I have a 67 SS396 and it runs hot. I can turn on the A/C and let it idle for 30 minutes and the temperature needle is pegged right in the middle of the word TEMP on the gauge. As soon as I start to drive the car under 50 MPH the needle rises to the top of the word TEMP. This happens with or without the A/C on.

The faster I drive over 50 MPH the hotter it gets until the needle is halfway between the top of the word TEMP and the "H" on the gauge. As soon as I slow down to under 50 the car cools off and the needle returns to the top of the word TEMP.

Here are the facts:
67 396 with 325 HP - bored .030 over. Engine was professionally rebuilt
A/C - Factory
Radiator is aobut 2 3/4" wide. I am not sure if it is a 3 or 4 core radiator. It was professionally disassembled and cleaned.

Chrome valve covers and original cast iron intake.

New fan clutch.

New water pump.

This drives me crazy and I would appreciate any advice you can give.

Tom

Make sure your radiator cap is holding 15 psi, the cap can be tested. Check the vaccumn advance to be sure it is working. Check timing to be sure is is correct and not too advanced.
4 row rad is 3" although I have had some 3 row converted to 4 row.

Tom
there are a few things that can make her run hot ...is the motor fresh and tight ,as a new rebuild ...not broke in yet
look in the fill cap .. you may have a 3 row rad ... 3 rows of half inch cooling tubes ...a 4 row will be 4 rows of 3/8 tubes .
I would recomend you hit a rad shop and have them shoot the stat housing with a infa red gun to get a true reading ... dont trust the gage ...It could be causing you worry over nothing .
200 degrees I wouldnt sweat. 220 I would worry .

If you need a up grade on the rad .. team members get a 10% discount on product .
see the signature line
Don

Don, thanks for giving me some things to check.

Tom

I have a similiar problem, keep us posted.

I have a 69 w/ a 502/502, griffin radiator, MarkVII fanand A/c..

When I drive the interstate, around 75, it creaps up, when I let off the gas, to pull off, it drops. That indicates the Rad is working. I suspect that my lower hose may collaspe. There is a spring in it , but there is a 8-10" gap,
%$#@$%!!!!!!!

Charles

Do you have a fan shroud on the car, and how close in the fan to the radiator ? Suprisingly a fan too close without a fan shroud can cause a circular flow through the radiator so you end up pushing hot air through it. This condition is very vehicle speed sensitive.

I agree with Don. The old gages were not necessarily very accurate. When my Autometer gage was showing 230°F( sensor in the thermostat housing), the factory gage was reading about 2/3 of the way up, nowhere near the red zone (sensor in the head).

Charles, do you have a converter with a high stall?, that can cause it. What you descibe is also the symtoms of a radiator that is too small/not efficient enough (ask me how I know).

Another thing to check is your initial timing,if too retartded will cause motor to run hotter too.

Most aftermarket non stock gm cams need approx 16-18 deg btd initial advnace so crank up the initial timing if your running a perf cam.

Also,are you running a vac adv,if not timing too retarded at cruise/part throttle can result in higher temps too.

Temp sending unit in head esp when using a mech gauge with the metal sender line that can get heat soaked form the exhaust manifold drives temps up too.


Scott

Thanks, Scott.

Tom

a 4 row will be 4 rows of 3/8 tubes

It will be 3/8 tubes if it's a HE type core. Standard cores will be 1/2" tubes.

Tom
I have the same car as you and till last month dealt with the overheating issue. Last year I put a new 4 row radiator in and it still overheated. New waterpump no help. Bought a aftermarket schroud from OPGI and replaced the stock 4 blade fan with a 18" flex-a-lite 6 blade with the fan half in and out of the schroud. The car now runs 180-190 in southern california weather. Thats what worked for me.

I do have a fan shroud. About 1/3 of the blade is outside the shroud and 2/3 is inside.

Tom

I tried putting my finger inside the radiator but could not feel any rows (tubes). All I could feel were fins that ran from the front to the back of the radiatior and were spaced about an inch apart.

Tom

The car has a stock cam and I do have a vacuum advance connected. I believe the intial timing is around 14 degrees.

Tom

I tried putting my finger inside the radiator but could not feel any rows (tubes). All I could feel were fins that ran from the front to the back of the radiatior and were spaced about an inch apart.

Tom
with some water drained you should be able to look in the fill cap hole and see the tubes to count them ...and see the size 1/2 inch 3 row or 3/8
4 row

Even though the water pump is new, you might want to check to see if the impeller in still "locked" to the pump shaft. Some impellers are a press-fit on the shaft and can come loose. I have seen this several times in recent years, even on new pumps, and it causes exactly what you are decscribing. It depends on how loose the impeller is as to how hot, and how soon the engine over heats.

FWIW, most applications that run hot at speed or cruising either are running too lean...or have a vacuum leak causing the same. The faster you go, the hotter it gets.

But, inadequate radiator capacity and poor timing can do the same.

Also, good advice...the factory gauges are NOT accurate! Do your homework here.

ps...I really doubt that your lower hose is collapsing if the thermostat is wide open. This sometimes happen, but rarely.