Originally Posted by Pre-Tuner
I have a street 406 small block with aluminum AFR 195 heads and I am having some temp gauge problems. When the truck is idling, the temp gauge moves up into the 220-260 area. When I start driving again, the gauge goes back down. It's clear to me that the engine is not actually overheating and I have verified this with an infrared thermometer at the water neck. At first I thought it was a bad sending unit, so I replaced it. It was old anyway, so I didn't bother testing it. The problem was completely unchanged. I also used the thermometer on the sending unit case and it also shows an increase in temperature when this happens.
The heads do have the steam holes drilled, but I am starting to think that the gauge is just picking up hot spots when the coolant flow is minimal. Does this sound correct? Should I move the sending unit to the intake? Not worry about it?
X2 on blwn31's comments. Chevy mounts the temp sender close to the adjacent exhaust valves on the head as this is the hottest place on the engine, so it's the first place to look for signs of trouble. Aluminum transfers heat from the metal into the coolant faster than cast iron so when idling the overall coolant temp goes up some but by these adjacent exhaust valves it goes up a lot and quickly, then drops when getting into motion and with higher coolant flows. As has been suggested if this really bugs you just move the sender by the thermostat housing where the temps average out because of thermal dilution into the larger coolant flow.
You might want to raise the overall coolant temp and increase the pressure limit from the cap. With aluminum heads this will improve combustion efficiency by putting more heat into pushing the pistons instead of warming the atmosphere and the higher pressure will help prevent localized boiling around the exhaust seats. It's important to keep them wet, when steam forms the local spot cooling in this area is greatly reduced. Many guys doing serious racing with the SBC tap a coolant source off the pump horns and deliver it into the head under the header and right between the exhaust ports as a means to keep this area cooler. Others enlarge the coolant transfer holes in the head and/or gasket at this point to facilitate the same end of getting some extra coolant flow on these adjacent valve seats.
In simple terms the faster heat transfer rate of aluminum will get your attention, as it has and yes your shot about the gauge just picking hot spots is right on.