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Well after 18 plus responses, there are still some things that have not been covered or clarified.
1) Lets assume that the back of the head is actually running hotter than the front. Then a) the air/fuel mixture could be leaner in #4, possibly due to a vacuum leak, or b) the timing could be different in #4 than in the other cylinders ,or c) you could have a build up of carbon in that cylinder or a hot spot that triggers detonation.
We normally only time # 1 cylinder and assume that the others will come out the same, but with a worn dist. that is not always true. If the bushings are worn your timing will actually jump around. Do you get a steady line or a line that moves around when you time with a timing light? Does it stay steady at both high and low rpm?
2) You can have a number of problems with the coolant tube in the head - the one that runs fore and aft and directs water onto the sparkplug side of the combustion chamber throug small slots in the tube. These tubes can be loose - shake the head and see if it rattles. They can rust out. The slots can become plugged. The only way to be sure is to pull the freeze plugs out of each end of the head and take a good look, and you might even have to pull the tube. When I rebuild heads checking and reinstalling a coolant tube is a normal part of the proceedure.
And without going into details, don't believe everything you see here about fans and thermostats. If you would like to do a little experiment and learn something, run the engine with and without a fan, and with and without a thermostat.
No more time now, I have to pull out two coolant tubes.
John
V-performance.com
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