Flathead V8 Engine Overheating Troubleshooting

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These are very general trouble-shooting ideas gathered from various sources and the author's own personal experience. These may not include all specific problems and may not include all possible solutions. This is intended to be a generic trouble shooting guide, but some areas may have references to specific Ford Motor Company products. This guide should only be used to help brain-storm ideas for potential problems. If you are not trained in auto repair, or do not fully understand the issues of service and safety, please consider hiring someone else to handle the repair of your vehicle. A professional mechanic may also be trained for other potential mechanical repairs that your vehicle may need, that are not addressed here.

In no particular order, things to look for and correct when your flathead overheats.

1. Be sure that thermostats are installed. The 1937-48 motors have thermostats installed in the upper radiator hose, just above the water outlets on the heads where the hoses are clamped. The 1949-53 motors have t-stats installed in the front of the heads, inside the radiator hose outlet castings.

2. Be sure that the thermostats are not installed upside down. The bimetal strip or wax plug should be facing the heads.

3. On 1948 and earlier engines be sure that the hose clamps keeping the thermostats in place and the hoses attached  to the heads are not too tight, preventing the thermostats from opening and closing.

4. Be sure that the radiator cap is the correct one for the system you have. Pressurized or non pressurized. If itís pressurized, replace it with a new cap of the correct rating.

5. Be sure that the water pump belts are tensioned correctly. Too loose, and the pumps will slip and not draw enough coolant around. Too tight and you prematurely wear out the bushings/bearings in the pumps.

6. Be sure that the radiator is clean. Shine a light through the fins. If you can't see the light, clean the fins by brushing the front lightly with a soft bristle brush to remove the exterior debris (mostly bug remains). Then, use the spray nozzle on your garden hose to  flush the rest of the debris out of the fins from behind.

7. Be sure that the radiator hoses are not kinked, preventing coolant flow. Check the insides of the hoses for delaminating flaps blocking the flow.

8. Back flush the old coolant until you get clean water. Note: if the block has years of accumulated corrosion and crud in the water jackets, a simple flush may not remove this material. The best method (and the most expensive option) is to remove the engine and tear it down to the bare block casting in order to have it acid dipped. This process will remove everything but the good cast iron. Once the block is CLEAN, it can be reassembled and then fresh coolant (50/50 mix) added to the system.

9. Be sure that the water pumps are not worn out (loose shaft/pulley, leaking seals, corroded impellers). Replace as needed.

10. Check to see if the radiator is plugged with years of rust and scale. Removing the upper and lower hoses can provide a simple visual inspection. If the radiator is suspect for these or any other reasons, have a professional radiator shop check it properly (including a pressure test AND a flow test). Have it repaired or replaced as needed.

11. Be sure that the ignition timing is correct. Timing that is too far advanced will cause the engine to overheat. Adjust the timing to within factory specifications. Pre 1949 distributors should be removed and adjusted on a distributor machine.

12. If the engine is coated with years of grease, oil, and dirt, clean the block by scraping and then pressure washing.

13. Be sure the brakes are not set too tight as dragging brakes make the engine have to work harder to move the car.

14. Be sure the tires are properly inflated. Low pressure creates more work for the engine and wastes fuel as well.

15. If the engine temp gauge still shows overheating, but visually it doesn't appear to be, the gauge could be reading wrong. Or, the temperature sender(s) could be defective. The sender can be tested in hot water with a volt/ohmeter to determine if it is functioning correctly. An infra-red thermometer can also help locate hot spots in the motor and indicate actual temps.

16. Use a coolant test strip (available at most auto parts stores) to check the coolant for evidence of exhaust by-products. This could indicate a leaking head gasket, or a possible crack in the block.  

17. A blocked exhaust system can definitely cause overheating. If the vehicle has been stored for some time, rodents may have climbed into the exhaust pipe and built a nest anywhere from the interior of the engine to the exhaust outlet. If you suspect this to be the case, and you can not feel normal pressure at the pipe outlet, disconnect the pipe from the exhaust manifold and try to blow compressed air though it. You may have to completely disassemble the exhaust system to locate and remove the problem.

Sometimes, over time, rust can gradually block the louvers inside the muffler baffles, resulting in serious blockage. Replacement of the muffler is necessary. Also, if the vehicle has a temperature controlled exhaust "warm up" damper at the manifold, this may be corroded to the point that it remains in a closed or partially closed condition. Remove and repair the valve if needed.

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