 95F

So, I have pulled off return rad hose and burped eng,
+ set horizontal brass coolant tank level 1/16"
+ ran w/o thermostat; same result
+ ran with box fan in front of Corvette with hood shut; same result
+ return hose is hot so I assume wp is pumping
+ fan clutch seems to be viscous and OEM big block fan spins.
+ will check timing again but had set at 4.BTDC
+ tried jacking up the right side of Corvette and adding coolant to eliminate air pockets

Next steps Now What??? change out water pump and install electric fans???

Any and all suggestions are welcome! Short of selling back to my neighbor as he knows all of this.

Much Thanks!

 

hmmm...
Sounds like you did all the right things and you have a very quick overheating problem. 15 min is really fast.
I do not think you even need a fan in the first 15 min.
"rebuilt by a professional shop but never started up"

First question:
How long ago was it built?
How long did it sit?
Where did it sit?
And was it all taped up while it sat?

A friend and dyno shop owner had a similar puzzling issue.
99 pt NCRS 65. Pulled it, disassembled it, and found a mouse nest behind a freeze plug because it sat for one weekend not taped up!

I am not saying that is your problem, just that you can't overlook anything unless you know all the history.

I would not even know how to go about checking water flow in the block while in the car.
Maybe do a thorough coolant flush, and examine output for any debris?
Use an infrared heat gun all over the engine and block and find the "hot spot" that makes the water boil in only 15 min?
Is it the same at the rad as the engine?
Triple check the water pump flow to make sure it is not a reverse rotation version. Even if it supposed to be right, does it pump water when spun the right direction?

How about a pic or two?
That might help.

 

On the Big Block engine you need to double check your timing. If your vacuum advance is disconnected or the timing is set to far retarded this can cause the engine to overheat. The timing number you have there sounds low compared to what my engine runs. Pull the thermostat and let the engine break in completely. A new engine is TIGHT and the first few hours of running will make it loosen up a bit and run a bit cooler once the friction is reduced inside the engine.

On my 427 I have 12.25-1 compression and I never use regular antifreeze in my C3. When I use anti freeze it will always be a 70% water 30% antifreeze along with a bottle of Redline WATER WETTER. The water wetter helps the engine transfer the heat from the metal to the coolant inside the engine. I use it in every water cooled vehicle that I own. It WORKS!

Check to be sure the engine has the proper direction for the water pump. There are water pumps made to spin the opposite direction readily available out there. I found that having a working radiator overflow make a huge difference in the cooling system capabilities. Also only use a new radiator cap as they don't hold pressure when bad.

What is the total timing you are using in the engine? I run 38* of total advance in my 1968 BB 427. You might want to read the paper Lars wrote about timing your engine. The timing is really important and I would check that first. The timing curve is equally important to help you make the power available in a 427!

 

Hi Leigh and excellent question!

Eng sat up close to 20yrs after rebuild and first start by me this year. Sat in a garage, pretty sure whole time. Taped up? I hope so, do not know.

My friend bought it in 85, took it apart in 87 and bought all kinds of new parts, and had professionally rebuilt most everything; eng, 4sp, rear end, new bc/cc paint, etc etc, etc. all the while raising 2 daughters and 1 son along with the rest of the usual life's events. Oh and BTW we have plenty of mice, rats, high humidity and dirt dabbers in south Louisiana, ha!

So maybe an engine pull and disassembly is in my future, ugh! Oh and the shop decked the original engine, BTW, ugh, ugh!

Thanks for the response! I might pull the water pump, engine drain plugs and see if I get good water flow first before full disassembly.

 

I would not even know how to go about checking water flow in the block while in the car.
Maybe do a thorough coolant flush, and examine output for any debris?
Use an infrared heat gun all over the engine and block and find the "hot spot" that makes the water boil in only 15 min?
Is it the same at the rad as the engine?
Triple check the water pump flow to make sure it is not a reverse rotation version. Even if it supposed to be right, does it pump water when spun the right direction?

Thanks will do some investigation!!!

 

Thanks for the response Christopher! Great info and will do some more homework.

 

1. Does it have a thermostatic clutch fan as opposed to centrifugal?
2. Use a 180 Stewart thermostat.
3. Set timing to 10 BTDC. This will give you 36 total but at 3,800 rpm. In this case we just want to lower temperature.
4. Connect distributor vacuum advance to manifold vacuum.

 

Thanks and will do, great advice.
-OEM style thermostatic clutch fan installed with the spring and valve (?) in front.
-Will try all.

Also someone told me drilling a couple of 1/8" holes in the thermostat flange (ss surrounding plate) helps eliminates air pockets, good idea?

And flex fan a better way?

 

Hi 20 Mercury,

First off, congrats on your '69!

It seems like you've done all the right stuff. Pics might help us. I have a couple of additional thoughts:

• Lower hose have the spring in it to keep it from collapsing?
• Other than the DeWitt's (nice choice), is the stock shroud there?
• All the foam there around the shroud and support to hood?
• Correct fan blade? (my experience and advice is a no to a flex fan)

Best of luck! Paul

 

Hi Paul!

Thanks, I think this 69 needs to be back on the road again I hope by me maybe!

+lower rad hose spring? forgot will look
+yes stock shroud with seals
+OEM BB bent tip (maybe that is the problem, ha!) fan blade.

Will check out, Thanks! Pics when available.

 

Flex fan defeats the thermostatic clutch, don't use it.
Again a Stewart 180 thermostat, look it up, bigger mouth to not restrict flow and you shouldn't have to drill anything.
As a last resort try a Flowkooler or similar water pump. The impeller design is better.
Assume radiator core is not blocked.

 

Thanks Mel for the helpful responses!

Will followup on your suggestions.

 

Have you tried running without thermostat to see what difference that makes? - I recall my engine with new heads would 'run-on' when switching ignition off. Engine was totally new and running perhaps a little rich at idle and fairly sooty plugs I was fairly certain that carbon build-up on sharp edges of combustion chamber caused it. The engine ran far hotter than expected even with Dewitt rad full shroud, viscous fan etc. but it did seem 'tight'. After some months when perhaps partly ran-in it improved significantly - even though I'm running 36/37 degrees total it has 21/22 degrees initial and seems to like it.
My old 427 ran ran better with 36 total but with 12 degrees initial. I also used the lightest springs and heaviest bob weights so that advance was all-in at 2000rpm.....L36 spec' engine loved it.

 

What is your total timing? Though a great scene in a great movie, the stock timing spec from My Cousin Vinny is not best practice.

Do you have an anti-collapse spring in your lower rad hose? Shouldn't matter at idle, but worth checking.

The high-flow thermostat change is great. I think Milodon made mine, should be the same as Stewart. I drilled a tiny hole to let air out.

I'll resist the urge to suggest an electric fan, as there are as many ways to get that wrong as right, but if the timing fixes don't solve your problem, that would be my next suggestion, for better idle performance.

 

1st post he stated ran without thermostat

 

No to the flex fan. They have blown apart on me before. Stock fan and clutch is your friend.

Grab both hoses when hot. They should not be one cold and one hot. Lack of water flow is what you would cause that. Infra red thermal gun is cheap.

 

Much thanks for all the great advice!!, gotta go do some homework now.

 

Well there's your problem - you're running so far retarded that you're going to be heating that engine up red-hot at that idle timing spec. Fix your timing, timing curve, and vacuum advance.

Lars

 

4 deg for smog.

damn, hopefully no tear down needed.
even at 4 deg burping and coolant blowout shouldn't happen.
especially with no thermo.
hate to say it but, head/gasket
fingers crossed

 

Thanks and will do!

I wonder why the GM spec is 4 BTDC ?