I browsed through some old posts and found similar problems but no solution. I have a freshly rebuilt d16y7 block with y8 head and it runs but has cooling issues. The cooling fan never comes on and Temps just keep going up and up. I've tested the coolant temp switch for the fan by jumping the terminals, that works. I've tested if the thermostat opens in a pot of water and that also works. The system has been filled, bled, and ran over and over but still no luck. I've used one of those radiator bleed funnels and filled it up, squeezed the hoses to try and get all the air out and just let it sit and run until it gets too hot or until coolant expands into the funnel and almost overflows. I am not aware of any leaks anywhere and the system holds pressure, but I don't think coolant is getting to the switch, although every time I take the hose off at the thermostat housing, it is filled to the brim with coolant. Not sure what else to do, currently waiting for it to cool down and then I'm gonna take the thermostat out, run it, and see if the cooling fan comes on, or if it will suck any more coolant into the system. Any suggestions help.

 

Are you using a GENUINE OE HONDA thermostat ?

 

Try a different fan switch. By bridging the plug you are taking the fan switch out of the equation, so unless you've tested the actual switch, replacing it would be the next thing to do

 

I am using a non oem thermostat, and I've switched out the fan switch and Still no luck. I took the thermostat out and ran it without and to my suprise everything ran right even having the fan kick on. Idk if this means, although the thermostat tested good in a pot of water, that the thermostat is bad but I'm most likely gonna replace that next.

 

In my experience, aftermarket thermostats typically don't have the proper flow through the plate to properly warm the fan switch in the thermostat housing neck. This is why the fan comes on when you remove the thermostat. In addition, most aftermarket thermostats do not have the proper rear plate that blocks off the bypass circuit when the thermostat opens... leaving the bypass circuit open creates a turbulent flow path which reduces cooled fluid from the upper radiator hose entering the head... causing overheating.

Get a Genuine OE Honda Thermostat.