Riddle me this! Turn on the heater, the motor overheats!

Sensor close to the return line from the heater core? Maybe its getting the hot water from there while the rest of the water in the system is mixing with cooled water from the radiator.

Posted using the Full Custom H.A.M.B. App!

 

"The heater is higher than the motor"...That's probably the problem. Vacuum the system to get the air out. I had a GM midsize that did the same thing, the hoses went way down, then back up, and the air couldn't get out. Most GM shops can do the job.

 

The heater hose at the manifold should have the heater control valve. You can purge the air by putting a radiator pressure tool in place. Pump to about ten pounds and walk away. The heater control will be needed come summer. Did you hook the Ac ground to the battery as VA recommends. I've heard of strange things happening if that ground is not real good. I've got several new manual heater control valves and the pressure pump. I'm in Hurst Pm if you need to use it.

 

That was my thought when I first read it. Is the motor running like a motor that is at 240 to 250 degrees?

 

This actually happened to me. I could turn on something that drew a lot amps, like the headlights and my temperature would start to climb.....on a cool night. Turns out I did not have a good ground on my gauges.

 

R Pope what your saying I think is Crazy " the Heater is High'er than the Motor"
That would be all 49-51 Merc Cars the Heater is High'er than the Motor
so they should run Hot when the Heat is on.!
I have a 50 Merc with a SBC & I have No Problem with the Heat on

Just my 3.5 cents

 

I fall in with the electrical issue crowd

 

Yeah, if your engine is running this hot you'll know.

 

First of all, you'd be puking out coolant and having to constantly add more.

 

I have had the same problems with my 51 chevy with Vortec heads I ran a by-pass thinking this would cure the problem. it still did't. I did figure out I mounted the heater core higher than the engine. This made the heater the highest point of the cooling system. I jacked up the front of the car and ran it with the heater on high and the cap off for a few min. bringing the RPM up and down to burp the system. After that it has seem to be OK. You may want to give that a try.

 

http://www.amazon.com/UView-550000-Airlift-Cooling-Checker/dp/B0002SRH5G

best way to fill cooling systems.

 

IHC Loadstar trucks are famous for needing the heater core bled to get water flow because the heater us above the rad cap. Some guys even put in electric pumps to circulate the water. A trick is to drill a 1/8" hole in the heater connection, so that it is covered by the hose. Then with the engine pressured up, work the hose back until the hole is exposed and the air can get out. The rad pressure will blow the air out easily that way.

 

You should read the article in Jan 14's Hot Rod Mag. I had a similar occurrence with my 39 coupe/350 sbc. Fixed/changed the water neck and installed a 180 thermo and drained and installed new antifreeze. Car runs 180 now with no elec fan on.
I think maybe I met you at a GP funeral viewing a couple months back.