Thermostat troubleshooting 1972/351c

angryman · Mar 7, 2011

The car is a 72 mustang with a 351c.

The temperature gauge was not working or connected to the temp sensor when I got the car. I have replaced the sensor and have not been successful at getting any readings from the gauge. The gauge is showing hot when grounded, but as soon as I hook it up to the sensor I get nothing.

I am trying to figure out if it could be something else. When I run the heater in the car, the air does not get very warm.

Could both of these problems be caused from a thermostat stuck open? I took off the radiator hose and confirmed that there is indeed a thermostat in there. I don't know how to determine if it is stuck open since I have never looked at one before. I sucked up the coolant in the housing above the thermostat with some paper towel and within a couple minutes it had filled up again. Other than that, I can't tell visually that there is anything "open" looking down into it.

I don't have a IR thermometer of any cool gadgets to use to check the engine temp. Should I replace the thermostat or is there something else I can do to check it? Could this be the reason I can't get a temperature reading from my gauge?

2+2GT · Mar 7, 2011

The 351C requires a unique thermostat that seals the bypass when it opens. If there's a generic 'stat in there, you have no hope, it will overheat. Even with the right 'stat, if the bypass restrictor ring is missing, it will overheat. As for your reading, if grounding the lead causes a "hot" reading on the gauge, but it does nothing when connected, either it's the wrong sensor, or the sensor is dead. Find a used one, all the currently available replacements suck, and read way too high.

horseballz · Mar 7, 2011

+1 on what 2+2GT said AND/OR did you happen to use teflon tape (or similar) on the threads of the sender? I so, you might not be getting a good electrical connection form the sender to ground. No ground, no worky gauge.
HTH,
Gene

pabear89 · Mar 7, 2011

Angryman, after reading both of your posts My guess is that your missing the restrictor plate and have the wrong or stuck stat.

Without the plate and proper stat for the C engine, your temp guage will not show what the temp is untill it starts running hot.

Look at the pic 2+2Gt supplied, the sending unit is in the upper corner and
without the plate and correct stat that blocks it off when open the coolant mostly goes back thru the block and not out to rad.
This will show as low temp reading till the coolant starts to do a heat transfer with the cooler coolant from the rad. By then the block is hot but not showing at the guage.

angryman · Mar 7, 2011

Thanks for the replies. I took the housing off and the thermostat out. Its a Gates 33129 which is a correct replacement for the 351c. I also see the restrictor plate inside the tank. Now that I know what I'm looking at I was able to determine that the thermostat was not stuck open either and was in the closed position which doesn't help firgure out my problem.

Back to square one then. Ill get another sensor but this time Ill order a motorcraft replacement and see if that helps. Ill also use some sealant paste instead of the teflon tape.

Ill also have to check the heater core now to see if that is the culprit for my lack of heat from the vents.

Ill remind myself that this is fun and I am learning before I go back out there.

pabear89 · Mar 7, 2011

Gates 33129....

Try using Flowcooler/robertshaw 333-180 stat. they are clev eng specific.

The gates brand shows W and will not close the bypass correctly.

Been there tried to go with the cheaper one and it didn't work.

angryman · Mar 7, 2011

I'll try that, thanks.
I am a little confused though about the restrictor plate and my eyes are starting to go crosseyed from searching the internet. I have a plate that looks like it is cast integral inside the block. The diameter off the hole is about the width of my thumb. Is there supposed to be a removable restrictor piece on top of this, or is that it? I have read where the integral unit was only on the 351M and that the 351C was supposed to have a removable plate.

pabear89 · Mar 7, 2011

The 351C has a removable brass plate with an approx 7/8 hole.
The 351M is cast into the block.
If the plate has been installed for some time they look cast.

While the Shaw stat is expensive, they work perfectly.
Summit racing sells them if you can't find one local.

Platonic Solid · Mar 15, 2011

I went through the same issue 2 years ago. Here's the thread I started then: http://www.stangnet.com/mustang-forums/783075-thermostat-necessary.html

Thermostat troubleshooting 1972/351c

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