Car: 70 2dr Malibu with stock 350 except for cam(unknown cam but lopey), performer intake, edelbrock 650 cfm carb and Headman headers. Due to cam had to add supplemental vacuum pump for power brakes because at idle cam won’t pull enough vacuum. Has TH350 trans.

Symptoms currently runs warm and even warmer going down the highway but open the heater control valve and temp comes down even without turning the fan blower motor on although when I do it drops the temp faster.
I know this is a lot of info but trying to be as accurate as possible to try and get the needed help to figure this out. If you’re willing please read on.

Things that have been done:

Intake resealed by professional prior to staring motor after sitting for 3+ years.
Replaced brass radiator with p30 van aluminum radiator. Again done prior to startup. Ran for two weeks, in town mostly with some highway and stayed at 190* according to gauge(AutoGauge, they were in it when car was purchased). Mechanical sender in passenger head. At the time it had a 160* thermostat in it. Alum rad had pinhole leak on top. Also radiator appears not to be flowing well, going to get another new one. This all started AFTER I went on the highway and floored it several times to try and get the kick down to work

Replaced the 160* thermostat that was in there with the correct 180* stat. Temp still climbs down the highway or in town if the heater control valve isn’t left open. Now instead of 190* stays “cool” at 210* at idle, or in town WITH heater control valve open. Close valve and temp will climb but I won’t let it go over 230* before cooling it down opening the heater control valve and turning the blower motor on. Seem to be able to run down the highway WITH heater going full blast and temp stays around 230*.

Replaced water pump, still does the same thing and it “appears” like the radiator isn’t flowing like it should.

Put old brass radiator back in and still does the same thing although it does leak around the filler neck(reason went with new aluminum radiator). Still exhibits the same symptoms.

Going to put another new p30 van aluminum radiator in(one that doesn’t leak) and try that. Carb it not running lean and timing is okay but will check again although not sure where to set it with the unknown cam. Oil is clean as is the antifreeze(50/50 mix).

What kind of fan is on the engine? If it's a clutch fan did you replace the clutch? Is the fan shroud in place? The thermostat isn't installed upside down is it?

Stock clutch fan with shroud. Fan is functioning properly. Thermostat is installed properly, spring/sensor down in intake. Thanks Pete

At highway speeds you don't need a fan because air should be going though radiator fast enough.I would take thermostat out and see if that helps flow. When you use your heater it's doing what your radiator should be doing.You might have clog in radiator hose or water pump inlet.

You should always check fan when having problems at highway speeds though, because the big 7 blade fans at highway speed if locked up (as they tend to do when they fail, can hinder and restrick airflow thru the radiator. I had to find this out the hard way.What's your rpm's when the temp starts to climb and how fast are you going at that rpm? You could be turning too much rpm's for the air flow thru your radiator. What rear end gears? How's your mixture at part throttle? Could be runing lean. How about timing? Is your vacuum advance working properly? Are you running too much advance for your compression? There are alot of things to check other than just the radiator and thermostat.

You should always check fan when having problems at highway speeds though, because the big 7 blade fans at highway speed if locked up (as they tend to do when they fail, can hinder and restrick airflow thru the radiator. I had to find this out the hard way.

Fan is functioning properly

What's your rpm's when the temp starts to climb and how fast are you going at that rpm?

Varies 1500-3000 dosen't make a difference

You could be turning too much rpm's for the air flow thru your radiator. What rear end gears?

Stock 273's I believe

How's your mixture at part throttle? Could be runing lean.

Mixture's good, not lean


How about timing?

14* BTDC intial


36* total Car ahd big cam(in it when we bought it)


Is your vacuum advance working properly?

Appears to be

Are you running too much advance for your compression?

Not sure how to check that, as far as I know stock 9 to 1 CR


There are alot of things to check other than just the radiator and thermostat.


Well after a lot of changing things and multiple heat gun tests I came to the conclusion that the best place NOT to put a mech temp sensor is in the passenger side head, HEAT SOAK.

Move the mech temp sensor to intake by thermostat housing stays a steady 190-200. Pete