If you use a spark plug with the wrong heat range, can it cause the engine to overheat? I'm restoring a 67 GT1000 - it was running when I got her a few months ago. Did the basic tune up and had to guess and cross reference the best I could for the plugs. Car does not miss and runs smooth and she only starting running hot since I changed the plugs. Both water and oil are high. Water pump and thermostat are new also. Any plug experts out there?
I've never come across a reference linking spark plug heat range to general average engine temperature (but, regardless, it's probably not a widely-studied subject because you can't be way off in the heat range for other limiting reasons). When you say "she only starting running hot since I changed the plugs" do you mean that even at a steady light cruise on a nice day it overheats? If so, you've got to confirm/deny ignition advance behavior and for exhaust gas presence in the coolant as the first two checks (assuming it's well-bled, and the radiator is "hot" when the engine is "hot"). Who is the manufacturer of your "GT1000"? The NGK part look-up for spark plugs is remarkably extensive -- is your make not listed there?
Using the wrong spark plug heat range can cause damage to the engine but in my 33 years of automotive engine experience, engine overheating is not one of them. A hot spark plug transfers less heat from the spark plug core to the cylinder head and, therefore, the coolant, causing the spark plug core to run hotter. This can lead to pre-ignition (fuel mixture ignition due to a hot spot in advance of ignition due to spark) which can punch a hole in the piston and cause catastrophic engine failure. A cold spark plug transfers more heat from the spark plug core to the cylinder head and coolant, causing the spark plut core to run colder. This can lead to plug fouling and subsequent misfire, but rarely causes any serious damage. It is likely that any cooling problem that you have is unrelated to the spark plugs, but don't underestimate the importance of using the right heat range plug. I think most spark plug manufacturers make color charts to tell you if the heat range is correct by inspecting the color and deposits on your spark plug tips. If you can't find one, email me and I'll try to find mine. Good luck! Tom
Check out www.ngksparkplugs.com. They's got a very good discussion of spark plug heat range and how it effects the car, the engine, performance, and related problems. They have an excellent look-up function to get the right plug for your application. BTW, the spark plug won't cause the engine to run hot, but if you are running lean, that can do it. Be sure to check the mixture if you think the engine in running too hot.
