Since I replaced my temp sensor many months ago in a different thread (it's a common problem finding a correct replacement) I also replaced my temp gauge while putting my interior back together this summer.

The instructions for the temp gauge said that it's more accurate than the old one and that I don't need the resistor on the back anymore. Well these days on my sb 350/270 my gauge is reading about 215 degrees or close to it. For my small, stock motor with a 26" DeWitts alum radiator, that seems awefully high. I don't have a heat gun to check the temp. So either the gauge is right and I have some other cooling issue, or the resistor is needed.

Yesterday after a couple miles of driving, I pulled into the driveway and put my hand on the top rad hose and gave it a squeeze to see how much pressure there was at idle. I was surprised that I could almost block off the hose by squeezing and that I could barely feel any water going through it. Granted, it was at idle, but I'm looking at that large hose and just thought there would be more water going through it. I probably should have revved up the engine and checked (with gloves as it's damn hot). I assume that there will be a lot more water going through it when the RPMs go up, making it tough to block off with just my hand.

Over a year ago I replaced the water pump, thermostat, and radiator in one fell swoop hoping to avoid overheating issues.

 

with an engine at 215 you should have a rock hard radiator hose! Have you checked the fluid level on a cold engine? Have you pressure tested your radiator cap? What type of thermostat did you put in standard or wide mouth and what temperature. Are you running a fan clutch and have you checked it to make sure it properly engages when hot? Do you know what your initial timing is set for?

 

I'd put my money on the rad cap. It isn't about water flow at idle, it's about pressure under temperature, if the engine is at true running temp (not just a 5 minute idle) then the hose should have roughly the same pressure on it as the pressure marked on the cap, it may be 16lbs, if the cap is working properly then you should be able to see water levels change on your overflow system, high when hot and lower when cold. For the price of a new rad cap I would simply replace it, if the pressure is too low the temp will be higher.

Dennis.

 

I had the same issue on my Street car - no pressure on top rad hose. It was a bad thermostat. That hose should build pressure pretty quick when the motor is started cold, then as it warms up the thermostat begins to open and the hoses get more pressurized.

I'd venture to guess that either your thermostat or water pump is bad......

Start the motor cold with the cap off and observe the coolant level and flow as the motor warms up. As the thermostat opens you should see good flow of coolant thru the radiator tubes. If you don't then the thermostat is not opening Or your water pump is bad.

 

Good morning all C3ers......

Informative posts....I have one ?.....My 78sa is pretty much all stock(L48) My rad cap is stamped with the #16.......I have never had an overheat problem...yet that is..may I asume that the #16 cap is appropriate???? I always thought that the stamped numbers were even....

mk's78

 

That's pretty much my assessment, either a bad thermostat or pump, both of which were just over 1 year old and < 700 miles. I've bought a replacement thermostat to put in today.

I know there's pressure in the system and the hose should be much firmer than it is. That's because before I looked at this problem I was putting in a block off valve for the water heater. I didn't wait for the engine to get cool enough and when I pulled the hose off the top of the intake, coolant squirted out on me and the car. Yes stupid mistake. Thankfully the water wasn't at full temperature.

I can't remember if I put a dirt cheap thermostat in there or not. My replacement is a hi-flow, 160 degree Mr Gasket #4363.

 

Pressure test the cap

 

So I replaced the thermostat yesterday and tested it on a 20 minute drive tonight. Upon first start up, the gauge slowly creeped past the 210 mark and then fell to just past the hash mark between the 100 and 210 markings, about 170 degrees? Once the thermostat opened, the gauge never got close to the 210 mark again. Looks like the problem is solved.

Now the thermostat I had in there was not a hi-flow and it was a 180 degree version. I tested it in boiling water and it opened fine. So either there was air in my system and just replacing the stat and making sure the coolant was topped off seemed to do the trick or maybe the stat spring was too weak to open up when it got under pressure?

When I got home after the drive, the upper hose had nice pressure on it like I was expecting.

Anyway, I'm a happy camper.