This article supersedes TSB
The majority of repeat heater core leaks are due to high flow rate or use of poor quality coolant. However, electrolysis should also be checked, especially when repeat repairs have occurred.
If the heater core is leaking, review the location of the leakage and check the condition of the coolant.
1.Review the location of the leakage and check the condition of the coolant:
1. If leaks are found on the inlet (or outlet) tubes entering /exiting the heater core, it is most likely due to due to high flow rate - replace the heater core and install a restrictor in the heater hose closest to the engine block, reference Workshop Manual, Section 412.
2. If leaks are found in the body of the heater core itself, and does not appear to be the result of physical damage like contact or puncture, check the coolant for possible electrolysis.
Testing For Electrolysis
Check for voltage in the cooling system by touching the negative contact of a voltmeter to the battery ground or a known good ground and suspend the positive lead in the coolant, making sure it is in contact with the coolant but not touching any metal part of the radiator or cooling system. Both AC and DC voltages must be checked. Vehicles normally have DC voltages; however, a faulty engine block heater or faulty diode in the alternator can produce AC voltages. It is understood that coolant is lost due to heater core failure but try to obtain a voltage reading on the old coolant in the engine block before addition to or replacement of. To keep more coolant from exiting the heater core clamp off heater core lines and measure coolant in the engine block. Try not to dilute the original coolant with new coolant during testing if possible.
1. Determine whether coolant condition is acceptable.
1. Remove both cables from the battery and ensure they do not contact each other or the vehicle.
2. Touch negative lead of voltmeter to engine ground and positive lead in the coolant.
NOTE:POSITIVE TEST PROBE IS IN THE COOLANT FOR TESTING.
3. Check the voltage in the cooling system. If less than or equal to 0.4 volts (V) OK, reconnect battery cables and proceed to Step 2.
4. If greater than 0.4 V, flush cooling system thoroughly.
5. Recheck voltage less than or equal to 0.4 V.
6. Reconnect battery cables.
7. Refill the system with appropriate Motorcraft® engine coolant.
2. Check for loose or missing grounds at static conditions.
1. Turn off all accessories. Turn ignition on but do not start engine.
2. Test with ground probe to battery ground, engine ground, and vehicle ground sequentially.
3. Voltage less than or equal to 0.4 V on all grounds OK.
4. Any one greater than 0.4 V, check and clean ground cable connections.
5. Check accessories without using the on off switch on the vehicle instrument panel, use a jumper wire to ground.
6. Plug in engine block heater, if equipped, and test.
7. Recheck voltage less than or equal to 0.4 V.
8. Unplug engine block heater, if equipped.
3. Check for loose, missing, or inadequate grounds.
1. Test with ground probe to battery ground, engine ground, and vehicle ground sequentially.
2. Crank engine but do not start.
3. Monitor voltage while cranking. less than or equal to 0.4 V OK
4. If greater than 0.4 V, ground or repair starter.
5. Start engine and run at about 2000 rpm.
6. Turn on all accessories including those customer only uses occasionally such as CB radio, cell phone, etc.
7. Test with ground probe to battery ground, engine ground, and vehicle ground sequentially.
8. Voltage less than or equal to 0.4 V OK
9. If greater than 0.4 V, turn off one item at a time until V drops to less than or equal to 0.4 V. Repair ground to the accessory just identified.
10. Recheck voltage less than or equal to 0.4 V
11. Turn the DVOM to AC volts.
12. Check for ANY AC voltage greater than 0.4.
13. If any AC voltage is present then try turning off each accessory one at a time including blower motor and any fan motors.
14. If AC voltage is still present then shut engine off and remove B+ from the alternator and tape it up then retest.
15. If voltage drop is gradual to less than or equal to 0.4 V, the ground straps may simply be overloaded by added accessories. Test by using heavy gauge jumper to ground. If indicated, install heavier gauge ground strap(s) and recheck.
NOTE:If vehicle is equipped with electric cooling fans, be sure they cycle during this testing and monitor voltage when they are on and when off.
CAUTION: DO NOT GROUND HEATER CORE. IF THE HEATER CORE IS GROUNDED, YOU HAVE PROVIDED THE ELECTROLOSIS A PATH THROUGH THE HEATER CORE. THIS WOULD CAUSE THE HEATER CORE TO BECOME AN ANODE OR RECEIVER AND IT WOULD PROMOTE THE ELECTROLOSIS, OR ANY STRAY VOLTAGE TO USE THE COOLANT AS THE GROUND PATH.
4. Refill the engine cooling system, reference Workshop Manual, Section 303-03.
NOTE: IF THE HEAT OUTPUT IS INSUFFICIENT, OR THE ENGINE DOES NOT REACH