Experience Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.
Organizations Associate member SAE
EAA member
Publications Import Car magazine
Education/Credentials ASE Master Auto with L-1 certification up to 2000
Question QUESTION: I have talked with you about my spit, I had an overheating episode. How is the best way to install a thermo switch, preferably with a digital monitor, so that the fan will continue to run when the car is off to drop the temp to an acceptable level? I want to prevent a temp spike after shut off. My current electric fan is on a manual switch.
thanks,
Mac
ANSWER: Hi Mac,
You can go to all that trouble if you like but, the temp spike you see on your gauge is coolant temp not engine temp.
Heat is energy and thus it is something and cold is just the absents of heat. As the engine runs some of the heat generated by combustion is absorbed into the head and block and the coolant is absorbing some of that heat and carrying it to the radiator to be transferred to the incoming air. When you turn an engine off there is no more heat being generated, so the hottest part of the engine (the head) starts dissipating the present heat it has to the coolant and the rest of the block. The coolant is colder than the engine so heat continues to transfer from the block and head to the coolant and since the coolant is now stationary it can continue to absorb heat. As it absorbs more heat the gauge reads that heat as a temp spike.
So, the engine is actually cooler as the temp gauge reads hotter. The only time that this becomes a problem is when something in the engine is wrong and it is on the edge of overheating. Then when the engine is shut off, the temp of the coolant raises so high as to cause the radiator to force open the pressure cap and overflow.
Many brands of cars have the fans run after the engine is turned off, not just to cool this spike down but to get engine room to cool down so as not to over heat the fuel in a carburetor and lines. However this then adds an element of possible failure to the system and what happens is a thermostat that controls the fans or a relay can stick etc. Now the car comes in to me on the hook with a dead battery. The fans continued to run until the battery was dead since the driver leaves the car with the fans running as he/she always does.
If you want to have the fans run after the key is off all you need to do is look on your wiring diagram and change the power to the relay to a direct battery power instead of from the ignition circuit. Then hope that the fan thermostat always works.
Howard
---------- FOLLOW-UP ----------
QUESTION: well, you caused me to reconsider. My fan is directly wired, with inline fuse. I also recently read an article online about a thermo gone bad-- dead engine... My spit has been running a little hot. I am getting a new temp sender and cable ( I doth question ye old rusty accuracy), I would like to install a digital thermometer. Any ideas?
(regarding the overheating episode)
I did a dry compression test.. 135 PSI in all four chambers on the dot, so I didn't do the wet test. The rushing air sound from the dipstick is actually a pretty high amount of vacuum. What amount of vacuum is normal? Do I need to install a smaller diameter (1/4 or 1/8 inch) vacuum tube line or nipple at the intake, or the valve cover. PCV valve? Maybe I could send you a picture of the set up?
thanks,
Mac
Answer Mac, I have no experience with digital thermometers so I can't comment on their accuracy. The fact that they are digital does not indicate they are accurate. Just that they give you a digital reading instead of an analog reading.
You lost me on why you didn't do the Wet compression test. The unusual Dry test of 135 on every cylinder is not a good reason not to do a wet test, in fact if it were mine, that would make me want to be sure to do the wet test.
Your car should have two large hoses connected to the carburetors in front of the throttle plates and connected to the valve cover and to the top of the charcoal canister. At idle this should burn off any excess oil vapors coming from the valve cover but should not cause a vacuum in the crankcase.
Do this simple test, let the car idle and remove the oil filler cap and tell me if you get an RPM change.
