Diagnosing Erratic Temperature Swings on the 1967 - 1973 Cougar or Mustang

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Erratic temperature swings?

Most likely you have air trapped in your cooling system. Air rises to the top of the cooling system and gets trapped, causing the cooling system to vapor lock. When the radiator is made the higher point in the system, the air will escape into the radiator and it will be vented out through the radiator cap and the overflow system.

There are a couple of ways to resolve this and free the air.

1. Free trapped air via the vent plug.

2. Fill the system as normal with your antifreeze mixture. Use a 50-50 mixture of antifreeze and distilled water. Auto parts stores will sell in-expensive to determine the concentration of antifreeze in your system.

3. Find the small threaded plug with a hex fitting on the thermostat housing where the upper radiator hose attaches to the engine. Loosen the fitting to the point where air starts to escape and inspect the thread sealant and re-apply as needed. Be sure to run your heater to allow water into the heater core.

4. Add antifreeze to replace the air being expelled through the plug.

5. As fluid starts to seep out of the plug tighten it up, top off the overflow tank, and you're done. If you continue to get erratic temperature readings, or fail to get heat from the heating vents, re-open the screw and let any residual air to escape.

6. To vent a system in this method raise the car such that the radiator is higher than the thermostat housing. You can do this with a jack and jack-stands. NEVER WORK ON YOUR CAR WHERE THE JACK IS THE ONLY THING HOLDING IT UP, THROW SOME STANDS UNDER THAT UNIT! You can also use a sturdy set of ramps or a nice steep hill where you can park with the nose of the car pointing up the hill.

7. You'll need to let the car warm up so that the thermostat opens for the system to vent in this method. AVOID BURNS AND PROTECT YOUR EYES, NEVER OPEN THE SYSTEM WHEN HOT! Let the car warm up from cold with the radiator cap removed. You may get some spillage while the coolant expands and the air bubbles out. This coolant will be hot.

8. Allow the engine to run, with the heater on (fan can be off or on low) until the thermostat opens and all the air is allowed to purge. Once the thermostat opens you will see the coolant level inside the radiator bubble and drop. Continue to add antifreeze to maintain fluid level. Again be careful as the coolant and any steam released by the system will be hot. Once the upper radiator hose becomes hot to the touch and no further air issues from the system carefully replace the radiator cap and ensure the overflow bottle is filled to the Max line.

9. Continue to allow the car to run to allow the temperature to stabilize. If you don't have a temperature gauge allow the car to run until the fan cycles on and off at least once. During this time ensure there are no leaks from the system and that the upper radiator hose gets hot to the touch (especially close to the radiator). If not then allow the system to cool, and repeat the steps above to purge any remaining air.

*NOTE* If you are not comfortable leaving the radiator cap off during warm-up then you can accomplish the same thing by leaving the radiator cap on and allowing the car to cool down after step four and then repeat steps one through four again, making sure the overflow bottle is maintained full. The heating and cooling cycle will push the air out through the overflow bottle and then suck coolant in to replace the air when the engine cools. The car must remain inclined for the whole procedure.

When should I check my cooling system?

The best time to inspect your heater and radiator hoses is cooler weather. Less obvious signs of decay can be seen and felt by grabbing hold of a cold radiator or heater hose and giving it a good squeeze. Brittle or cracking material, a spongy feel, or a hose sticking to the inside of itself are bad signs.

Why is my car overheating?

There is no one answer for this question but here is a list of places to start to isolate the problem.

1. Before you do anything else, tune up the car. Many overheating cars are out of tune, running lean or with retarded timing. A lean fuel mixture will overheat your car. If your engine runs lean you can chase your tail looking for problems in the cooling system and never figure it out. The easy way to do this is richen your jetting a couple of steps. If the overheating is better, you're on the right track.

2. There is a lot of misinformation about ignition timing and cooling. Retarded timing contributes to overheating. Advanced timing helps cooling. Advance your initial timing a few degrees and see if it helps the car run cooler. However, if you advance to much you risk detonation and that too will cause you to overheat. If you start to detonate back off the timing. Overheating cars should always run vacuum advance. Vacuum advance helps cooling.

3. Radiators: Your radiator is the primary means to bring the engine temperature back to the optimal temperature. They can be dirty, clogged, poorly designed, too small for the engine, fin density too great for the low rpm airflow...you get the picture.

4. Airflow. Inadequate airflow can cause overheating. You have to get the air through your radiator and out of the engine compartment. Obstructions to that airflow can cause a cushion of hot air to build around your block and engine compartment. A properly fit fan shroud sealed to the radiator helps to channel the air through your radiator.

5. Inadequate coolant flow. If your overheating at idle, stop and go traffic, on grades or towing you might benefit from higher flow rates through the radiator. Hi Flow water pumps and hi flow thermostats allow more water to pass through the radiator where it has a chance to cool.

6. Modified gear ratios: Generally speaking lower ratios give slower acceleration, higher top speed and less braking power. Higher ratios give more acceleration, less top speed and more braking power. Changing gear ratios may generate more heat in the engine and potentially cause overheating.

7. Coolant composition. A 50/50 mixture does not help cooling it raises the boiling point. Run 100% distilled water with water pump lubricant or distilled water with about 15-20 % antifreeze. Both cool better than 50/50 and still lubricate the water pump to prevent corrosion in system. When the weather cools return to 50/50.

8. Cooling the transmission is added work for a poorly constructed radiator. Do not obstruct airflow or heat the air by placing the cooler in front of the radiator where it will overheat the cooling air. Transmission and engine oil coolers constructed within the radiator can also tax your cooling system. Using external coolers may help.

9. The elusive manifold vacuum leak. Trouble at idle may point to a manifold vacuum leak. If you find you're too fast an idle speed, rough idle or stalling, misfiring on acceleration or adjustments to your carburetor seem to have limited effect than you may have a vacuum leak causing overheating.

10. Use a better grade of gas. If you are not running premium fuel and experience overheating a higher grade may help. If there is no improvement, try advancing your timing a few degrees. A little extra octane will allow you a little extra timing without getting into detonation. Earlier engines were designed for better gas than is sold today.

11. Head gasket leaks from the cylinder to the water jacket are a definite cause of overheating. That's bad news and a whole lot of work.

12. A stuck thermostat. Many thermostats are designed to fail in the open position but thermostats have also been known to stick partially open and impede coolant flow.

13. A slipping water pump belts can slow flow rates from the water pump and impede coolant flow resulting in overheating.

14. A fan blade with insufficient blade count or incorrect pitch may not draw enough air resulting in overheating.

15. Headers without thermal coating can cause overheating.

16. A stretched timing chain or belt can cause overheating.

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