I have a 94 Civic DX Sedan (D15b7) and it idles fine during regular driving but when I turn on the heater it goes up and down, until I turn heater off.

Any ideas?

 

does it happen with the A/C on too? with the blower on, your revs will drop b/c of the added drain on the altenator. the ecu compensates for this by upping the idles speed a bit.

also when your heater is on, your car's engine temp will cool a bit. your idle is also engine temp. sensitive. it its cold (like when starting up in the morning), your idle will be high. when my engine coolant temp wiring got messed up, my idle bounced too