I just swapped in my d16z6 in my 1990 Civic Si. Just put in New Fluids and warms up fine at idle and fan kicks in as well, but when i drive the temp starts to drop to cold. Warms up fine if stopped and at stop lights. Anyone know the problem? I'm told that the vtec won't engage when temperatures are cold. Any suggestions ? thanks.

 

did you check your thremostat? aftermarket thermostats like mugen (when they go bad) are meant to fail open, means that the fluid is always surging throughout the system

 

All thermostats are designed to fail open. This will prevent over heating. They don't always fail open though. I would replace your thermostat. Don't forget to clean the ground on the housing while you are at it.

 

and do a search about "burping" the coolant system...

 

awesome thanks. i'll giveit a shot and let u guys know.